The Magnitude and Impact of By-catch Mortality by Fishing Gear Robin Cook FRS Marine Laboratory Aberdeen UK.

The Magnitude and Impact The Magnitude and Impact of By-catch Mortality by of By-catch Mortality by

Fishing GearFishing GearRobin Cook

FRS Marine Laboratory

Aberdeen

UK

When sea-gulls follow trawlers, it is because they know sardines will be

thrown into the sea

Eric CantonaManchester United and France

• Fishing operation produces unwanted by-product

• Discarded material is of interest to other organisms

Incidental Catch

Discards

Target catch

DefinitionsBy-catch

Discard Weight by World Region

Northwest Pacific (33.81%)

West Indian Ocean (5.45%)

Northeast Pacific (3.42%)Southwest Atlantic (2.97%)

East Indian Ocean (2.97%)East Central Pacific (2.84%)

Northwest Atlantic (2.54%)East Central Atlantic (2.20%)

Mediterranean and Black Sea (2.09%)Southwest Pacific (1.09%)Southeast Atlantic (1.03%)

Atlantic Antarctic (0.13%)Indian Ocean Antarctic (0.04%)

PacificAntarctic (0.00%)

Northeast Atlantic (13.59%)

West Central Atlantic (5.93%)

Southeast Pacific (9.63%)

West Central Pacific (10.28%)

% of total discards

Discard Rate by Region

0 0.1 0.2 0.3 0.4 0.5 Kg Discarded per Kg Landed

Indian Ocean, EasternMediterranean and Black Sea

Atlantic, NorthwestPacific, Eastern Central

Pacific, NortheastPacific, Southeast

Atlantic, Western CentralAtlantic, Eastern Central

Atlantic, NortheastPacific, Western Central

Atlantic, SoutheastIndian Ocean, Western

Pacific, NorthwestPacific, Southwest

Atlantic, Southwest

Regio

n

0 2 4 6 8 10 12 Kg Discarded/Kg Landed

Pelagic Fish Trawl

Purse Seine

Danish Seine

Longline

Pot/trap

Non-Pelagic Fish Trawl

Shrimp Trawl

Mean Discard Weight per Landed Weight

Fate of fish interacting with a fishing gear

E sc ap e s

D e a th s

D isc a rd sS u rv ivo rs

L a n d in gs

C a tc h

F ish e n te r in gg ea r

Reasons for by-catch and discardingTarget

species: legal restrictions

Target species: individuals of low economic value

Species of no

commercial value

By-catch Mortality: non-target species

• Shrimp fisheries implicated in severe reduction of croakers.

• May result in local extinction e.g. common skate in Irish Sea.

• May result in mortalities as high as fishing mortality on target species, e.g. 50% exploitation rate in North Sea.

By-catch mortality: target species

• Technical interaction: unwanted bycatch in one fishery may reduce yield in another fishery, e.g. fish by-catch in shrimp fisheries.

• Discards of target species can reduce spawning stock biomass and yield in target fishery.

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Yie

ld p

er

Recru

it (

Kg)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 Fishing Effort

North Sea Haddock: Effect of Discards

Withdiscards

Nodiscards

By-catch Mortality: Reptiles, Birds and Marine Mammals

Birds: Has been a problem for albatrosses in drift net and long line fisheries

Dolphins and porpoises: Even low mortalities enough to adversely affect populations

Turtles and snakes: many released unharmed but can add to mortality of threatened species

Ecosystem Effects

• Reduces abundance of large individuals, especially predators

• Increases the relative abundance of small early maturing species with high reproductive rates

• Favours the increased abundance of scavengers

Heavily Exploited

Why are so many fish discarded?

Lightly Exploited

Mitigation of by-catch

Closed Areas: Areas where juvenile fish concentrate can be closed

Technical measures: A variety of gear modifications can reduce unwanted by-catch

Fishing effort: Reducing effort results in larger fish and decreased incentive to catch juveniles

Conclusions• By-catch mortality is one component of total

fishing mortality which can have effects beyond the target species

• By-catch mortality is difficult to quantify but failure to account for it in stock assessment can result in bias which affects management decisions

• Successful achievement of traditional fishery objectives would go a long way to reducing the problem of by-catch and its impact.