Dan Minchin Marine Organism Investigations, Ireland ERASMUS Course: Coastal Research and Planning...
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Transcript of Dan Minchin Marine Organism Investigations, Ireland ERASMUS Course: Coastal Research and Planning...
Dan MinchinMarine Organism Investigations, Ireland
ERASMUS Course: Coastal Research and Planning Institute, University of Klaipeda, Lithuania, October 2012
Growth varies throughout the year and rings can be laid down:
In winterWhen spawningWhen disturbedOften not clearFirst ring can
be difficult
Different types of scale
Not always easy to age
Spawning checks can occur
Can be removed during the life of the fish
Otoliths are paired and the fish to be killed because the bones lie in the brain
The bones are used for balance
The ear bone is a crystalline structure with clear rings.
This bone may need to be sectioned for some fishes
A microscope may needed as the bone can be small
Some otoliths do not need to be sectioned
Many flatfish have flat easy read-bones
More recently daily rings can be found.
Also the chemical make-up may tell where the fish has been
Ceyham et al., 2007. ICES J. Mar. Sci. 64(3): 531-536
Other bones can be used to age fish
This bone is where the jaw hinges
Other head bones can be used.
The vertebrae can show rings along the centrum
This can be a useful way to age sharks
Can even be used to are and estimate growth of extinct species
Geffen et al., 2010. High latitude climate variability and its effects on fishery resources as revealed by fossil cod otoliths. ICES J. Mar. Sci. 68 (6): 1081-1089
Some sharks have spines
These can be used to age
The spur dog Squalus acanthias grows faster in the Atlantic than in the Pacific
ATLANTIC PACIFIC
The European freshwater eel spends the first part of its life in the sea where the strontium to calcium ratio is high.
Eels living a marine, brackish or freswater existance can be distinguished
Careful population stidies use the nose to the end of the hypural plate
Fork-length is often used
Total length mainly used in fishery biology
Some fish have special measurements
Some species can be aged in this way but older age groups most usually merge.
Best with following the youngest stages
Some species such as crustacea grow abruptly following the castings of their ‘shells’
In some fish a very large number are born in a particular year
These can be followed through in subsequent years
Haddock a special example.
Depends on water temperature
Spawning may interfere with growth and cause shock marks on shells
As an organism gets larger the variability cause such effects to ‘blurr’
Fish have different growth preferencesThis is usually reflected in the habitat they occupy
Do not breed until 30-35 years
Slow growing
Captured when they aggregate to spawn
K is the value that is used to represent the rate of growth. Most ‘r’ strategists have a high K value
‘r’Small and short livedHigh fecundityShort generation timeHigh rate of dispersalVariable population
densityOpportunisticProductive
‘K’Large and long livedLow fecundityLong generation timeLow rate of dispersalStable population
densityStable habitatsEfficient
Fish in a given year are presumed to have a birthdate of 1 January
Fish length gradually slows down with age, but weight may be increasing considerably
The maximum theoretical length is known as Lɷ (infinity)
Develop age-length relationships
This can be applied to a population from a known region
Sexing of fish will determine sex ratios
Sexes may grow at different rates (i.e. plaice)
Deep water surveys are difficult and time consuming
May take hours to set a net on the bottom
Such surveys also take commercial sized fish
As a result the full range of fish from pre-recruit to marketable fish can be surveyed
Good fishing practice does not involve the capture of juveniles
Most fishing equipment will capture fish after a certain age
Young fish surveys look to capture fish at a smaller size
Beam trawls with a fine mesh are used to capture fish that are not marketable
Used for most flatfishes or for fish living just at the bottom. i.e. red mullet
Must be the same stations each year and from this an index of recruiting fish is developed
Often use a mid-water trawl or benthic trawl in shallow water
Many have several spawning periods in a year
Often occur in dense shoals
The total weight of fish in a stock able to reproduce
Important for the sustainability of a stock
Critical levels according to species are set
North Sea cod
Different indicies according to egg development or gonad appearance
Different fishes can have different indicies
Can vary in their index number
Maturity stage DescriptionFemales
1.Immature Ovary small, firm, no eggs visible to the naked eye
2. Maturing virgin or resting Ovary more extended, firm, small oocytes visible, giving ovary grainy appearance
3. Developing Ovary large, starting to swell the body cavity, colour
varies according to species, contains oocytes of two sizes
4. Gravid large, filling or swelling the body cavity, when opened large ova
spill out
5. Spent Ovary shrunken, flaccid, contains a few residual eggs and many small ova
The age at which 50% of the individuals become mature is termed the L50.
Reproduction can vary considerably according to temperature and food availability
The larger the fish the more young produced
Gulf II sampler
Travel at 5 knots using an oblique tow
The water passing through the device is measured
The numbers per unit surface area is calculated
The amount of time spent fishing
The fishing mortality (F) is the fish deaths from fishing
Fishing effort and fish mortality are not linearrelationships
Octopus grow rapidly
Difficult to age
Their size changes according to Fishing Mortality (F)
Schaefer curve
The proportion of the catch declines with increased fishing effort
Optimal return is – fishing at the peak of the dome.
Once one fishes past the peak of the dome the return is less
Taking into account fuel and maintenance costs there is a tipping point where return equals outgoings
The yeild (catch) varies according to the effort.
At the maximum return fish should have spawned
The yeild increase with increased fishing effort
Trends in the decline of fish stocks
Concerns over the future of fisheries
Management measures urgently required but unpopular