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