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Collaborative Action 8:Purse seine

Terje JørgensenInstitute of Marine Research

Berge, Norway

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10/4/09

MORTALITY OF NORTH SEA HERRING CROWDED AND SUBSEQUENTLY SLIPPED IN THE PURSE SEINE FISHERY

PRINCIPAL RESEARCHERS: Maria Tenningen &, Aud SoldalInstitute of Marine Research, Bergen

Slipping in Norwegian purse seine fisheries

• Norwegian herring catches mainly caught with purse seine

• Most of the catches used for human consumption and the price paid for a catch depends on the size and quality of the fish

• Slipping of fish of unwanted size or quality or if the catch is too large (e.g. exceeds vessel capacity or the allocated quota)

Norwegian legislation prohibits slipping of dead or dying fish

How do we know if the slipped fish will survive or not?

• Instant mortality, e.g.:- lack of oxygen- exhaustion- physical injuries

• Delayed mortality, e.g.:- Skin damageinfections, osmoregulatory problems

A major problem for the fisheries managers to find a concrete ”point of no return” after which slipping is not allowed

Main Goals

• Crowd herring at densities corresponding to those in a late phase of purse seining and quantify mortality rate

• Handling procedures that resembles crowding in a commercial purse seine as closely as possible

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METHOD

Transfer of fish from the purse seine into the net pen

Three net pens were filled from the same purse seine haul

• 5 – 10 tonnes of herring• One control net pen• Two crowded groups

Catching herring and arrnging the net pens

Crowding and slipping simulation

• Density increased by lifting bottom of the net pen • The volume in the net was calculated• The crowding density was maintained for 10 min

1°E 2°E 3°E 4°E 5°E 6°E

59°00'N

59°30'N

60°00'N

60°30'N

61°00'N

• Net pens allowed to drift freely in the sea for 4-5 days

• Cameras installed for observation of fish behaviour

• Argos transmitters allowed the localization of the net pens

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In varying weather conditions…

At the end the net pens were emptied and dead and alive fish were weighed and counted and length sampled

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RESULTS

Herring mortality: results from 2008 and 2009

2008 A 2008 B 2009 A

-100 0 100 200 300 400 500

Crowding density (kg/m 3)

-10

0

10

20

30

40

50

60 M

orta

lity

%

control

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< 2 %

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28 %

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51 %

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52 %

Many of the herring that were alive in the most crowded groups had abrasions and bruises at the termination of the experiment after 4-5 days

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CONCLUSIONS AND FUTURE PLANS AND CHALLENGES

Conclusions

• Herring is vulnerable to high crowding densities in the purse seine.

• Slipping at a late phase of hauling the net should therefore not be allowed

• If the fish are slipped carefully in an early phase of hauling there is a good chance for a high survival rate

Future challenges

• Are the injuries received in the experiments the same as in commercial fishing (10 tonnes vs 100s of tonnes)?

• What are normal crowding densities in commercial fisheries before slipping?

• How do variation in catch size, weather conditions, day-night variation and seasonal variation affect the survival rate?

B. Improved identification before shooting the purse seine or in an early phase of hauling

C. Development of responsible purse seine gear• Seines that allow slipping at early stage of fishing•Seines resistant to bursting•Seines that allow control of quantity

A. Unaccounted mortality• Quantification of mortality in commercial purse seine fisheries of pelagic spesies•Assessment of unacconted mortality caused by purse seine fisheries on fleet level.

Development of environmental friendly purse seine fisheries for pelagic fish

D. Fisheries management & stock assessment

For more details see:

Teningen, M., Vold, A., Saltskår, J. and I. Huse, 2009. Mortality of North Sea herring that is crowded and subsequently slipped from a purse seine. ICES CM/M:16.