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

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

2. When sea-gulls follow trawlers, it is because they know sardines will be thrown into the sea • Fishing operation produces unwanted by-product • Discarded material is of interest to other organisms Eric Cantona Manchester United and France

3. By-catch

4. Discard Weight by World Region

5. Discard Rate by Region

7. Fate of fish interacting with a fishing gear

8. Target species: legal restrictions Species of no commercial value Target species: individuals of low economic value Reasons for by-catch and discarding

9. 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.

10. 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.

11. Birds: Has been a problem for albatrosses in drift net and long line fisheries Turtles and snakes: many released unharmed but can add to mortality of threatened species Dolphins and porpoises: Even low mortalities enough to adversely affect populations By-catch Mortality: Reptiles, Birds and Marine Mammals

12. 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

13. Lightly Exploited Heavily Exploited Why are so many fish discarded?

14. Technical measures: A variety of gear modifications can reduce unwanted by-catch Closed Areas: Areas where juvenile fish concentrate can be closed Fishing effort: Reducing effort results in larger fish and decreased incentive to catch juveniles Mitigation of by-catch

15. 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.