Fishing and Gear Technology (2+1)

Unit-1 Introduction to fishing gear

1.1. History and development of fishing gears

Fishing is the art of catching fish and other aquatic animals. Many years ago man started using

various type of gear used for hunting the terrestrial animals and for fishing also. It is very difficult to

say which started first, but some time it is opined that fishing is younger. Reason behind it is easy to

catch animals in the land than in the water. In olden days fishing was not having much importance as

there was no demand for fish. Earlier fishing was restricted to a particular community but it is not so

now. In order to meet the increased demand, fishing is now carried out industrially. Fishing

technology not only concerns fishing gear, fishing methods and vessels but also concern Biological

and Environmental factors.

Fishing technology has developed continuously by utilizing improved and larger fishing vessels, more

sophisticated fishing equipment and preservation techniques. With the increase in population, larger

quantities of seafood became necessary hence individual harvesting was replaced by groups fishing

with larger and more effective fishing gear.

Mechanization started in the fishing industry during the second half of the nineteenth century with the

use of steam driven capstans. Steam power was successfully used in towing a trawl for the first time

in 1877. Steam gradually replaced manual and sail propulsion. This was later superseded by the

internal combustion engines. Today, the diesel engine is universally favoured for larger vessels and

petrol/kerosene driven outboards for small boat operations. Development of electrical and hydraulic

power systems led to complete mechanization of fishing which use larger nets and lines, or more pots

and traps.

During the last 50 years, synthetic fibres, which are virtually rot-proof, gradually replaced natural

materials in the fabrication of nets and lines. These materials have greatly extended the useful life of

fishing gear. This along with the introduction of mechanized net and rope making increased the size

and complexity of nets. Developments in new synthetic materials and fabrication techniques are

continuously improving fishing gear effectiveness and efficiency.

Traditionally fishermen detect fish through local knowledge, record keeping, observation of schools

breaking to surface, and the behaviour of birds and other sea creatures. In deeper waters fishes were

detected from the vibration of signal lines dropped from the vessel. The introduction of electronic fish

finding equipment a half century ago has revolutionized the process of finding fish to catch. Aircraft

enable much greater areas to be searched visually, while satellite and laser technology are being

utilized as aids for identifying suitable environmental conditions. Fish finding sonar and echo

sounders rapidly analyse signals from high performance transducers and display detailed information

to the fisherman.

1.2. Evolution of fishing gears

Before the invention of any fishing gear, men used their hands (hand picking) to catch fish and other

aquatic organisms from the shallow water lakes, rivers and seas. However, it was only possible to

collect sedentary, stranded slowly moving organisms by hand picking. To prevent the escape of fish,

methods were developed to stun and stupefy the fish either mechanically or electrically. The capacity

to capture live fish or the stupefied ones was limited by the length of the arm. To overcome this

problem and to extend the range of the fisherman, spears or lances were developed. The reach of

spears pushed by hand can be extended by using them as a casting gear. To prevent the fish from

escaping the thrown gear, facilities for retrieving the catch are necessary which is characteristic of

harpooning

The use of spears for catching fish implicates some injury to the prey. To catch delicate organisms

without causing damage, a number of grasping instruments have been developed. Eg. clamps, tongs,

rakes etc. These gears like the spearing and shooting gear have been developed to extend the range of

the human arm and to secure the catch more efficiently.

Luring the fish with a bait and catching them led to the development of line fishing. The bait is

presented in such a manner that the fish is neither able to take away the bait nor it can escape once it

takes the bait. Various types of fishing lines have developed. The simplest form of fishing line is the

hand line. The number of hand lines operated by a single man is limited. In order to operate more

lines, set lines have been developed. From simple fishing lines capable of catching a single fish, long

lines with thousands of hooks which extend to considerable length have been developed. The need to

operate over a larger area using a movable fishing line led to the development of trolling lines.

Fishing with primitive gear is confined to shallow water areas. Permanent and semi permanent and

barriers were erected in the areas with changing water level for catching fish. This is practiced in the

areas of fresh waters and the ebb and flow areas along the sea coast. The barriers prevent fish from

escaping when the water recedes. This was the beginning of filtering fishing gear.

After the invention of net making, netting made of fibers of different origin were used for gear

construction. First, fishing gear was made of wooden material like sticks and flexible branches. There

was a transition from wooden material to netting material in the construction of traps and barriers and

bag nets. The bag nets are held open by means of a frame. Scoop nets, scrape nets and gape nets come

under this category.

From the primitive rake, the development lead to the creation of the dredge. From the dredge first the

dragged stow net and the trawl was developed. Trawling can be considered as further development of

dredges. The desire to operate on wider areas and to catch more fish than was permitted by the

maximum length of the frames of dredges led to the development of towing gear like trawls. With the

advent of fish detecting devices and other electronic equipments, mid water trawls were developed to

catch pelagic species.

Seine nets without bags have evolved from a net wall, while the seines with a bag developed from a

bag net like stow nets. The desire to catch the shoaling fishes without allowing them to escape led to

the evolution of surrounding nets. These gears are designed in such a way that they surround the fish

shoal from below as well as sides to prevent their escape in any direction e.g. purse seines. On the

same principle as scoop nets another type of gear called lift nets were developed which are dipped in

water and then lifted as soon as the fish have gathered over them. The lift nets made of netting had

their predecessors made of twigs and bast. Contrary to the lift nets, certain gears called falling gears

secure their catch from above. Gears have been developed to catch high individual value fish without

causing damage to them using the principle of gilling e.g. gill nets. Idea of catching fish by entangling

them in water plants in earlier days led to the development of present day entangling nets like single,

double, or triple walled (trammel) tangle nets.

1.3. Classification of fishing gears of world and India

A. Von Brandt classified fishing gears based on these principles of capture

Without gear – This includes hand picking, fishing with feet, simple tools for catching fish

etc.

Grappling and wounding gear – In this method, the fish are caught by grappling, squeezing,

piercing, transfixing or wounding. The implements used for this are spears, clamps, tongs,

rakes, harpoons etc

Stupefying devices – they prevent fish from escaping by stupefying or stunning. This is done

either by mechanical, chemical or by electrical stupefying.

Lines – In lines, the fish is offered a bait which once taken makes them unable to escape.

Traps – A fish enters the trap voluntarily but is prevented from coming out by means of

certain arrangements.

Fishing in the air – Aerial traps – Fish that jump out of the water when faced with an obstacle

are caught in the air by special devices.

Bag nets – These are kept open vertically by the frame and horizontally by the current. Fish

enter voluntarily and are caught by filtering.

Dragged gear – These gears are towed through the water either on or near the bottom or on

the surface and manner of capture is by filtration

Seine nets – gears with long wings and towing warps with or without bags. The manner of

capture is by surrounding a certain area and towing the both ends of the gear to a fixed point

on the shore or on a vessel.

Surrounding nets – Nets that surround a shoal of fish not only from the sides but also from the

bottom.

Drive-in-nets –The fish to be caught are driven into the gear by swimming or diving

fishermen.

Lift nets –Sheets of netting are lowered into the water and lifted when the fish swim or creep

on to them.

Falling Gear – The manner of capture is to cover the fish with a gear and are normally

operated in shallow waters

Gill nets – Principle of gilling is made use of in gill nets. Fishes are caught by their gills

(gilling) in the meshes of a gill net.

Tangle nets –Fishes are caught by entangling any protruding part of their body to the net.

1.4. Fishing Gears of India

Fishing gears of India can be broadly classified into

Fishing implements.

Traps and barriers.

Hooks and lines.

Nets

Fishing implements – These consist of most primitive types of fishing gears like fish spears, sickle,

blow gun and others.

Fish traps and barriers – Traps of different types like basket traps, box traps, traps for jumping fish

etc. are used in different parts of the country. Screen barriers are also used which are set in shallow

tidal backwaters.

Hooks and lines – These are effective in catching large predatory fishes. Rod and line is used in sports

fishing. Long lines and trolling lines are used to catch high individual value fish.

Nets – Cast nets are widely used in inland and marine waters of India to catch fishes. These are used

in clean waters with even bottom. There are stringless cast net and also stringed cast net.

Fixed bag net - Stake nets are conical bag nets set in streams and tidal waters to filter out fish coming

along the current. They are held in position by stakes driven into muddy bottom. A stout coir rope

encircles the mouth.

Dip net - Chinese dip nets are highly specialized nets used in Kerala. It is stationary net balanced by

counter weight so that it can be dipped and lifted.

Among the shore seines the most popular one is the Rampani net of Karnataka. This is also operated

in certain parts of Goa and Maharashtra.

Single and two boat seines are used in many parts of India to catch shoaling pelagic fishes.

In the case of gill nets, the length, depth and mesh size varies depending on the type of fish to be

caught. The anchored gill nets made of hemp were used along the South Canara coast. Coconut husks

are used as floats and heavy stones as sinkers. However, at present the gill nets make use of synthetic

twines. Drift nets are one of the most efficient indigenous gears used in India. Surface drift nets are

extensively used in Malabar and South Canara coasts e.g. Kola vala. Bottom drift nets are commonly

used on the Veraval – Porbunder (Dhakal) for catching pomfrets. Besides these, trawls and purse

seines are operated for bulk fishing in different parts of the country.

Unit 2 - Fishing gear materials

2.1. Classification of fibres

Netting is defined as a meshed structure of indefinite shape and size composed of one or more yarns

twisted or interlaced or joined. The basic material for the construction of fishing gear is composed of

fibre. A fibre may be defined as a unit of matter of hair like structure/dimensions whose length is at

least 200 times greater than its width.

Fishes are classified based on the sources from which they are obtained as natural fibres and

manmade fibres. Natural materials are obtained either from plants i.e. vegetable fibres or from

animals -animal fibers. Vegetable fibres have a cellulose base and are prone to rotting in water as they

are attacked by cellulose digestive microorganisms. Vegetable fibres are obtained from different

source of plant such as leaves, stalk, fruit and seeds. These fibres are further classified as seed fibre,

fruit fibre, bast fibre and leaf fibres.

Seed fibre

Cotton fibres are the seed fibres derived from the cotton plant. Before the introduction of synthetic

fibres, the cotton fibre was the most popular. They are extremely fine with a length of only 20-50 mm

and a diameter of 0.01 to 0.04 mm. being dull white colour and owing to their fineness; they were

used in light and heavy fishing gear.

Bast fibres

Bast fibres extracted from the bast tissue of stems. Eg: Ramie fibre also called as china grass has a

length of 8-25cm and diameter of 0.024-0.070 m.

Hemp is derived from the bast of the plant cannabis sativa.

Leaf fibre

Leaf fibres are hard fibres obtained from long leaves. Eg. Sisal, Manila

Sisal is from the leaves of sisal plant. It is 75-100 cm long and 0.01-0.03 mm dia. Manila is obtained

from the Abaca plant. These fibres have large diameter compared to other natural fibres and hence it

is used in heavy netting.

Fruit fibre

Coir is a fruit fibre obtained from the coconut husk. The fibre length varies from 15-25 cm and

diameter 0.01-0.025 mm. This is mainly used for ropes.

Animal fibre

Animal fibres are composed of protein. Eg. Silk is a very costly fibre so it is used only for some

specialized fishing gears

2.2. Rotting

Vegetable fibres are parts of dead plants and consist of cellulose. When exposed to humid condition

they are attacked by cellulose digesting microorganisms, especially bacteria. This process of

decomposition of dead organic material releases the inorganic nutrients for new plants. Unfortunately,

effect of decomposition on fishing net leads to increased labour and financial loss. This was the main

reason for the development of synthetic fibres. In rotted fibre there is a direct relation between the

corroded film in a fishing gear and breaking strength.

Factors responsible for rotting

Kind of fibres: Type of fibre smooth or rough

Water temperature: The activity of cellular digestion bacteria depends on the extent of

temperature. During cold season and decay of vegetable fibre is considerably slower than

during the warm season. Tropics net become useless faster than in temperate climates.

Rotting power of water: Running water generally have greater rotting power than stagnant

waters. Fertile water contains high percent of organic materials; lime, and phosphorus, natural

fibres quickly deteriorate due to presence of microbes.

Duration of immersion in water: Fishing net continuously immersed in the water for a long

time is more liable to rotting than a net used only temporarily or occasionally.

Depth/place of net operation: When a net kept in the putrid zone or in contact with mud the

rate of deterioration is quicker than a net set on the surface.

2.3. Synthetic fibre

The development of synthetic fibres was started around 1920 by H. Staudinger, a winner of Nobel

Prize for chemistry in 1953. He found that all fibrous material consists of chain of molecules in which

a great number of equal simple units are linked together. Synthetic fibres are man made fibre

manufactured by chemical synthesis of simple substance like phenol, benzene, acetylene, prussic acid.

Depending on the type of polymerization, 4 different groups of compounds can be identified.

Poly condensation: Water is eliminated. eg. Polyamide (PA), Poly ester (PES), Terrylene, etc.

Polymeric compound: water is not eliminated. Polyvinyl chloride (PVC), Polyvinyl Alcohol

(PVA), Polyvinylidin Alcohol (PVAA).

Mixed polymers: These are formed by co-polymerisation of mixture of vinylidene and vinyl

alcohol. PVD, Polyamolidine chloride.

Poly additive compound

Physical properties:

Density: It is the mass per unit when and is expressed (g/cm3). Fibres with low density like

PE(0.96), PP (0.91) float in water and hence hold more weight. Low density material gives

more length/weight and vice versa. This property of the material is significant in construction

of certain gear like purse seine. The sinking speed of the gear increase with the increase in the

density of the material.

Breaking strength: This is the maximum force applied to a specimen causing it to rapture or

break. It is expressed in kg and special equipment known as Breaking strength

testers/Dynometer are used for determining is expressed in terms of tenacity and tensile

strength.

Tenacity: It is the Breaking strength per unit denier and Tenacity = Breaking loading / Total

density

Tensile strength: It is the Breaking strength per unit area and expressed as kg/mm2.In wet

condition PA & PVAA loose breaking strength and PES, PE and PP remain unaffected.

Elastic property: Elasticity is a property of a material by virtue of which it tends to recover

its original length and shape after removal of the stress.

Extensibility: This is the property of a material to undergo extension/elongation under the

influence of a force.

Water absorption: This is the property of netting material to absorb water when it is

immersed in water and expressed as % of dry weight. Natural fibres absorb more water where

as synthetic materials do not absorb water.

Shrinkage: This is the reduction in length of a material due to wetting. Shrinkage is not a

major problem in synthetic fibres.

Abrasion resistance: This is the resistance of a material to wear off any part when rubbed

against any surface. The abrasion resistance of vegetable fibre is comparatively less. Staple

fibre show lower resistance to abrasion than continuous fibres.

Sinking speed: This is the speed at which material sinks through the water. It is important in

purse seine.

Visibility: It is the property of the netting material to be seen in water during operation. This

property is relevant for gears like gill net. Veg. fibre is more visible than synthetic fibres.

Mono filament is almost invisible as it as transparent.

Melting point: This is the property of heat resistance of the material. The melting point and

shrinking are important factors while dying the fibres. PA & PES are more heat resistant than

PE & PP.

Weather resistance: This is the property of the materials to with stand the effect of sun light,

rain, wind, smoke and gases and other atmospheric element. Natural materials are more

resistant than synthetic. The rate of deterioration is measured in terms of breaking strength.

2.4. Identification of fishing gear materials

The synthetic fibre groups have well-defined characteristics which help us to distinguish it from other

groups. These characteristics determine the suitability of a fibre for a particular fishing gear. The

fisherman should be aware of the chemical group his net material belongs. Unfortunately it is very

difficult to differentiate various kinds of synthetic fibres. There are less visual differences between the

various kinds of synthetic fibres than there are in vegetable fibres and synthetic netting material can

therefore rarely be determined by its appearance alone. If you know the trade name it is possible to

find out the chemical group from trade-name lists. If this is not the case, several methods of

identification may be applied. In the following, only those are considered which can be carried out

easily and without elaborate techniques and apparatuses (with the exception of the determination of

the melting point.). This excludes for practical reasons more elaborate methods which require

expensive equipment and specific expertise such as microscopic examination and micro-photographs,

staining with special dyes or reagents, determination of fibre density, as well as infrared spectroscopy

gas chromatography and differential thermal analysis.

Finishing agents (pigments, colours, delustring and stabilizing agents adhesive substances, oil, starch,

resin) used for dyeing stiffening and stabilizing the netting yarn, the netting, or its knots, may change

the test results to be expected from raw and unfinished materials. They should therefore be removed

from the specimen by a suitable simple treatment, taking appropriate care that the fibres are not

unduly damaged. For instance, boiling in distilled water will often be sufficient.

2.4.1. Water test

This is the first step in the identification of a method. A small piece of netting yarn is put into a vessel

filled with water after tying a simple overhand knot. Air bubbles in the material must be squeezed out

by hand under water. Preferably a wetting agent (e.g. 1g 1 Lissapol N or Nekal BX) should be added

to the water. The water test serves to classify the netting material in to two groups. i.e. those synthetic

fibres which float in water (PE and PP) and those which sink (all other kinds of synthetic fibres)

2.4.2. Visual inspection

The preliminary identification of the material is possible by observing the types of fibres used for the

construction and appearance of the netting yarn used. The type of single yarn used for the

construction, whether it is shiny or soft, smooth or hard will give an idea about the material used. For

example PE is not produced as continuous filament and staple fibres. Similarly PP is produced only as

split fibres for fishing nets.

2.4.3. Burning test

The changes in the material when it is nearer the flame and after removal from

the flame are noted. For this test only a clean flame and two forceps are needed.

The best source of flame is a Bunsen burner. Alternatively alcohol lamp or even

a cigarette lighter can be used. A piece of netting material is bought near the

flame and then removed. The melting and burning patterns and the nature of the

residue produced if any are observed. In the early days of the introduction of

synthetic fibres into fishing the burning test was very often used by fishermen

as it is the simplest test for distinguishing synthetic fibres from cotton.

Vegetable fibres and most of the man-made fibres made of regenerated

cellulose burn rapidly in the flame and continue burning after removed from the

flame. They have an afterglow, the smell of smoke is similar to that of burning

paper and the residue consists of a small amount of fine ash. The synthetic

fibres being thermoplastic shrink and melt in the flame; the melting substance

drips from the flame, mostly forming a bead or a hard irregular residue.

PA 6.6 and

PA 6 PES PE PP PVC Saran PVA(A)

In flame Melts and then burns

with light

flame.White smoke,

Drops of yellowish

melting drip down

Melts and

burns with

light flame.

Sooty,

blackish

smoke, drops

of melting

drip down.

Shrinks,

curls,

melts and

burns with

light

flame.

Drops of

melting

drop down

Shrinks,

melts and

burns with

light

flame,

drops of

melting

drip down

Shrinks

rapidly and

melts does

not burn,

sooty,melts

into a

crumbling,

black

substance

Melts and

burns with

light flame.

Shrinks, curls

and burns

very rapidly

with light

flame.

After leaving

flame

Stops burning if

melting drops fall

down. Small bead on

the end of the

sample. Hot melting

bead may be

stretched into fine

thread

Stops burning

if melting

drops fall

down. Small

black bead on

the end of the

sample. Hot

melting bead

may be

stretched into

fine thread

Continues

to burn

rapidly.

Hot

melting

substance

cannot be

stretched

Continues

to burn

rapidly.

Hot

melting

substance

may be

stretched

into fine

thread

Hot melting

substance

cannot be

stretched.

Stops burning

immediately.

Hot melting

substance

may be

stretched into

fine thread

Continues to

burn rapidly.

Melting

substance

cannot be

stretched

Residue Hard, round,

yellowish bead; not

crushable

Hard blackish

bead; not

crushable

No

melting

bead; like

paraffin;

crushable

Hard,

round

brown to

blackish;

not

crushable

Hard,

blackish,

crumbly

irregular; no

bead

Porous,

blackish

irregular

substance;

crushable; no

bead

Hard, brown

to black,

irregular; not

crushable

Smell of the

smoke

Celery-like; fishy

odour; faintly like

pyridine.

Oily, soot

odour; faintly

sweet; similar

to scaling-

wax;

aromatic

Similar to

burning

asphalt;

like wax or

paraffin

Similar to

burning

asphalt;

like wax or

paraffin.

Sharp, sweet;

sweetish-

sourish

Sharp, acrid Sharp,sweet

like chlorine.

Table shows the reactions to the burning test of the different synthetic fibres by which they can be

identified. However, in practice the differences are not always so clear as shown in the table. For an

accurate identification it is necessary to verify the results of the burning test by the solubility test

and/or the melting point test.

2.4.4. Solubility test

This relatively simple chemical test does not require any particular skill in chemistry, or any

equipment or complicated equipment. Safety precautions common for chemical laboratories should be

observed, especially when working with boiling reagents. The requirement for the test are 25 ml test

tubes, a heat source (e.g. Bunsen burner of alcohol lamp) a holder to hold the test tube into the flame,

and the chemical reagents quoted in the table. If an electric hot plate is used instead of test tubes, glass

beakers of 50 to 100 ml in size and glass stirring rods are used.

The fibres of the sample to be tested should be loose as possible. Therefore, the netting yarn is

untwisted and the fibres are cut in to pieces of about 1cm in length. Coarse material, like split fibres,

and monofilaments, should be reduced to very small pieces. Small samples of the material and 10 to

15 ml of the solvent should be put into the test tube. Further directions for the application of the

solubility test may be taken from Table. The reagents quoted have been selected so that only one

reaction needs to be observed i.e soluble or not soluble.

In most cases it will not be necessary to conduct the test with all reagents given in Table

Polyamide (PA) fibres are soluble in the reagents (a) and (e). If it is desirable to separate the

two types from one another, (c) can be used, it which PA 6 is soluble but not PA 6.6

Polyester (PES) fibres are not soluble in (a),(f) and (g).

Polyethylene (PE) and polypropylene (PP) fibres are not soluble in (b) and (c).

Polyvinyl chloride (PVC) (not after-chlorinated; see note(3) in table 2 is the only synthetic

fibre here mentioned which is soluble in reagent (g) at room temperature.

Saran (PVD) may be identified by its solubility in (f) and (c).

Polyvinyl alcohol (PVAA) (after – treated with formaldehyde; see note (4) in Table 2) is

soluble in (a) but not in (e).

PE and PP cannot be distinguished from one another by a solubility test. The burning test

(Table) shows some differences in the reactions of these two synthetics but it is not always

sufficient for identification. The most reliable method to distinguish PE from PP is to

determine the melting point.

Table-identification of synthetic fibres by solubility tests

Reagent\kind of fibre PA

6.6 PA 6 PES PE PP PVC Saran

PVA

(A)

(a) Hydrochloric acid/HCL (37%)

30 minutes at room temperature

+ + O O O O O +

(b) Sulphuric acid/H2SO4 (97-98%)

30 minutes at room temperature

+ + + O O O (+) +

(c)Dimethylfomamide/HCON(CH3)(1)

5 minutes boiling

O + + O(2) O(2) +(3) + O

(d) Formic acid/HCOOH(96-100%)

30 minutes at room temperature

+ + O O O O O +

(e) Glacial acetic acid/CH3-COOH

5 minutes boiling

+ + O O O O O O

(f) Xylene/C6H4(CH3)2

5 minutes boiling(inflammable!)

O O O + + O(2) + O

(g) Pyridine

30 minutes at room temperature

O O O O O + (+) O

+ = soluble O = not soluble; (+) = soluble only if boiled, but not at room temperature.

(1) Dimethylformamide is decomposed by exposure to light even when store in a brown bottle. It

should be stored away from light and preferably in a cool place. (2) Destroyed but not soluble. (3)

Soluble already at room temperature.

2.4.5. Melting point test

The melting points of the thermoplastic synthetic fibres are significantly different. The determination

of the melting point of an unknown net material is, therefore, a very reliable identification method.

Unfortunately the equipment needed is not quite simple, although the actual test with suitable

equipment is easy to perform. The melting point test is mentioned here, mainly because it enables one

in relatively simple manner to determine type of material.

Unit 3 - Construction of twines

3.1. Introduction

a wide variety of materials ranging from natural fibres, wood and metal to synthetics are used in the

construction of different fishing gears. Most of the widely used fishing gears such as trawls, purse

seines and gillnets make extensive use of netting in the process of capture and for retraining the catch.

Due to their superior strength properties, service life and rot resistance, man-made synthetic fibres are

almost exclusively used for construction of fishing twine, ropes and netting in recent times.

3.2. Basic fibre forms Continuous filaments: These are fibres of indefinite length and they are silk like in appearance and

are produced with different degrees of fineness. Generally much thinner than 0.05 mm diameter.

Fishing gear is usually made of filament a 1000 m of which weigh between 0.6-0.2 g. eg.PA.

Staple fibres: These are discontinuous fibre, usually prepared by cutting filament into length suitable

for the yarn spinning process. Fineness is similar to that of continuous filament but length varies for

40-120 mm. These short fibres are held under pressure to form continuous strand which is called as

single yarn. Netting yarn made out of staple fibre has a rough surface owing to the numerous loose

ends of fibre sticking out from the twine. This hairy nature decreases the slippage of knots. Staple

fibre has lesser breaking strength than continuous filament.

Monofilament: This is a single filament which is strong enough to function as a yarn; without any

further processing. Transparent P.A monofilament is used in gillnets. These are stiff, larger in

diameter, transparent and less visible.

Split fibres: are developed recently. These are originated from plastic tapes, which are stretched

during manufacture to the extent that tape split longitudinally when under pressure. Yarn made of

these filaments contains fibres of irregular fineness.

3.3. Single yarns Single yarn is the simplest continuous thread composed of fibres

Types of single yarns

Single spun yarn

Single Filament yarn

Single mono-filament yarn

Single split fibre yarn

3.4. Netting twine

Netting twine is manufactured by one single twisting operation of two or more single netting yarns or

monofilaments. Netting yarn is the simplest continuous thread composed of fibres. Based on their

construction twines are classified into cabled or twisted and braided types.

Construction of braided netting yarn

Braided netting yarn is produced by interlaying a number of strands in such a way that they cross each

other in diagonal direction to the edge of the fabric. This braided material is in the form of a tube.

This braided material composes of core, number and kind of strands. Twisted netting twines are more

widely used in fishing gears.

Cabled or twisted netting are manufactured by combining two or more netting twines, by further one

or more twisting operations. The single yarns are first twisted into folded yarns in the S-direction

(left-hand laid). Most twisted netting twines are with three-folded yarns.

Structure of the braid:

Structure of the braided twine can be one strand over one and under another. One strand over one and

under two others and one strand over two and under two others.

Construction of Core:

Core can be prepared by using a single yarn, folded yarn and monofilament. Mostly monofilament

material is being used up to 30 numbers depending on the thickness of the material. It also gives

strength to the product of braided material is prepared without core, knot stability is reduced to get

knot stability soft braided material should be treated with bending agent/heat setting.

Braided netting twine is manufactured by plaiting three or more netting yarns. Braided twines are

further grouped into crossing-laid which is without a core and tube-shaped in which braiding is done

around a central core. Braided twines are less prone to kinking compared to twisted twine.

3.5. Ropes

Ropes are extensively used in the fishing gear construction and operation. They provide the frame

work and linear strengthening components in the fabrication of fishing net such as gillnets, trammel

nets, seines and trawl nets and constitute main line and buoy lines of large longlines. Fibre materials

used are Polyamide (PA), Polyester (PES), Polypropylene (PP), Polyethylene (PE), Polyvinyl alcohol

(PVAA), manila, sisal, coir, hemp and steel wire. Continuous filaments, staple fibres, monofilaments,

film split fibres and natural fibres and combination of different fibre types are used in construction of

ropes.

Construction of ropes

Rope generally consists of three or more strands cabled or braided together, with or without a core,

with a finished diameter of not less than 4 mm. Based on construction, ropes are grouped into laid and

braided. In the laid rope, fibres are twisted into rope yarn and two more yarns are twisted in the

opposite direction to produce folded yarns. Three or four folded yarns are laid in the direction

opposite to the twist of folded yarns to produce the laid rope. Three strand constructions is the most

commonly used and it is called hawser laid or plain laid rope. Depending on the number of twists per

unit length, hard, medium and soft-laid ropes are available. A four folded yarn construction is called

shroud-laid rope. Three or more laid ropes are combined by twisting in the opposite direction, to

produce cable laid ropes. In the braided construction, the strands are plaited together to produce

different types of braided ropes. Ropes are specified by material, nominal diameter and construction.

Fibre ropes

Fibre ropes used in fisheries are manufactured from natural fibres such as manila, sisal and hemp and

synthetic fibres such as polyamide (PA), polyester (PES), polypropylene (PP), polyethylene (PE) and

polyvinyl alcohol (PVAA). Continuous filaments, staple fibres, monofilaments, film split fibres are

used for manufacture of fibre ropes. Fibre ropes are widely used for framing and strengthening of

fishing gears and also as warps, bridles and sweeps in small-scale trawling operations. Fibre ropes are

vulnerable to stretch under sustained strain. Natural fibre ropes are mostly substituted by synthetic

ropes, due to their superior rot resistance, long service life and high breaking strength. Considering

ropes of same nominal diameter, polyamide (PA) rope has the maximum breaking strength, followed

by polyester(PES), polypropylene(PP), polyethylene (PE), polyvinyl alcohol (PVAA) and natural

fibre ropes.

Construction of fibre rope

Single fibres are twisted to form rope yarn. Two are more rope yarns are twisted to form folded yarns.

Two or more folded yarns are twisted to form fibre rope. Construction of synthetic ropes – Here, the

rope yarns may be single yarns or folded yarns and two or more folded yarns are twisted to form

cabled yarns. Two or more cabled yarns are twisted to form cable netting yarns. Two or more of these

cable netting yarn are twisted to form synthetic rope.

Knotless webbing

Japanese twisting technique

Raschel technique

Braiding technique

Advantages and Disadvantages of knotless webbing and knotted webbings are same. But knotted

netting has some principle disadvantage. (1) Knot decrease in breaking strength (2) knots are

protruded and abrasion is more. (3) Substantial % of twine is consumed for knots (4) The increase of

weight (5) Hydrodynamic resistance is increased

As dirt does not adhere so much to knotless netting it is easy to clean and dry.

Unit 4 - Yarn numbering systems/designation of netting yarn

4.1. Introduction

Size of netting yarn is designated as linear density by yarn numbering system, as it cannot be

described by thickness or diameter due to practical difficulties. Numbering to designated the thickness

of yarn is expressed either as mass per unit length (direct system) or as length per unit mass (indirect

system). Important direct numbering systems in use are the direct tex and denier systems and the

indirect metric numbering system, British cotton count and runnage systems.

4.2. Numbering systems

Direct system

In this system the length of the material is constant but weight varies. eg 1000m of yarn weigh 1 g. is1

Tex ( The tex system (Tex) specifies twine size in terms of weight in grams per 1000m of single

yarn.) The tex system is recommended by international standards organization and is widely adopted

and is expected to eventually replace other systems. The denier system (den or Td) which is also

widely used specifies the size as weight in grams per 9000m of the yarn. Denier value is converted

into tex by a multiplying factor of 0.111 and tex into denier by multiplying factor of 9.

Indirect system

this is usually used to express the size of the finished products. In this system the weight is kept

constant and length varies.

The metric system

The metric system (Nm) designates the size as number of kilometers of yarn which weigh one

kilogram is known as 1 Nm. Tex value is obtained by dividing 1000 by metric count and denier value

by dividing 9000 by metric count.

The british cotton count

The british cotton count (Nc) is the number of hanks (840 yards) which weigh one pound. Equivalent

tex and denier value to british cotton count can be obtained by dividing 590.5 and 5315 by British

cotton count respectively.

Runnage system

Runnage system is expressed in terms of meters/kg, ft/lb, yards/lb etc and is mainly used for the

finished products only.

4.3. Conversion formula

since different systems are followed in different countries there is a formula which can be used for

converting from one system to another.

4.4. Calculation of Resultant tex The final product and netting yarn may be designated by the resultant density indicated by symbol

“R” to be put before the numerical value. Thus R tex means the mass in gm per kilometer of the final

product. Size or linear density of twine and ropes are designated by resultant tex (R-tex). It is roughly

10-15 percent more than the combined tex value of the constituent yarns in the finished product.

Example for calculation to be included

20 tex Z 700 x 2 S 400 x 3 Z 200; R132 tex

4.5.Complete designation of netting yarn or finished product

According to ISO the complete designation of the above netting yarns as follows.

20 is the Tex value of single yarn

2 No. of single yarns in the first fold

3 No. of folded yarns in the finished product

S and Z are the direction of the twist

700, 400 and 200 are the no. of twist

R tex value of the finished product

Another system which is in common use for finished netting twine s and ropes, is runnage. It is

expressed as m.kg-1

/ feet lb-1

, yard lb etc and is the reciprocal of Rtex.

Unit 5 - Construction of netting/webbing

5.1. Types of knots

There are different types of knots and hitches used in making netting, mending, joining and mounting.

They are as follows

Knot:

It is a tie made by one end of a rope or twine at some parts of its own body to make a loop or

overcome other object to get it fastened to it.

Hitch:

Hitch is made of series of knots by which one rope is joined to another or made fast to some object

such as spar.

Sheet bend knot/English knot – It is made by passing the end of one line through the eye or

bight of the other around the eye and under itself.

Double sheet bend knot – It is a similar knot of single sheet bend but with the end rope led

twice underneath the eye instead of once.

Fisherman’s knot – This knot is made of two overhand knots which slide together when the

ropes are pulled forming a firm knot.

Overhand knot – It is formed by passing the end of the line over the standing part and

through the eye.

Square knot/Reef knot – This knot is formed of two overhand knots with the ends always

falling in line with outer parts.

Bowline – It is a knot tied in such a way as to produce an eye or loop in the end of a rope.

Figure of eight knot – It is a knot made in the end of a rope by passing the end of the rope

over and round the standing part, over its own part and through the bight.

Sheep shank – It consists of two long bights in the rope and a half hitch over the end of each

bight made in the standing part of the rope.

Clove hitch – Bend formed by two half hitches, the second reversed so that the stand part is

between the hitches.

Rolling hitch – It is made like clove hitch with an intermediate round turn.

Round turn and two half hitches – it is made by taking a full turn round the post or bollard

and finishing the knot off with two half hitches round the standing part of the rope.

5.2. Netting

Netting or webbing is a meshed structure of indefinite shape and size produced by interlacing or

joining one or more systems of yarns. Most commonly used netting materials have quadratic or

diamond shaped meshes which is either knotted or knotless. Of late, netting panels with square and

hexagonal meshes are also used in fishing gear construction. Different types of fishing gears such as

trawl nets, seine nets, surrounding nets, gillnets and entangling nets, lift nets, falling gears, stow nets

and traps, extensively use netting panels as a restrictive barrier in their design and construction. There

are two ways of net making namely machine making and hand making.

Hand braiding

Construction of webbing by hand is initiated either by using chain mesh method or by using loop

method. Knotted netting panels can be hand fabricated but are normally machine-made. Different

types of knots are used in construction of netting. The most popular knot used is the English knot

which is also known as weaver’s knot or sheet bend. Reef knot is useful in preventing knot slippage.

Knot breaking strength in knotted netting varies with knot type, material type, treatment and thickness

of knotted twines. In general knot breaking strength decreases with the angle of the loop in the knot

and increases with the number of loops in the knot. Double weaver’s knot has the strongest knot

breaking strength, followed by single weaver’s knot and reef knot.

Braiding is initiated by laying a foundation line which is a strong, thick twine stretched between two

supports. Clove hitches are made on the foundation line. The number of clove hitches made is one

extra over and above the required number of meshes. When this is over, a mesh gauge must be used to

obtain meshes of uniform size. The type of knot made depends on the requirement. The topmost edge

of the webbing and the lowest edge of the webbing are called as Head and Foot of the webbing.

In chain mesh method a big loop is formed on the foundation line. This loop act as base for the further

net making. Then series of required number of meshes are formed. After that each meshes should be

stretched then reeved on to the main line. Further fabrication is carried out according to the number of

meshes required in depth of the webbing.

Machine making

The introduction of net making machines nearly one hundred of years ago made efficient production

of webbing, this accelerated the fishing industry. In this method one can prepare the webbing having

indefinite length but the width of the net is restricted to the width of the machine. By adjusting the

machine we can prepare the webbing of required mesh size. However, the main drawbacks of

machine made nets are: they cannot be baited – decrease the net – or creased – increase the net, and

machines can’t braid fly meshes on to net pieces which is necessary for the construction of trawl nets.

But the above set-backs are not so serious in practice because if it is necessary to decrease or increase

the net, this can be done by cutting to the shape required and braiding the edges with twine later. The

fly meshes can also be braided by hand once the net has been made and cut to shape. In the process of

making nets in such a manner, a certain amount of wastage will be there over but this can be reduced

with careful initial planning.

Knotless netting:

Two types of knotless netting are used in fisheries (i) Japanese twisted type and (ii) Raschel type.

Advantages of knotless netting are higher mesh breaking strength and low material loss due to

absence of knots and lower weight and hydrodynamic resistance compared to knotted netting.

Knotless webbing:

Japanese twisting technique

Raschel technique

Braiding technique

Advantages and disadvantages of knotless webbing and knotted webbings are same. But knotted

netting has some principle disadvantage. (1) Knot decrease in breaking strength (2) knots are

protruded and abrasion is more. (3) Substantial % of twine is consumed for knots (4) The increase of

weight (5) Hydrodynamic resistance is increased

Advantages of knotless webbing:

For the same area knotless netting would be lighter and bulk are greatly reduced

Easy to handle

Production cost is lower due to the considerable savings of material (25-30% cheaper)

Knotless netting can have a lower towing resistance

Abrasion resistance is higher

Breaking strength is higher

Constant and accurate mesh size

Dirt does not adhere so much to knotless netting which is easier for cleaning and quick

drying.

5.3. Shaping of webbing

There are two ways of tapering with hand braided webbing i.e. baiting and creasing. However in the

case of a machine made webbing, cutting has to be invariably followed to obtain the required taper.

5.3.1.Baiting

Baiting derived from the word abate which means decreasing It is reducing the number or meshes in

concerned rows at regular intervals to shape the webbing. For this two half meshes of the previous

round are picked up in a single knot of the succeeding round. The degree of taper required decides the

rate of baiting. It is calculated in the following way.

Where

a/b is the baiting ratio

M1 is the number of meshes at the top of the webbing

M2 is the less number of meshes at the bottom of the webbing

D is the number of meshes in the depth of the webbing

5.3.2. Creasing

Creasing is the word derived from the word increase. It is altering the shape of a net by increasing the

number of meshes in concerned rows at regular intervals. This is normally done by making an

additional loop after the completion of the mesh in the same knot. This loop acts as full mesh for the

next row. After which normal braiding is followed. The degree of taper required decides the rate of

increasing. It is calculated in the following way.

Where

a/b is the increasing baiting ratio

M1 is the more number of meshes on the webbing

M2 is the less more number of meshes on the webbing

D1 is the number of meshes in the depth of the webbing.

5.4. Fly meshing Fly mesh is a full sized mesh but having only 2 knots instead of the usual 4 knots. These are made on

the edge of the webbing by missing out the last mesh of the previous round. This is also one of the

methods of shaping a piece of webbing.

5.5. Tailoring/cutting of webbing In certain types of fishing gears, the webbing has to be tailored considerably to acquire the required

shape of net especially in trawls, seines etc. Braiding is not feasible when large scale manufacture of

nets is needed as it is time consuming as well as uneconomical. In this case, machine made webbings

are cut to required shape with minimum wastage. To accomplish this, proper methods of cutting are to

be followed.

Methods of tailoring Point – Bar system: Each mesh consists of four bars (legs) and four points (knots). There are two

ways of cutting a mesh.

Point cut: Cutting of 2 legs in each knot. All point cut gives a right angle webbing

Bar cut: Cutting of only one leg at each knot. This results in a right angled triangular piece

where the two sides are equal.

Tailoring rates:

Varying degrees of taper in the webbing is brought about by a combination of point and bar cuts. If

‘N’ is the number of meshes at the base and ‘L’ the number of meshes in height of a triangular

webbing, the number of points (P) to be cut is (L-N). The number of bars (B) to be cut is always (2N)

irrespective of whether point cut is included or not. Therefore to have triangular webbing having ‘N’

meshes at base and ‘L’ meshes in height the calculation is as follows:

Where,

P - point cut

B - bar cut

L is the number of meshes in the depth of the webbing

N is the number of meshes in the longer side of the triangle

Unit 6 - Mounting of webbing

6.1. Introduction The process of hanging the webbing to the main lines like head rope and foot rope is known as

mounting. Actual shape of a mesh or netting panel is determined by the process of hanging. While

hanging one should apply proper hanging coefficient to get required horizontal opening of the mesh

and proper shape shape to the net. The shape and looseness of netting depends on the hanging

coefficient. The hanging ratio is defined as the length of the frame/head rope to the stretched length of

netting and it is expressed in terms of decimal, percentage and vulgar fraction. The hanging ratio is

also expressed by another term, hang-in or take up ratio. This refers to the amount of excess webbing

expressed as percentage of total length. The hanging coefficient varies from net to net and in some

cases it varies in different parts of the same net itself

6.2. Methods of mounting

Reeving – This is the method where all the end meshes are passed through the main line to which the

webbing is to be hung. These meshes are either fixed individually or after few meshes at the required

distance at regular intervals the uniform distribution of these meshes along the main line is again

depends on the hanging coefficient.

Stapling – This method of mounting is widely used in commercial fishing. In this method, a stapling

line (a thicker twine) is hitched to the head line near one end. The end of the mounting twine is tied to

the main line or head rope by rolling hitch with first mesh. The mounted line (staple line) is then

passed through particular number of meshes and hitched on to the main line at regular intervals by

means of clove hitch. This is continued till the other end of the netting where it is stapled with the

main line by rolling hitch. The number of meshes taken and the distance between each staple depend

upon the hanging coefficient.

Norselling – This is a method of mounting in which the hanging line is attached to the head line by

means of short pieces of twine or line called as Norsel.

Setting – It is the method of spreading or spacing the meshes of the netting on the head line to give a

definite degree of openness to the meshes.

Rigging - Rigging is the process of fitting the necessary ropes and accessories so as to make a net

ready for fishing. Rigging affects the efficiency of a gear. A well constructed net may give

unsatisfactory results due to defective rigging. All auxiliary components like ropes, floats, sinkers etc

should be of the right type and fitted in the correct position so that the net acquires required shape

during its operation.

Unit 7 - Floats and sinkers

7.1.Introduction Floats are essential components of fishing gears. They help in keeping the gear in the desired depth,

obtaining the proper shape during operation and maintaining vertical opening in gears like trawls.

Fishing floats should have the following essential qualities.

High buoyancy

Should withstand the pressure of water

Strong enough against rotting, shock and abrasion

Easy to manufacture

Low cost

7.2. Types of Floats

Low density floats made of materials like wood, cork, thermocole, sponge plastic etc.

High density floats made of materials like glass, aluminium, steel, high density plastic and

Fiber reinforced plastic (FRP)

7.2. Types of Floats

7.2.1. Low density floats

Wood: This is used in Indian fishing gears like gill nets. The density of wood is 650g/1000 cc.

They are relatively cheap. However continuous usage lead to absorption of water and crack

even after drying. Hence reuse is not favorable.

Cork: It is easily available and have a high buoyancy . They are mostly used in surface gill

net and purse seine. Generally they cannot withstand high pressure.

Thermocole: (Expanded polystyrene) is light in weight and highly buoyant. It cannot

withstand high pressure. Should be used only in surface.

Sponge plastic: These are light, do not rot, do not absorb water, have high buoyancy, but

pressure resistance is poor. Mainly used in gill nets.

Synthetic rubber: This is having Low water absorption and high buoyancy. It can withstand

high pressure.

7.2. Types of Floats

7.2.2. High Density Floats

Glass: They are made in the form of sphere. This is covered by netting to facilitate fixing and

for protection. However increases the drag while towing. They do not absorb water and can

withstand pressure upto400 m depth. Big glass floats are used in lines and traps.

Aluminium floats: These are made up of two halves and joined by means of welding at

seams. They are more buoyant than glass and can withstand pressure up to 80m. Mostly used

in trawls and gillnets.

Steel: They are made in two halves and are welded together at the seams. They are not very

popular. The buoyancy is reduced due to thick walls. Rust also get accumulated.

HDP: These are useful in deep sea fishing up to 250 m. Not corrosive.

GRP: Like HDP, GRP floats are expensive and not readily available. They are highly buoyant

and can withstand pressure up to 400 m.

7.3. Special Floats

Trawl plane float: As the size of the vessel increases, there is a tendency to tow the gear at a speed

more than 3 knots. While towing in faster speed all the ordinary spherical floats have a tendency to go

downwards thus reducing the mouth opening of the trawl. The trawl plane floats have low resistance

and more lifting at speed before 3 knots. These types of floats are suitable for midwater trawling.

Siamese twin float: is made by joining two spherical aluminum floats with a curved collar.

Hydro foil floats/Upthrust floats: It is also a spherical float, but the difference is it has a collar

around one third of the lower portion known as stabilizer. In experiment it is proved that when towed

at 5knots, the lift of the float is equal to that of 10 floats and drag is of only 3 floats.

Inflatable floats: Used in pelagic trawls. They are becoming popular for all floatation as storage is

not a problem due to their inflatable nature.

Hydrodynamic floats: Hydrodynamic float has a streamlined top, and V shaped bottom .They are

very efficient.

7.4 Calculation of buoyancy for floats

Buoyancy: - For spherical shape F = KD3

W= wt. in g. in air

F= Buoyancy

D= Diameter in cm.

K= Constant

Value of K varies with material

7.5. Determination of buoyancy of a float

A suitable piece of netting is attached to an iron frame to form a scoop net. Three lines of equal length

are tied together to which sufficient weight to keep the float submerged completely in water is

attached. Two more lines are tied to the frame at diametrically opposite points of the frame, the free

ends of which are tied to a wooden piece. A container of sufficient size with enough water is taken. A

wooden platform with a pan balance is placed on the container. The scoop net with a weight is

suspended in the water without touching the side and bottom of the container. The initial weight of the

balance is noted. The float to be tested is inserted into the scoop net from below. Adjust the weight so

as to submerge the float under water. Find the reading. The difference between the final and initial

reading is the lifting capacity of the float.

7.6.Steel wire rope

Steel wire ropes are used as trawl warps, sweeps and bridles and as foot rope and head rope in large

trawl nets, as purse wire in purse seines. Steel wire ropes have high breaking strength and resistance

to sustained loading with low elongation, unlike fibre ropes. Steel wire ropes are, however stiff and

heavy. Steel wire ropes used in fisheries are generally manufactured from steel wires with a tensile

strength of 1570-1770 N.mm-2. Most common construction used as trawl warps, sweeps and bridles

is six strand ropes with a fibre core (6 x 19s-cf). There are 19 steel wires in each strand including a

steel wire core (9-9-1). The diameter of the finished rope used for fishing gear construction generally

ranges from 9 to 32 mm and the breaking strength from 43 to 497 Kg. Six-strand steel wire ropes with

a central fibre core (6 x 24s cf) and each strand having 24 steel wires each with a fibre core (15-9-

fibre core), ranging in diameter from 8 to 30 mm are in used as purse wires.

Construction of wire rope – The basic materials forming the wire ropes are small wires extending

lengthwise in the rope. These wires are twisted into strands and these strands are twisted to form a

wire rope.

7.7. Combination ropes Combination rope or combined rope with or without a central core is manufactured by twisting

strands which are formed from a combination of natural fibre or synthetic fibre yarns and steel wires.

In this construction steel wires are completely covered by fibre yarns. Steel wires of 0.5-1.5 mm dia.

with a carbon content of 0.4-0.9% are generally used and are either galvanized or ungalvanised. The

steel wires provide high breaking strength and low extensibility while fibre component reduces

stiffness and protects the steel wires from wear and tear. Polypropylene is the most widely used fibre

material in combination ropes. Combination ropes are used in the construction of large bottom and

midwater trawls and as sweeps and bridles.

Unit 8 - Other accessories

8.1. Thimble It is a grooved ring set in the eye of a rope or cable. Thimble prevents chafing of the rope or cable and

deformation of eye. It facilitates easy connection of rope or cable ends. It is made of galvanized iron

or steel, brass or gun metal. The size depends on size of rope.

8.2. Shackle Shackle is semicircular bar of metal having an eye at each end to take a pin. It is used as a connecting

link or device for fastening parts together; so as to permit some movement. It is made of galvanized

iron. Shackles of different shapes are used for different purposes.

Anchor shackle: It is used for attaching anchor cable to the anchor. This is also used for

connecting the brackets of the otter boards with the wrap.

D-shackle: This is used for joining ropes. In this pin does not project beyond the width of

shackle.

Eye bolt shackle: For joining all the types of lines and tighten or loosen the pin with the help

of spike. The size of the shackle is given by the diameter of the pin.

8.3. Swivel A swivel is having two links that turn round independently on a pin or neck. Swivel serves as

a connection between two parts which are liable to cause twists and kinks. There are different

types and sizes swivels which are meant for different purpose. It is made of bronze,

galvanized iron or steel. In a fishing gear it is used to prevent twisting and to secure normal

gear operation. In trawls they are used in between the towing wrap and otter board behind the

otter board between the back straps. To prevent the twisting of lines, swivels are used

between the branch line proper and snood wire in a long line and between the line and snood

wire in a trolling line.

8.4. Kelley’s eye and Stopper link

The Kelley’s eye is a combination of two metal rings one is big and the other is small. The small ring

is connected to the double end of the back strops through a shackle. The other ring is for jamming the

figure of “8” link. Eight link is attached in between the independent piece and the sweep line. 8 is also

called stop link/stopper link. The sweep line is taken through the big rings of the kelley’s eye. During

trawling strain of the net is taken by the kelley’s eye and the back stroops. The independent piece

connects the sweep line with the wrap when the otter boards are disconnected from the wrap, so that

the sweep line and the net can be hauled by the winch. The material used for kelley’s eye is

galvanized iron.

8.5. G-Link Assembly

It is a clip link shaped in the form of the letter ‘G’ and is made of galvanized iron. This is attached to

the brackets of the otter board. The end of the towing wrap of a trawl is connected with the recessed

link through a swivel. The ‘G’ link can be joined to the recessed link. This method of attachment of

trawl wrap to the otter board helps to save time in connecting and disconnecting the doors and wraps

while hauling and shooting the net.

8.6. Purse ring It is an essential part of the purse seine net. Different types and shapes of rings are in use. Purse rings

are made of bronze, stainless steel or galvanized iron. The rings are attached to the sinkers line of the

net through the V shaped bridles. The pursing cable pass through the rings and the bottom of the net is

closed by pulling the cable.

8.7. Buoys

These are used in fishing for marking fishing ground, locating fishing gear in operation and also for

keeping the fishing gear at the required depth of operation. The different types of buoys are empty

watertight tins, wooden barrels, bigger floats made of glass, aluminium or plastic. Some of the special

types of buoys used n fishing are light buoys, radio buoys and dahn buoys.

8.8. Anchors

It is an implement for retaining a ship in position at sea by chaining it to the sea bottom. It is also used

to secure fishing gear in stationary position at sea. This consists of a strong member with curved arms

terminating with a fluke at one end which get embedded in the ground and at other end a ring to

which a cable is attached. The position of an anchor is indicated by a buoy called anchor buoy

connected to the anchor by a rope. It is also a guide to position of a lost anchor. Three types of

anchors are:

Stock anchor.

Single fluke stock anchor.

Stockless anchor.

Unit 9 - Fishing hooks

9.1 Hooks

Line fishing consists of a line and a hook. Hooks are made of galvanized or aluminium coated iron,

brass and stainless steel. They are manufactured in different shapes and sizes. In addition to simple

hooks, hooks of more complicated designs such as double hooks, triple hooks and jigs are also in use.

A simple hook generally consists of a head or eye, by which it is connected to the line, shank, bend,

point and barb. Barbless hooks are used in pole and line to facilitate easy removal of fish. Kerbed

hooks which are twisted in the bend part make the baiting easier and increase efficiency. Double

hooks or triple hooks are used in troll line with artificial baits. These hooks hold in fish more securely

than the single hook. Jig hooks have a single or double whorl of barbless hooks around spindle shaped

coloured /metallic/plastic body. Size of the hook is denoted according to numbering system followed

by different manufacturer. Generally size of the hook varies inversely with the number of hook. Mean

selection length of the fish caught is proportional to the size of the hook represented by its breadth.

Smaller hooks are reported to have higher catching efficiency.

9.2 Different parts of hooks

Fish hook: Hooks are of different types, shapes and sizes. The hooks are made of steel and wrought iron.

Wrought iron is commonly used. It is shaped, tempered and plated with tin. Fish hook is fastened to a

line for the capture of fishes. The principal part of a hook are eye, shank, bend, barb and point.

Eye: Purpose of eye is to attach the hook to the line. There are various types of eyes to facilitate easy

attachment.

Shank: It is the second part of the hook. Eye is followed by the shank. Shanks are given different

shapes and length, depending upon the fish caught.

Bend: Bend is next to the shank. There are various types of bends. The shapes and size of the bends

will vary depending upon the fish to be caught.

Barb: The function of the barb is to prevent the escape of fish after hooking. There are hooks without

barbs. Barbless hooks are used for catching tuna by pole and line.

Point: The point of the hook is given different shapes. Point is the part which pierce into the mouth of

the fish. The fish caught may try to escape after hooking. During struggling the single hook maybe

straightened, helping the escape of fish. If double hooks used the hooking is at two different portion of

the fish hence the chance of escape is less.

Numbering of hooks: Norwegian method is followed for numbering of hooks. The size of the hook

becomes smaller as the number increases. In addition, the length of the hook, the diameter of the

shank, the radius of the bends etc. are also important.

9.3 Artificial baits or jigs

For enticing or luring fish to take the hook, various types and shapes of artificial baits are used.

These types of artificial baits are more effective for catching swift swimmers, feeding on small

moving animals. These artificial baits attract fishes either by their shape, color or by their reflection.

In troll lines the attraction is not only due to appearance but also by their movement similar to those of

exhausted prey.

Tin, lead, brass, horn of oxen and buffalo, whale bone, feather with different color, plastic fishes and

squids etc. are used as artificial baits.

Unit 10 - Maintenance and storage of gears

10.2. Preservation Rotting can be stopped only by completely drying the net, even the inside of knots.

Many preservation methods have been developed by fishermen and fisheries research institute,

chemical and textile industry.

Fishermen usually use coal tar, wood tar and tanning solution. Tanning solution is made from cutch-

an extract from wood.

Chemicals: Various chemicals used for preservation are - Potassium bichromate, copper nepthenate,

copper sulphate, cuprous oxide (Testalin).

10.4. Disadvantages of preservation

High degree of preservation can only be obtained for cotton

Number of treatments are required causes high cost, time and labour

Preservation may have side effects on the physical properties such as stiffness, flexibility,

extensibility, elasticity and breaking strength.

10.5. Commercial fishing methods

A wide array of fishing gears and practices ranging from small-scale artisanal to large-scale industrial

systems are used for fish capture. Over the years, traditional fishing gears have been upgraded and

newer more efficient fishing systems have been introduced. Most important among these fishing gears

are trawls, purse seines, lines, gillnets and entangling nets and traps. Among the most significant

developments which affected the historical evolution of fishing gear and practices are (i)

developments in craft technology and mechanization of propulsion, gear and catch handling (ii)

introduction of synthetic gear materials (iii) developments in acoustic fish detection and satellite-

based remote sensing techniques (iv) advances in electronic navigation and position fixing equipment

(v) awareness of the need for responsible fishing to ensure sustainability of the resources, protection

of the biodiversity and environmental safety and energy efficiency.

The fundamental objective of responsible fishing is to maximize economic returns to the fisherman

without affecting the long-term sustainability of the fisheries resource and with minimum impact on

the ecosystem. Most fishery resources are considered to be exploited at levels close to or beyond their

sustainable limits. Fossil fuels used for vessel propulsion and gear handling in active fishing systems

is known to be non-renewable and limited. In recent years, increasing emphasis has been placed on

adopting responsible fishing practices which seek to minimize waste by reducing the level of discards,

optimize energy use and protect the environment from negative impacts.

10.6. Basic principles of Fishing Gear Design and Construction

Fishing gears evolved on a trial and error basis and until recently, only empirical approaches have

been used to determine design paramteres rather than analytical procedures. Design and development

efforts based on fish behaviour, engineering studies, system analysis and model studies taking into

consideration resource conservation, ecological and economics issues have been taking place in the

recent decades. With the development and wider availability of synthetic gear materials, recent

advances in vessel technology, navigational electronics, gear handling machinery, fish detection

methods and fish behaviour studies, large-scale changes have taken place in the design, fabrication,

operation and catching capacity of modern fishing gears such as trawls, purse seines and long lines.

Widely used traditional fishing gears such as entangling nets, hook and lines and traps have also

benefited by way of design upgradation and efficiency improvement in the recent years.

Choice of fishing gear and its design primarily depends on biological, behavioural and distribution

characteristics of the target species. There is no universal fishing suitable for all fishing conditions

and resources. Fishing gear has to be selected or designed based on the presence of maximum number

of attributes suitable for the particular fishing condition and resource and trade-offs may be necessary.

Principal mechanisms used in fish capture are (i) filtering e.g. trawls, seines, traps; (ii) Tangling e.g.

gill nets, entangling nets, trammel nets; (iii) Hooking, e.g. hand line, long line, jigging; (iv) Trapping,

e.g. pots, pound nets; (v) pumping, e.g. fish pumps. Main behaviour controls used in the fish capture

process are (i) attraction, e.g. bait, light, shelter; (ii) repulsion or avoidance reaction, e.g. herding or

guiding by netting panels as in set nets and trawls or sweeps and wires as in boat seines and trawls.

Model testing is increasingly used for design evaluation of the existing commercial fishing gear

designs with a view to optimize their design parameters and for development of newer designs. In

model testing, a scaled down model of the fishing gear is tested in a flume tank in order to study its

behaviour and estimate working parameters. Principles of similarity are then used to assess the

dimensions, specifications and characteristics of the full-scale version based on model studies. The

fishing gears are further evaluated using full-scale version through statistically designed comparative

field trials with a gear of known fishing efficiency and operational parameters are verified through

gear monitoring instrumentation and underwater observations.

10.7. Factors affecting fishing gear design

Important factors which influence the design of fishing gears are discussed below:

10.7.1.Biology, behaviour and distribution of target species

Choice and design of fishing gear is greatly influenced by biological characteristics such as body size

and shape, feeding habits and swimming speed; behaviour in the vicinity of fishing gear and during

capture process; spatial distribution and aggregation behaviour of the target species.

Body size and shape determine the mesh size required to enmesh and hold the fish in gill nets and the

mesh size to retain the target size groups of the species with out gilling in the trawls, seines and traps.

This is also related to the tensile strength requirements for the netting twine in gill nets and hook size

and lines in hook and line. Body size is again directly proportional to the swimming speed. Swimming

speed is directly proportional to the body size which is a significant attribute to be considered in the

fishing success of a dragged gear. Feeding habit of the target species is more important in passive

fishing methods like hook and line and traps where the fish is attracted by the bait, and in the active

fishing methods like troll line used for catching predatory fishes.

Swimming speed of the target species is important particularly in the active fishing methods like

trawling, seining and trolling. Fishes are known to sustain a cruising speed of 3-4 body lengths per

second for long periods without fatigue and burst speeds of 10 body lengths per second for short

duration. During burst speeds reserve energy supplies in the fish muscle is used up. Fish in front of

the trawl mouth will be eventually caught if the trawling speed is greater than the cruising speed of

the fish. Behaviour of different species might vary when they turn back into the trawl. It is reported

that flat fish and cod turn back in the horizontal plane close to the bottom; whiting turn back at a level

higher than this and haddock rise and turn at a still higher level. Such differential behaviour makes it

possible to separate the different species using separator panels inside the trawl. Selective capture of

the slow moving crustaceans providing opportunity for the fast swimming non-target fin fishes to

escape, could be possible by controlling the towing speed and minimizing the longitudinal length of

the trawl net.

Behavioural differences between fish and crustaceans and size difference between them, could be

used in the design of selective trawl designs. In such designs rigid grids are placed at an angle, before

codend. Small sized prawns move through the grid into the codend while fish and other non-target

species are deflected by the grid and are released through an escape chute. Such devices are

sometimes called Trawl Efficiency Devices as the they reduce the sorting time and thus increases the

efficiency of operations. Protected species like turtles are allowed to escape in a similar way using

Turtle Excluder Devices(TEDs)

Large mesh trawls and rope trawls, in which front trawl sections are replaced with very large meshes

or ropes in order to reduce drag, make use of the principle of repulsion of herding to guide the finfish

into trawl codend. In the conventional trawling systems, herding effect by the otter boards, wires and

sweeps and sand-mud cloud created by the boards on finfishes in between the boards, is made use of

to improve the catch rate by increasing the effective sweep area. Long leader nets placed in the path of

migratory fishes guide them into large set nets operated in Japan. Tendency of some fishes to

aggregate towards light is used in squid jigging, light-assisted purse seining and dip net operations.

Behaviour of fishes like tuna to aggregate around the floating objects, is utilized successfully in Fish

Aggregating Devices (FAD)-assisted purse seining.

Catching efficiency is maximized when the vertical opening of the trawl mouth, vertical dimension in

gill nets, and the catenary of the main line of the long line with branch lines and hooks, coincide with

the vertical range of the layer of maximum fish abundance. Hence knowledge of the vertical

distribution of the target species could be used to optimize the horizontal and vertical dimensions of

the netting panels in gill nets, main line catenary in long line and mouth configuration in trawls. Some

species of fish are sparsely distributed either singly or in small groups and thus exhibits a pronounced

patchiness, while some others form dense schools. Sparsely distributed and scattered fish are more

efficiently caught by passive fishing methods such as gill netting and long lining, where as schooling

fishes are effectively caught by purse seining and aimed midwater trawling.

10.7.2. Fishing depth, current and visibility

Hydro-acoustic pressure increases approximately at the rate of one unit atmospheric pressure (1 bar)

for every 10m depth. Buoyancy elements used in the deep sea fishing gears such as deep-sea trawls,

gillnets and bottom vertical lines have to be strong enough to withstand the high pressure at the

fishing depth. Compressible buoyancy elements that are simple light and cheap can only be used in

surface operated gears such as seines and surface gillnets as they absorb water and loose their

buoyancy in deeper waters.

Prevailing strong currents in the fishing ground may restrict the choice of fishing gears to longlines

and gillnets which are less affected by currents. Light levels at the fishing depth could influence the

fishing success, as vision of fish is affected by light levels. In passive fishing gears such as gillnets,

visibility of netting panel adversely affects fishing efficiency. Negative impact of visibility is

observed in hook and line operation while in light-assisted jigging controlled lighting has a positive

impact. Visibility is also important in effective herding during the capture process in trawls and in

large pound nets and trapping enclosures where leader nets are used.

10.7.3. Sea bottom conditions Rough sea bottom conditions limits the operation of most of the fishing gears close to the ground

except handlines, vertical longlines, bottom vertical long lines and traps. Trawling on rough bottom

requires special rigging such as bobbin rig or rock hopper rig, improvements in trawl design to

minimize gear damage or loss and selection of appropriate otter boards.

10.7.4. Other factors Choice of fishing gear and their design features will also be influenced by the scale of operations, size

and engine power of fishing vessel, energy conservation objectives, selectivity and resource

conservation objectives, catch volume requirements, operational and handling requirements of the

gear, prevailing weather conditions, skill required for fabrication maintenance and operation, material

availability, local traditions and economic considerations.

Unit 11 - Trawl fishing gear and operation

11.1. Trawling

Fishing is the art of catching aquatic living beings. Various methods are adopted for the capture of

fishes. Due to advancement of Science and Technology many progressive changes have been

introduced in design, construction and operation of fishing gear. The introduction of trawls and

trawling is one of the important development in the history of fishing technology. The modern trawl

net is basically a large conical shaped; bag netting which is drawn along the sea bed just above the

bottom or in the water column to catch bottom fishes or shell fishes. Initially trawling gear were all

for bottom fishes and in later years more complicated innovation were deviced. The present day

trawls may be classified as under.

11.2. Parts of a trawl net

The size of a trawl net may be defined by the length of the foot rope, headline or the number of

meshes around the fishing circle, and the mesh size at that point.

Mesh sizes are usually at their largest in the mouth of the trawl and progressively reduce towards the

codend.

11.2.Trawl Diagram

11.2.2.Trawl net parts Top Wing - The two sections of netting forming the upper pats of the mouth of the trawl, to which the

Headline is attached along the inner edges.

Square – The upper trapezoids panel of netting leading forward of the baitings, to which the top wings

are attached. This forms the forward running cover to restrict the upward escape of fish.

Baitings or upper Bellies – The upper part of the tapered funnel of the net that guides the fish towards

the codend.

Lower Wing – The lower part of the trawl mouth that is directly below the top wings. The inner edge

of this is attached to the fishing line beneath the square.

Bunt – This is a continuation of the lower wing that is attached beneath the square.

Lower bellies – The lower part of the funnel of the net that guides fish towards the codend.

Extension or Lengthening Piece – This is a continuation of the belly, and is usually rectangular like

the codend. It may be inserted to improve the flow of the net, close the meshes in front of the codend

or to make the hauling of the codend easier.

Codend – This is the final section of netting, usually constructed from a heavier or double netting.

That is used to hold the catch of fish.

Flapper – A panel or panels of netting inserted inside the trawl to prevent or restrict the escape of fish

from the aft part of the trawl. This is usually positioned in the after part of the belly or extension.

Lifting Bag – A cover over the codend that is normally made of a larger mesh size and is used to

strengthen the codend whilst lifting the catch aboard.

Chafers – Pieces of netting; rubber matting or hide that are attached to the lower side of the codend

and bellies to reduce damage from ground contact.

Dolls – Pieces of stranded rope that are cow hitched through the meshes on the underside of a trawl or

chaffer to help protect the net from damage. These are usually used on nets that are being worked on

stony ground, and are most commonly found on the underside of beam trawls. Dolls also help to block

the meshes hence reducing the effective mesh size.

Wing Tips – These are triangular sections of netting found at the forward ends of the mouth of some

trawls.

Side Panels – Sections of netting that are placed between the upper and lower parts to change the

cross sectional shape of the trawl and give it 4 seams.

Headline – the upper rope framing the mouth of the trawl to which the headline floats are attached.

Fishing Line – The lower framing rope at the mouth of the trawl to which the foot rope, bobbins or

ground rope is attached.

Bolsh – A rope that may be inserted between the wings and a framing rope.

Wing Lines – Ropes that frame the wing ends of a trawl.

Belly Lines and Selvedge Lines – These are ropes that run along seams in the trawl to give additional

strength. The trawl is held open vertically by the floats attached to headline. It retains contact with the

sea bed by the weight of the ground rope. The trawl is held open laterally by the otter boards, which

also act as a major weight force to keep the gear on the sea bed.

A series of sweeps and bridles connect the otter boards to the trawl. These may have considerable

length and are used to herd the fish to the mouth of the trawl. Tensions in these wires may also be

adjusted to control the engineering performance of the gears.

11.2.3. Selecting Netting Materials

The material most widely used in demersal trawls is Polyethylene. This is usually blue, orange or

green in colour, and may be constructed in a twisted or braided form. The codend section of the trawl

is most commonly constructed from braided twine, and is often made in the form of a double netting.

Polyamide or Nylon netting is widely used in the construction of Beam trawls, Mid water trawls and

Shrimp trawls.

All synthetic materials are subject to shrinkage, which may have several causes, and could lead to a

net contravening a fishing gear regulation.

Polyethylene is subject to shrinkage caused by prolonged periods of storage in direct sunlight.

Polyamide may be subject to shrinkage by submersion in water unless adequate heat setting processes

have been undertaken during the twine and netting manufacture.

Contact with chemicals, hot water pipes etc. may also cause problems. Another form of shrinkage is

caused by dimensional change. This is particularly common with braided twines that are used in close

contact with a sandy sea bed. The sand particles find their way into the lumen of the twine causing an

increase in diameter and reduction in length of the twine bars.

Rigging of otter boards:-Otter boards are rigid sheer devices which are used to keep; the trawl

mouth, bridles and warps horizontally open. They keep the bottom trawl in contact with the sea bed

and help to maintain the fishing depth of the mid water trawl .Otter boards were first used trawling in

1894 in Scotish waters. By the end of the century otter trawler had become popular. Originally otter

boards were attached directly to the wings of the net. By around 1920, Vigneron-Dahl system was

introduced. In this system otter boards were attached to wing by means of sweep lines and bridles.

(Fig.page 69 Modern F.G.). There was an increase in the catch rate, the effective swept area and

through the herding effect of sweep lines, bridles and otter boards on fin fishes. Otter boards

contributes about 25% of the total drag of trawl system and is responsible for about 16% of total fuel

consumption in trawling operation.

11.2.4.Ground gears used during trawling

Floats and sinkers are used all along the head rope and foot rope to give the fishing gear the desired

shape, keep the mouth of the gear vertically open and maintain a good ground contact. Floats are

buoyant material which gives an upward thrust due to which the head rope is lifted off. Whereas

sinkers are provided on foot rope to give a downward thrust. The vertical mouth opening of the trawl

net is achieved due to these opposite actions.

When the trawl net is dragged in the sea bottom, specially made ground rig is attached of the foot rope

to offer protection from snags. Rubber discs of appropriate diameter, threaded together in a rope used

for trawling in a moderately rough ground. Rubber bobbins of cylindrical or spherical shape and

appropriate diameter and weight (230-450 mm dia and 4-15 kg in air) and steel bobbins (230-600 mm

dia and 13-155 kg) with a spacer and washers packed on to the steel wire ropes are used for trawling

in hard rough ground. Rubber rock hopper disc (400-600 mm dia; 7-21 kg) with rubber spacer packed

together using iron link chain are connected to foot rope are also used in very rough grounds. Rock

hopper gear causes less wear and tear to the ramp and deck of the vessel when compared to bobbins.

11.2.5.Scope ratio

It is the ratio between length of warp line released to the depth of water in which a trawl is operated. It

varies with towing speed, type of ground, depth and type of species to be caught.

For bottom trawling the towing speed ranges from 1.5 to 3.5 knots depending on size of the vessel.

The bottom towing speed generally ranges from 1.5-2.0 knots for shrimps, 2-3 knots for fishes. For

mid water and pelagic trawls it is 3.5-6.0 knots.

Towing a particular trawl too slowly may cause the otter boards to close together, resulting in

insufficient spreading of the net which may sag on the bottom. On the other hand, towing too fast

could result in the net lifting off the bottom and floating, which may lead to its turning over and

becoming foul gear. Scope ratio is around 1:5 in shallow waters less than 20 m with smooth ground.

The warp length could be 5-10 times the depth in deep water for operating a heavy gear and in rough

grounds length of warp could be 2.5-4 times the depth. Scope ratio is mainly depending on the nature

of the fishing ground for e.g.

For smooth fishing ground : slightly reduce the scope ratio

For sandy bottom: slightly increase the scope ratio

For rough ground: greatly reduce the scope ratio

Swimming speed of the fishes to a large extent determines the required trawling speed. As the trawl

drag depends on the towing speed, maximum dimensions of the trawl will be determined by the

available towing force of the vessel and the towing speed requirements for the target species.

Swimming ability of fish depends largely on their size and varies with different species. If the

trawling speed is above the cruising speed, the fish in front of the footrope would eventually tire and

drop back to be caught in trawl codend.

Typical towing speed for beam trawls 1.0 - 2.5 knots

Typical towing speed for bottom trawls 2.0 - 4.5 knots

Typical towing speed for mid water trawls 3.0- 8.0 knots

11.3.Bottom trawling

When a trawl net is dragged in the bottom or just above the bottom it is known as bottom trawling.

Stern bottom trawling was first introduced in the late 1940s and has become widely popular since

then. The advantages of stern trawling is the maximum utilization of the towing power and ability to

set gear in straight line.

11.3.1. Stern trawling

In the stern trawling the wheel house is placed forward leaving the aft deck clear for the fishing gear

and fish handling. Split winch is placed abaft the wheel house and each drum is wound with about 300

m of steel wire rope or synthetic wire rope. This forms the warp line. The entire warp line is marked

at regular intervals. This marking is required to know the amount of warp line released during bottom

trawling. Mast and derrick are provided just abaft the winch. Mast is used for hoisting the

navigational signals and derrick is used to lift the cargo/cod end. Gallows with towing blocks are

provided on either side of the stern deck part of the trawler. In large stern trawlers towing blocks are

fixed on the stern gantries instead of gallows. A clear view of the aft deck from the wheel house is

provided so that the fishing master can observe and ensure increased safety during the operations.

Large stern trawlers are provided with a stern ramp which facilitate hauling up of the gear. In this case

the transom of the vessel is cut and deck is lowered to form a smooth gradient up to the water level.

Shooting and hauling of the gear are thus made easier and requires less man power. In large trawlers

net drums are used to haul up, pay out and store the sweeps bridles and net with rigging.

11.3.2.Preparation of the trawl net:

The trawl net is to be rigged and kept ready in the fishing deck or in the net drum. Required number

of floats should be attached on the head rope at regular intervals. Check the foot rope and tickler

chain. The head rope and foot rope are connected to the bridles and sweeps which is then connected to

otter boards. The winch warp line is connected to the other side of the otter boards through the

brackets. Net is arranged on a stern part in such a way that codend part of the net should be on the top

of the net for easy releasing. The open end of the codend should be tied by using self securing knot

and keep ready for further operation.

11.3.4.Shooting of the net As soon as reaching the fishing ground, one should observe the meteorological and hydrographical

parameter such as direction and speed of wind, temperature, surface temperature, direction and speed

of current, type of ground, depth of water, fishing position etc. Bring the vessel into lee word side and

slow down the vessel with the trawler moving release the codend with forward motion of the trawler

and then release throat portion belly followed by head rope and foot rope. Once the net is in the water,

the sweeps and bridles connected to the otter boards are released with the vessel slowly steaming

ahead so that net sweeps and bridles spread out and open properly. The otter boards are lowered to a

few meters below the surface of water and kept for a few moments to ensure that they are spreading

properly. The gear is then lowered to the desired fishing depth by releasing sufficient length of the

both warps. The length of warp released during bottom trawling depends on the depth of the fishing

ground, nature of sea bottom and towing speed of the boat. The ratio of depth of fishing ground and

the warp released is known as scope ratio and is typically around 1:6 in shallow water up to less than

20 m, beyond 20 meter it would be 1:5 and in deeper waters it is 1:3.

11.3.5.Towing speed and duration

Towing speed of bottom trawling should be more than the swimming speed of fishes. Usualy fishes

will swim at a normal speed of 3-4 times of its body length and will increase the speed up to 7-8 time

of its body length per second and fishes will increase further up to 10-11 times of its body length per

second when they are in danger known as burst speed which will last for few seconds. For bottom

trawling towing speed should be in the range of 1.5-3.0 knots. They again vary with targeted species.

For e.g. shrimps and soles 1.5-2.0 for other bottom fishes 2.0-3.5 knots.

Normally towing duration is one hour but commercial trawlers are dragging for 2-4 hours. In order to

get good quality fishes one should drag for a short period. If dragging duration is increased fishes will

get damaged and deteriorate fast.

11.3.6.Hauling

After dragging the gear for a prescribed duration of 2-4 hours depending on the catch, the hauling

operation begins. Decelerate the engine and engage the winch clutch. The winch drums start rotating

and the main warps are heaved in evenly on to the winch drums until the otter boards reach the

gallows. As soon as otter boards reach the gallows stop the hauling and fix the otter board to the

respective gallows. Then the net is dragged with full speed for some time so that fishes present in the

body of the net are driven into the codend. This also help in cleaning the net. After decelerating the

engine, sweeps and bridles are hauled in followed by the main body, throat and finally codend of the

net. If the codend is heavy with the fish catch, mast and derrick is used for hauling up the codend.

Further, the codend was full of fish catch then catch may be split in to two parts by using split rope

provided in the middle of the codend and then part by part catch is taken aboard.

In large trawlers, a system consisting of Kelley’s eye, independent wire and back strap is used for

transferring the load back to the powered hauling system or net drum after otter boards have reached

the gallows.

11.4.Side Trawling

In side trawling the net is set over the side. This method has been gradually replaced with stern

trawling. There are only very few side trawlers operating throughout the world. Side trawlers have a

forecastle- the forward part of the vessel and wheel house at the aft with the engine room below. The

working deck is forward with one or more hatches to the holds below. A two drum winch with towing

warps is situated in the forward of the wheel house which also has two warping heads or gypsies for

handling running lines. The winch may be placed anywhere on the fore deck to allow convenient runs

of wire ropes with the help of deck bollards. Two gallows are provided on port the side of the vessel.

Forward gallow is situated just abaft the forecastle and aft gallow is placed in the side of the wheel

house. A towing block is situated at the aft port side of the vessel were warps are clamped together

while towing the trawl gear. A well stayed mast is situated at the forward end of the working deck and

in a second mast is situated in the aft.

11.5. Arrangement of trawl gear for side trawling The towing warps from the vessel is connected to the brackets f the doors with g link so that it can be

easily engaged and disengaged. The net is connected to the otter doors through a short chain and

ground cables. The end of ground cable passes through the Kelloy’s eye and is connected to a stopper.

The end of the stopper is connected to the ground cable and is small enough to fit into the eye of the

figure of 8 link and transmits the pull from the door to the ground cable. The section of the stopper

that is forward to the eye is fastened to a wire or chain known as the independent wire. Quarter ropes

are fitted to the trawl net and is connected to the ground rope at the point where the wings merge into

the body of the net. They then pass around the outside of the net and through the rings attached to the

headline. These ropes are used in bringing the net aboard the vessel.

11.6.Stern Trawling

The side trawler sets and hauled the trawl gear over the side and tows trawl gallows and towing block

on the side. The stern trawler carries out these operations over the stern.

11.6.1. Gear handling in stern trawling

A number of basic methods after handling the gear have being developed and became associated with

vessels of different size ranges. Although the method for gear handling may differ, the general layout

of most of the vessels are similar. Small stern vessels are usually single decked with a transum stern,

while medium size vessel have a full or partial sheltered deck. The large vessels, which have the

capacity of freezing their catch at sea have full sheltered decks together with extensive

superstructures. All stern trawlers are characterized by a wide, clear, working deck at the aft and a

wheel house or living quarters forward. The engine room is usually forward of amid ship below the

wheel house. Some medium sized vessels have the engine room aft with exhaust uptakes on each side

of the working deck. The fish hold normally lies below the working deck or in the case of shelter deck

vessels, below the main deck or amid ship so that trim does not change markedly as fish is taken

aboard and stowed below.

The main winch is situated abaft the deck house or superstructure and the gear towed from gallows

sited aft on each side of the stern. Stern trawlers offer two striking advantages when compared with

other trawlers.

Gear is worked over the stern.

The vessel maintains a straight course during hauling, setting and pulling along the direction

of motion.

Most cases the vessel can therefore be headed directly into the wind and able to continue

fishing under severe sea conditions than a side trawler.

The working area at the after end of the vessel is much steadier as it is not affected by

pitching. The forward deck house provides protection from pulling. Thus the stern trawler

provides greater safety and comfort for the crew.

The main methods of gear handling:

Use of quarter ropes

Net drum

Stern ramp

Various combinations of the above

Equipment: for stern trawler winch should have at least two main drums and two warping heads. In

order to ensure a working deck clear of running wires the winch may be mounted at forecastle level so

that the wire run over the heads of crew members on the working deck.

11.6.2. Towing and Handling warps and doors

A smooth run of warps from the winch to the gallows is necessary. The required blocks and sheaves

must be arranged in the working area to permit easy spooling on the winch and a smooth run over the

gallows with minimum changes in the direction.

A pair of gallows to be mounted at each side of the working deck at the aft and alternative method for

towing support is to have a gantry structure which will leave the working desk completely free of

towing blocks.

11.6.3. Net drum

In this method the net is wound aboard on to the drum. The deck space required is limited when

compared to other method. Here the space required to mounting the drum plan the area needed to

dump, clean, sort trawl catch.

Size of vessel: 40ft-90ft.

11.6.4. Equipment

Winch: Requires at least two main drums for the warps and two warping heads for handling the

codend.

Trawl drum: Must be large enough to wind on the trawl and the bottom gear. This may be fitted in

between the gallows.

Gallows: Situated on each side of the aft deck, and are usually tripod in structure. This is often are

made portable and pinned into socket at their base.

Boom: The boom must be of sufficient length to bring the codend aboard in the required position. A

separate topping winch for the boom will be useful.

11.6.5. Operation

I. While towing, the warps pass atwartship from the winch round rail mounted sheaves to the gallows

and doors.

II. During hauling, otther doors are pulled up to the gallows and are attached to the warps.

Independent wires are disconnected from the towing brackets and is connected to leads from drum.

Drum rotates to take pull and ground cables are disconnected from blackstrap and wound on to the

drum.

III. The mouth of the net is wound on drum. A ring strap is used to lift body and vessel turns to star

board. Bull rope being taken forward to bring codend along side.

Trawl along side, the body is lifted to fill codend. Codend lift is taken by second whip.

Full codend is lifted and dumped and return to the water and filled again.

11.6.6. Stern Trawling With gantry

Operation is similar to that of side trawling but carried over the stern.

Small stern trawlers are usually single decked with transform stern

Wide, clean working deck aft

Wheel house and leaving room forward

Forwarded engine room

Fish holds are below the working deck

Abaft the fish hold fuel tanks are located to allow control of trim

Some vessels will have engine room aft and fish hold forward

The winch is sited abaft the wheel house

Gallows are situated aft on each side of the stern

11.6.7. Advantage over side trawler

As the gear is worked with stern, the vessel is maintained on a straight course while hauling

and shooting.

The pull is along the direction of motion. In most cases the vessel can therefore be headed

directly into the wind and sea.

11.6.8. Operation It is similar to that of side trawling. However, some of the points to be remember

Trawling speed should be slightly increased on heavy ground (at least 3 knots)

Minimum warp scope possible

Trawl without quarter rope is a good protection in heavy grounds.

In all areas whether bottom is unfavourable for trawling, all drags should be made a straight

courses. Since change of al course may result in an overturned codends clearing lower wings

and trawl board digging into ground.

11.7. Mid water Trawls

Mid water trawls are used for pelagic trawling. These are conical bags with wide mouth made of light

netting yarns and large meshes. Based on the construction of the trawls can be grouped into

Two seam pelagic trawls

Four seam trawl (with all the four equal panels for pair trawling)

Four seam trawl (The upper and lower panels are identical and the two identical side panels

are narrow)

True mid water trawls are four seam trawl net with short wings. These nets considered are not fit for

bottom trawling due to their lightness and get easily damaged on contact with sea bottom. This gear is

known as “fly catchers” among the fishermen.

Gear technologist are in the process of developing new gears like rope trawl, hexagonal mesh trawls,

big mesh trawl, Danish trawl etc.

11.7.1. Rope trawl

Rope trawl was developed during 1970 in Poland, Germany, Norway, France and Netherland. In all

designs rope can be found in three or four panels running from; head rope, foot rope or side rope

down to approximately one quarter of the length of the net.The biggest concern in the design was

what shape the leading ropes were going to take. Unlike normal netting, the ropes are independent of

one another on the basis of equal tension in each of the ropes. Wings are narrow at the top and wider

at the bottom, with a very small bosom resulting in a very noticeable reduction of drag. But this is not

superior to conventional gears in the quantity of catch. The ropes do not have herding effect on fish

that a normal net do. An explanation of this can be that these ropes which are stretched in the

direction of towing seems to be static to fish. But the sound and the vibration that goes along with the

movement of the ropes makes the fish to get away. Between the ropes, water does not move and it is

safe and easy for fish to escape. As a comparison, these make a whole wall of vibrating water between

the twines drive the fishes into the net.

11.7.2. The Danish Trawl This is a modification of the rope developed in Denmark. In this trawl ropes and twines are at an

angle to the direction of the flow. The ropes are positioned like the normal netting bars. Therefore,

they appear to transfer the forces acting on the net very well on to the headlines and provide excellent

herding effect.

11.7.3. The Line Trawl The line trawl developed in Newzealand to reduce the drag. In this design rather than avoiding water

flow into the net like in previous designs. This net is based on the theory that it makes easier for water

to flow out of the net. Once it leads the fish into it, a series of lines are as far apart on the meshes

would have been which they replaced. The reduction of drag in this design is not as high as in the

previous two designs. Advantage is that all the lines are of the same length, which makes it easy to

repair.

11.7.4. The Big Mesh Trawl

This design was introduced in France. Mesh size at wing and first part of the net is around 18 meter.

This net is very successful in catching of shoals of clupeids, anchovies, sea bass and whiting.

Netherland modified this net with 20 m mesh size and producing community.

11.7.5. The Hexagonal Mesh Trawl Mesh size is around 30 m was developed in Norway and Netherlands. There is a moderate reduction

of drag and superior efficiency for catching mackerels.

Unit 12 - Purse seine design

12.1. Netting material

The choice of material plays an important part in designing a purse seine. The performance of the

gear will depend upon the speed of sinking. It is proved that hard laid twines of continuous filament

has less hydrodynamic resistance than soft laid twines of staple fibres.

12.2. Net size and depth

the length and depth must be determined by the size of vessel, the species behavior of fish species,

fishing method and condition. Operation is easy when the depth is between 10-15% of the length.

However, in deep water purse seines the depth goes up to 30-50% of the length.

12.3. Mesh and twin size

The mesh and twine size is directly related to the size of fish and the quantity of fish caught .

If the mesh size is too small it increases the cost and slow sinking, but large mesh results in

loss of catch and gilling which is not desirable.

12.4. Selvedge

these are large meshes made of tickler twines along the border of the main webbing. Selvedges are

providing at on top, not on and side of fishing gear.

12.5. Main webbing

Vertical strips of uniform mesh size of different depths laid one after another with the central

portion having the maximum depth from the main webbing. This tapers gradually to both wings.

12.6. Float line

there will be two lines, a double cord 12mm rope will be attached to the upper strip of the top

selvedge along with a hanging coefficient its entire length giving of 0.3-0.35. The second rope is the

actual float line; floats pass through it and are attached to the entire length of the former at regular

intervals.

12.7. Lead line

Lead line is also a double cord of same specification. One of which is attached to the lower strip of

the bottom selvedge along its entire length giving with a hanging coefficient of 0.25-0.30. The

sinkers are passed through a second rope and is attached the first rope at regular intervals.

12.8. Breast lines

there are two breast lines one each at hauling and skiff ends. These are also of the same

specification as float line and lead lines but of single cord attached to the head rope and foot rope

and are stapled along the respective breast selvedge. This forms loop at either ends.

12.9. Bridles

Bridles are attached at a gap of 5-10 meters depending upon the size of the net, along its foot rope.

They can be either single as in case of Japanese purse seine or “V” form as in the American and

Norwegian type purse seines. The length of the bridle is 2-4 m. The upper end of the bridle is

attached to the foot rope and the purse ring attached to the lower end. Purse rings are generally

made of 12-18mm dia galvanized rods with a ring diameter of 60-100 mm and an way for attaching

bridle. In “V” shaped bridles there will be an additional single short bridle attached with a shackle to

which the open type or closed purse rings are attached.

12.10. Purse line

Purse seines for sardine and mackerels for the type of net mentioned require 12-18 mm rope where

as deep sea purse seine require 30-50 mm rope as purse line. The purse line passes through the rings

and is attached to the skiff end foot rope with an easy detaching arrangement on one end. The other

end is attached to the pursing winch through the gallow.

12.11. Deck Equipment

12.11.1.Gallows and Davit

Gallows are used in trawlers and purse seiners. Trawl gallows are “V” shaped heavy steel

structure fixed on the either side of the stern part of a trawler. This is used in hitching up of

otter boards as well as for passing of towing warks. The position of the gallows is very

important in order to avoid fouling of warps during a sharp turn.

Purse seine gallows are used to lead the purse line and tow line to the winch in larger

vessels and davit with blocks is fitted on bulwark for passing purse line.

12.11.2. Winches

Different types of winches are used. They may be placed fore and aft direction or athwart ship

depending on the requirements. They are commonly used in trawlers, purse seiners and Danish

seiner for releasing and hauling purpose.

Trawl winch can be single drum split type, two drum on one shaft or two or more drums on parallel

shaft. The last type is most suitable for double rig trawling and purse seining. A winch may or many

not have warping heads. Warping heads are used for auxiliary hauling requirements such as hauling

of sweep lines, wings and split winches are common in large trawlers.

Winch can be driven either mechanically, electrically or hydraulically usually mechanical drive is

preferred in smaller vessels. Power take – offs are used to take the power from the main engine to

drive the winch. It is simple, easy to maintain and less expensive. Electrical drive is confined to large

vessels. Hydraulic drive is popular in only larger vessels.

12.11.3. Others

Windlass: - This is a special type of winch used for handling anchor chain and rope.

Power blocks: - These are used in purse seining and gill netting. It is a labour saving device.

The purse seine power block is so designed as to pass the bundles of webbing, corks, weights

and rings through the sheaves during hauling operation. It is a self powered free swinging

“V” shaped sheave made of cast aluminium along with zinc and it is hydraulically driven.

Triplex: - It is a net winch used in purse seiners instead of power blocks for hauling large

nets without damage. It consists of three hydraulically driven rollers fixed parallel to each

other on independent shaft and adjacent rollers rotate in the opposite direction.

Line hauler: - Line haulers are used for heaving lines. Capstan and line haulers are similar in

many aspects. Line haulers or gurdies are used in hauling the trolling lines, long lines, jigging

lines and tuna long lines.

Unit 13 - Gills nets

13. Gillnets A gillnet consists of a long, rectangular web of netting with head rope, footrope, floats and

sinkers are fixed on it, which is set in the water to form a vertical wall. Gillnet can be set on

the bottom, in midwater or at the surface. Fish are caught in gillnet in one of the three ways.

Wedging: The fish is held tight around the body of the mesh.

Gilling: The fish enters the net but it is too large to pass through the mesh and it is

prevented from retrieving by the twine that has caught the fish behind its gills.

Tangling: The fish has not necessarily penetrated a mesh but caught to maxillaries or

other projections.

Gillnet selectivity is mainly influenced by its mesh size, but other factors like twine and gear

design and environmental factors are also have some influence. Capture by gilling, wedging

and tangling is dependent on the shape of the particular species of fish encountered. Besides

its well known size selectivity properties, gillnets are therefore also species selective.

Fishes are commonly caught in a gillnet are by wedging and gilling. The smallest fish caught

has a maximum girth equal to the perimeter of the mesh and the largest fish caught has a head

girth equal to the perimeter of the mesh. There is an optimum size at which a fish is most

likely to be entangled in the net. Fish that are smaller or larger than this optimum size but

within the capture range are less likely to captured.

Unit 14 - Long line gear

14.1 Long line gear Long lining is a common fishing method throughout the world.The gear consists of baited

hooks attached by branch line to a main line. The bait attracts fish either through chemical,

visual or taste stimuli. Size selectivity curvers of long line resemble those of gillnets,

although their selectivity ranges usually are wider and sometimes smaller fish are taken. Long

line catch is comparable to those of non-selective type of gear like small meshed bottom

trawl.

14.2. Traps and pots The most commonly used traps are baited traps or pots to harvest crustaceans like crab and

lobster. Baited traps are sometimes used to catch fish. The capture process of the traps is

assisted by the tide or netting leader guiding the fish into the trap. Once the fish enter the trap

through a door, the fish continue to swim around the circumference of the trap. At this stage,

smaller fish may escape through the larger meshes of the net wall. However, very small fish

continue to swim inside the trap rather than escape.

When the trap is lifted, the fish are concentrated in a small section of webbing at the back of

the trap. At this point, the fish may attempt and succeed to escape through the meshes.

From this description of the catching process, it is obvious that the main selection process

takes place during hauling.

The bait attracts the fish and crustaceans to the traps. The organisms try to enter the trap

through the entrance. In this phase size selection may occur if the entering organisms are too

large to pass through the opening.

Unit 15 - Selectivity in fishing gear 15.1. By-catch reduction devices in trawl gear In shrimp fisheries, much effort has been directed to improve selection performance of by-

catch reduction devices in the trawl gears.

The first design consisted of larger mesh panel in the side panel of the trawl which guided the

fish to escape outlet while shrimp passes through the mesh opening of the panel to the

codend. The separator trawl was developed on this principle.

The recent by-catch reduction devices developed in Norway and Canada use a grid in the

shrimp trawl. This system consists of a rectangular metallic grid slanting outward and

upwards, placed in the aft belly in front of the codend (Fig.2) and a fish outlet on the top of

the trawl in front of the grid.

15.2. Turtle Excluder Device Through experiments CIFT developed a Turtle excluder device known as CIFT-TED. Marine

Products Export Development Authority is popularizing the turtle excluder devices. Most of

the experiments were conducted in the east coast of India as occurrences of turtles are more

in the east coast.

15.3. Separator Trawl

several methods have been developed for the separation of fin fishes from shrimps and

escapement of juveniles during trawling operation. The separator trawl has a dividing panel

that extend far enough ahead and be constructed of a mesh size small enough to prevent fish

in one side leaking into the other as may happen as the catch accumulates. It is further

advantageous that the extension section is modified so that codend of normal length and

circumference can be used, rather than dividing an extension section designed for the single

codend and fitting into two half sized codend.

15.4. Trouser Trawl A conventional single trawl with normal rigging is modified to a trouser trawl by the

installation of a vertical dividing panel beginning somewhere aft of the fishing circle and

dividing the trawl body into two equal side by side chambers leading up to two parallel

codends. Trouser trawls are commonly used in some commercial fisheries.

15.5. Square Mesh Codend in Trawl

The size and shape of meshes in the codend are main factors which determine the selectivity of trawls.

The filtering ability of the trawl can be enhanced either by enlarging the mesh size or by restricting

the lumen of the mesh under stow. The size of the mesh cannot be increased beyond a certain limit.

Hence, the other alternative is to effect changes in shape of the mesh to keep the meshes open during

trawling operation. Square mesh (Fig.3) webbing can be prepared from the ready made webbing and

can be attached to either in the throat part or on the upper panel of the codend (Fig.4). The method of

maintaining square mesh shape for the codend meshes is to increase the selectivity of the trawl gear

and to ensure that juveniles and undersized fish can escape from the net. Moreover that they will not

be damaged in the process and thus survive and grow.

Many experiments conducted under commercial condition, showed that the use of square mesh

webbing in the codend meshes reduce the catch of small fishes. The main reason for improved

selectivity is simply the large area of open meshes, through which fish can escape. The square mesh is

also more stable. The diamond mesh on the other hand, tends to distort due to longitudinal and

transverse tension, depending on catch size, current direction and other factors.

15.6. Advantages of Square Mesh Codend

Reduce the catch of juvenile fish

Less time to sort the catch

Improved quality of the catch

Increased survival of escaping fish

Cleaning codends made of square mesh webbing is easier when there is some gilling

Unit 15 - Electronic equipments for fishing

16.1. Introduction

Introduction of acoustic fish detection devices in the early 1950’s is one of the most

significant developments in modern fishing. As a result many modern fish detecting devices

were developed. The use of modern electronic equipments in fishing has greatly reduced the

searching time and enhanced the effective fishing time.

16.2. Principles of acoustic fish detection

all acoustic fish detection devices are based on echo-sounding or echo ranging techniques in

which a pulse of sound energy is transmitted through water and received back from a

reflecting object. Sound is transmitted through sea water at a nominal speed of 1500 m per

second at a temperature of 13ºC and 35.5 ppt salinity. The speed ranges from 1450 – 1570

m/sec. depending on temperature, salinity and pressure. The speed increases with temperature

at the rate of 4.5 m/sec/oc, with salinity at the rate of 1.3 m/sec. per ppt and with pressure at

the rate of 1.7 m/sec. per 100 m depth. Speed in sea water is about 3% higher than in fresh

water.

16.3. Echo Sounder

The echo sounder is the most common fish detection device. It consists of four main

components, the transmitter, the transducer, the receiver and the recorder. The function of the

transmitter is to produce energy in the form of pulses electrical oscillations. The transducer

converts the electrical energy is to sound in the water and conversely the sound waves of the

returning echoes to electrical energy. The function of the receiver is to amplify the weak

electrical oscillations produced in the transducer by the echo, so that they can be recorded and

displayed. In modern fish finding echo sounders various signal processing controls are built

into the receiver. The echoes from nearest targets are stronger than those from distant targets.

From the echo gram or display one can understand the relative abundance of fish at a

particular depth. To read the depth of occurrence of the fish or sea bed directly from the

echogram or display, the time delay is calibrated to a scale marked in meter or fathom or

both.

16.4. Position and Installation

The position and installation of the transducer are often decisive in ensuring proper performance.

Normally the transducer should be placed between one-third and one half of the vessel length from

the bow and approximately one meter away from the keel. In a vessel smaller less than 30 m the

transducer may be closer to the keel. The ideal position of the transducer is at the deepest and at the

same time most noise-free. The transducer is usually housed in a streamlined shoe or blister to reduce

the noise and turbulence.

16.5. Uses of echo sounder

Echo sounder are used for :

Depth recording

Detect the types of sea bed i.e. to distinguish soft mud, sand, gravel rocky bottom etc.

Determination of sea bed contour

Location of fish and determination of its depth of occurrence.

16.6. Different types of Echo sounders

Different types of Echo sounders

Different types of echo sounders are now produced for different purpose and requirements.

They are available in the following specifications

Low frequency sounders 10 – 40 KHz: This gives good ground discrimination and can

detect the schools of fishes without air bladders. Eg. Tuna, mackerel, squids etc. This

is usually used for bottom trawling operations.

Mid frequency sounders 40 – 80 KHz: The performance in between the previous two

and is preferred by the gill netter and hand liners.

High frequency sounders 80 – 200 KHz: These are very good for determination of

fishes without air bladder but they do not give good ground discrimination. This is

suitable/chosen for detecting Tuna and Mackerel. Mainly used in purse seiners, squid

jigging vessels and liners.

Dual frequency sounders 50 – 200 KHz: This combines low and high frequency and is

very popular in fishing vessels.

16.7. SONAR The sonar is a term used in World War II. It is an acronym for Sound, Navigation and

Ranging. Lewis Nixon invented the first Sonar type listening device in 1906. The first

Sonar devices were passive listening devices in which no signals were sent out. In

active SONAR signals are sent out and received back. Sonar is capable of transmitting

and receiving the sound pulse in horizontal direction. As in echo sounder the sonar

also has four main components, the transmitter, the transducer, the receiver and the

recorder or display.

Sonar can be described as a horizontally scanning echo sounder. Its transducer is fixed

to a pipe which can be lowered or raised within a longer cylinder located below the

hull. This can scan either side of the vessel or through 360º with free sight in all

directions. Sonar can be tilted up and down from +10º to -90º. The train and tilt are

remotely controlled from the bridge. Information is displayed in a plan position

indicator (PPI) display as in; radar or in a Cathode Ray Tube (CRT).

16.8. Omni Sonar Omni sonar transmits sound pulses which effectively radiates from the transducer through a

complete 360º and electronically scan the entire area while listening for echoes. The display

shows the complete picture around the vessel for each transmission. Sonar performance is

affected by retraction of sound, thermocline, density gradient and by the pitching and rolling.

Audio output facility is not usually found in the echo-sounder but in SONAR audio out is

through loud speaker. This is normally provided for operator’s convenience as he can work

continuously without watching the display (eyes free operation). A skilled sonar operator can

detect from the sound coming from the loud speaker potential fish targets, sea bed echoes

including wrecks and other echo returns caused by vessel wakes.

16.9. Types of SONAR

In vertical echo sounders the frequency determines the maximum detection ranges, the

SONARs are available in the following specifications

High frequency sonars 60 KHz and above usually have a effective range limitation

between 800 – 1200 m depending on the target strength, density and depth of water.

Low frequency SONARS 20 – 60 KHz can detect up to 2000 m. This also depends on

the target type, density and depth of water.

16.10. Net Monitoring System/Trawl eye The net sounder is an aid for trawl fishing.The knowledge on the position and state of the

trawl can contribute to a successful fishing.This requires necessary sensors/transducers to be

located on the net and the indicator or receiver installed on board the ship along with a

linking medium, either by cable or radio.The transducers give inputs regarding trawl opening

both horizontal and vertical, vertical depth from trawl to sea bed and sea surface, water

temperature, catch details etc. Other uses of the system are as follows

Detects when the bottom trawl touches or lift off the sea bed

Detect the amount of fish caught in the cod end

Measures the depth from sea surface to the sensors

Measures the height from the sensor to the sea bed

Can detect if the gear is torn or damaged

Measures the distance between the doors

Measures the water temperature at the gear