The fine art of gill protection

Protec Gill - designed to support gill health and recovery during disease, environmental and treatment challenges.

PROTEC GILL

GILLS – AN IMPORTANT MULTI-FUNCTIONING ORGAN

Fish gills are the most important site of contact between the fish and the water. They are not

only responsible for the exchange of the respiratory gases (oxygen and carbon dioxide) they

also play a part in maintaining salt balance and excreting waste. Skretting’s gill handbook has

been developed so you can learn more about this life sustaining organ, and how maintaining

good gill health will enable fish to perform to their full potential.

GILL STRUCTUREGills lie in what’s known as the branchial

cavity and are protected by a hard bony

flap called an operculum. If you lift the

operculum, you will see four gill arches.

On each gill arch, there are lines of frills

called filaments. On the top and bottom

of the filaments, there is a series of

secondary folds called lamella. Inside the

lamella, you find tiny blood vessels. The

lamella walls consist of one cell layer,

and the entire lamella is about 10 microns

thick; a little more than half the thickness

of a strand of human hair. This branching

structure gives the gills its large surface

which is about 50 times larger than

the area of skin covering the body. Gill

surface area declines with increasing fish

weight which is why larger fish often are

the first to succumb to oxygen depletion.

Illustration showing how blood vessels coming from the heart (in blue) and going to the body (in red) runs through the gill arch. The water flows between the gill arches, through the gill filaments, passing each gill lamella.

Filament

Lamella blood vessels

GILL FUNCTION

When the fish opens its mouth, water flows in, past the gills, and out of the gill operculum in one

direction, while the blood in the gill flows in the opposite direction through the blood vessels in the

lamella. This process is called the counter-current principle. The combination of the large surface

area, the counter-current principle and the short distance between blood and water makes the gills

very efficient. Gills can extract up to 80% of the oxygen from the water passing through them.

CONTROLLING THE BODY’S WATER CONTENT

Chloride secretory cells located in the gill lamella play an important role in the transportation of

salts to and from the blood into the surrounding water; this process is known as osmoregulation.

In freshwater, water will continuously diffuse into the fish so it has to actively uptake salts from the

water to get enough ions in the body for it to function properly.

In seawater this process is reversed

and fish will have a lower concentration

of salts than that of the surrounding

seawater, so it loses water and gains

salts. Fish need to get rid of this excess

salt to avoid dehydration.

ELIMINATION OF WASTE

The process of getting energy from

feed produces waste and a large

amount of this is in the form of

nitrogen. Nitrogen combines with other

molecules in the body to produce toxic

compounds such as ammonia. About

80 to 90% of a fishes nitrogenous

waste is excreted as ammonia by the

gills, with the remainder excreted by

the kidney as urea. Again, the short

distance between the blood and the

water the fish swims in is useful, as

ammonia can easily be transported

from the blood stream to the water via

the thin lamella wall.

The counter-current principle: A small segment of one of the gill filaments where the lamellae are located. Water and blood flow in opposite directions past the lamellae making oxygen uptake very efficient.

Lamella

GAS EXCHANGE

WHAT HAPPENS WHEN GILLS ARE COMPROMISED?

There are many potential threats to gill integrity and function such as disease, poor water quality and

the side-effects of treatments. The very design that makes gills so efficient means that they are also

vulnerable to damage.

The most common response of the gill to any disorder are inflammatory responses in which the

distinguishing feature is either.

1) Hyperplasia - the enlargement of an organ caused by an increase in the reproduction rate of its cell

2) Hypertrophy – the enlargement of an organ or tissue from the increase in size of its cells.

Both hyperplasia and hypertrophy will lead to the lamellae becoming stuck or clubbed together.

Chronic irritation is also likely to cause a thickening of the mucous layer. These inflammatory symptoms

can significantly reduce the surface area available for gas and salt exchange as well as excretion. The

effects on farmed fish populations can be reduced appetite, a lower tolerance to stress and in severe

cases mortality. Today, gill disease is one of the most common causes of production loss in aquaculture.

AGD – AMOEBA CHALLENGES GILL HEALTH

An increasingly common challenge to

gill health is from the parasitic amoeba

Neoparamoeba perurans which causes

amoebic gill disease (AGD). Amoebas are

single celled organisms. In the human intestine

we have several naturally-occurring amoebas,

while other amoebas that end up in food or

drink can cause illness, for instance dysentery.

N. perurans only infects fish in sea water

and was first observed in Tasmania in 1985.

By 2015 nine of the twelve Atlantic salmon

producing countries worldwide had reported

confirmed cases of the disease. Even though

amoebas are small, they are large compared

to the gill filaments, with a length of up to

50μm. N. perurans can float well in water and

actively move over the surface of gills. They

reproduce quickly and easily by cell division.

When the amoeba attaches to the gills, its

presence causes a severe increase in the

number cells (hyperplasia) within the gill tissue

which in turn leads to the fusion of the gill

lamellar. These are the white spots visible

when you open the gill operculum and study

the gill filaments. If you look at the damage

under a microscope, you can see that the

lamellae are clumped together. Therefore,

the gills lose their surface area, and the gill

capacity is poorer. When the gills are severely

attacked, the fish can suffocate.

In Northern hemisphere salmon production

regions, AGD is most common in the autumn

when the temperature starts to drop from the

summer highs. The severity of the disease

reduces when the temperature falls to its

lowest winter levels.

It has been proven that N. perurans can

survive for at least 14 days in open sea water,

and further spread one kilometer from the

source. It also exists on other species living

in the sea, such as cleaner fish, shellfish and

hydroids. The risk of disease is likely to be

higher in farmed salmon, as the density of fish

and nutrients is higher.

A common scoring system has now been

adopted almost uniformly across the industry

to monitor the gills to see if the fish has been

infected by P. perurans. The damage is scored

in individual fish on a scale of 1 to 5. More

information about gill scoring can be found

on Skretting’s AGD poster and scorecard. A

score of 5 indicates the most serious damage,

where most of the gill tissue is altered and

there is a higher risk of mortality. It is therefore

common to treat the fish before the average

gill score of the unit has escalated. AGD is

treated with hydrogen peroxide or freshwater

baths. This requires time and resources, and

the treatment can be stressful for the fish.

Skretting research has shown that the

combination of several of our tried and tested

functional ingredients can modulate key

immune system parameters which are known

to support gill structure and function.

Amoebas may form runners used for movement and to catch food.

THE FUNCTIONAL FEED DEVELOPMENT PROCESS

At Skretting our rigorous process of functional feed development starts by screening

substances in the laboratory to establish the potential of the nutrient to either affect

pathogens directly or to modulate the immune system to achieve a positive effect. As these

innovative substances will be added to feed formulations the ingredients with potential are

then assessed for aspects such as stability, yields and bioavailability in finished feed products.

DEVELOPING PROTEC™ GILL

The role that functional feed ingredients play in supporting farmed fish health and in turn to

sustainable and economic production has been a major focus for Skretting Aquaculture Research

Centre over the last 25 years. In 1992, Skretting launched its first functional feed called Response,

whose ingredients were designed to strengthen the fish’s immune defense systems. Since then,

our researchers have discovered more functional ingredients that have been added to the feed that

work synergistically to improve fish health. With these upgrades, the name of the feed was changed

to Protec™. Today, the Protec™ solution is used for a number of species worldwide, and is effective

in preparing the fish for upcoming stress and challenges in farm conditions, such as handling,

vaccination and diseases.

With an extensive array of laboratory research, and repeated infection trials, a new feed that better

supports gill health and recovery during environmental, treatment and disease challenges such as

AGD has been developed. The feed is called Protec Gill and, as the name implies, it contains the full

Protec™ package, with added components that ensure better gill health. Protec Gill has produced

higher survival rates in three controlled AGD infection trials, and analysis of these results has shown

that the fish fed Protec Gill had significantly lower rates of mortality.

Protec ingredients inhibit the growth of amoebawhen exposed to mucus from fish fed different functional ingredients:

Protec Protec Gill Pos. ControlNeg. Control

Gro

wth

Pe

rce

nta

ge

97

96

95

94

93

92

9 1

90

89

88

87

Results show that when P. perurans amoeba are exposed to the mucus of Protec gill fed salmon

the growth of amoeba is inhibited.

Protec Gill

Protec Gill Increases Survivalin fish challenged with P. Perurans

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Days after infection

Control

Pe

rce

nta

ge

Su

rviv

al

100

90

80

70

60

50

40

30

20

10

0

Challenge trials using Protec Gill against AGD infections were replicated three times to

confirm results. The fish that received Protec Gill had the lowest mortality rates every time,

and showed 27.1% 20.3% and 42.4% higher survival rates.

Protec Gill

Protec Gill supports osmoregulationin fish challenged with P. Perurans

Control

Blo

od

So

diu

m (

mm

ol/

L)

174

173

172

171

170

169

168

167

166

165

164

In seawater fish must replace the water which they constantly lose through osmosis.

However, drinking seawater brings with it a large quantity of salt into the blood and this has

to be removed by chloride secretory cells in the gills, which actively transport salts from the

blood back into the surrounding water.

Measuring blood sodium levels gives an indication of the integrity of gill structure and the

efficiency of gill function. Unhealthy or damaged gills will be less efficient in removing salts

from the blood. Research shows that Protec Gill fed fish are better able to maintain osmotic

regulation during an AGD challenge; the principle cause of death in AGD infected fish.

An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly

by plasma cells that is used by the immune system to identify and neutralise pathogens such

as bacteria and viruses.

Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. It is

the first antibody to appear in response to initial exposure to an antigen. IgT is the first

monomeric immunoglobulin to be shown in teleost fish. Recently discovered, IgT is produced

following exposure to toxins, pathogens or other foreign substance and is predominantly

found in the fish mucosal surfaces, suggesting a possible role of IgT in mucosal immunity.

IgT

Protec Gill increases the expression of antibodies

IgM

Re

lati

ve

Exp

ressio

n

16

14

12

10

8

6

4

2

0

Control

Protec

Lysozymes are enzymes that increase the rate of destruction of the cell walls of certain

bacteria. Lysozyme is one of a number of immune related components occurring in fish

mucus and indicates the importance that mucus plays in providing the first line of defence

against a broad spectrum of pathogens. Research shows that the mucus produced by the

skin and gills in Protec Gill fed fish have higher levels of lysozyme and anti-bacterial potential.

Protec

Protec Gill increases anti-bacterial potential

Control

Iyso

zym

es (

U/m

l)1000

900

800

700

600

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300

200

100

0

Protec Gill

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www.skretting.co.uk

Skretting is the global leader in the production of innovative and sustainable nutritional solutions for aquaculture. We deliver high quality feeds and services to farmers around the world, so that they can produce healthy and delicious food in a sustainable manner. Skretting produces feed in 18 countries and has an annual production of 2 million tonnes of high quality feed to a wide range of fish and shrimp species in all life stages.

SKRETTING’S RECOMMENDATIONS FOR MAINTAINING GOOD GILL HEALTH

Proven functional feed solutions like Protec™ should be included as part of a structured approach to

performance and health management and alongside other strategies such as vaccination, disease

monitoring, and water quality management. When specific known gill stressors do develop, a

change to Protec Gill will focus the benefits of functional nutrients to where they will have the most

impact.

RECOMMENDATIONS FOR PROTEC GILL USETypical recommendations for Protec Gill use in Scottish seawater salmonid production:

Before and During Gill Health Challenges

Feeding Protec Gill 2 to 4 weeks ahead of known gill health risk periods and/or at the commencement

of gill disease outbreaks until its resolution, will provide additional mitigation against the effects of the

challenge.

Before and After Bath Treatments

Feeding Protec Gill for 2 weeks before and 2 weeks after Hydrogen Peroxide (H2O2) treatments will

support the regeneration of gill structure and function.