Large industrial vessels, small artisanal boats, coastal fisheries, high seas fisheries... all around the world, fishing catches far more than is needed or wanted.
Wherever there is fishing, there is bycatch
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.
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.
The good news is that some fisheries have enforced their regulations, resulting in increased bycatch reductions in recent years. Seabird deaths around South Georgia in the CCAMLR zone of the Southern Ocean have declined by 99% since regulations were enforced.
South Africa achieved a 85% bycatch reduction in its foreign-licensed fleet in 2008, when putting a limit on the number of seabird deaths permitted.
In April 2011, Brazil also passed a law requiring the use of stringent seabird bycatch measures in their domestic tuna longline fleets.