The main reason that bycatch occurs is because modern fishing gear
is very strong, often covers extensive areas and can be highly unselective - meaning it catches not only the target species but also many other creatures as well.
Furthermore, poor fisheries management
in certain countries does little to minimize bycatch. For example:
- Although devices to minimize turtle bycatch are required in some fisheries, they are not always used due to lack of enforcement or political will.
- Many Asian tropical shrimp trawl fisheries generate massive bycatch of "trash fish" - juvenile and small fish - that is often marketed. This provides little incentive for fishers to implement the bycatch reduction devices that will allow these fish to escape.
- Some policies actually create incentives to discard unwanted fish. For example, under some management systems fishers can only land fish species for which they have a quota - and so dump the smaller, less valuable fish overboard, often already dead.
- Due to different fish populations often living together, the directed catch of one species may well result in non-allowable catches of another! This is a particular problem in the Grand Banks off Canada where, despite a long-time ban on cod fishing, cod recovery is prevented as cod juveniles are caught by fishers targeting other fish.
Widespread pirate fishing
adds to this problem by ignoring regulations on net mesh sizes, quotas, permitted fishing areas, and any bycatch mitigation measures.