Presentation on theme: "The Magnitude and Impact of By-catch Mortality by Fishing Gear Robin Cook FRS Marine Laboratory Aberdeen UK."— Presentation transcript:
The Magnitude and Impact of By-catch Mortality by Fishing Gear Robin Cook FRS Marine Laboratory Aberdeen UK
When sea-gulls follow trawlers, it is because they know sardines will be thrown into the sea Eric Cantona Manchester United and France Fishing operation produces unwanted by-product Discarded material is of interest to other organisms
Reasons for by-catch and discarding Target 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.
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.