The incidental capture (or by-catch) of dolphins and porpoises in fisheries presents one of the most acute threats to cetaceans in many parts of the world, and has been identified as the most serious threat facing the harbour porpoise.
Worldwide attention has been given to the high mortality of dolphins associated with driftnets and certain purse-seine tuna fisheries in the Eastern Tropical Pacific. Other types of fishing gear also endanger cetaceans but have not received as much publicity.
Data from around the world indicate high numbers of dolphins and porpoises are also killed by gill nets and in mid-water trawl fisheries.
Dolphins and porpoises are not the only cetaceans being harmed by fishing gear. Fishing gear can injure and kill large cetaceans as well - even the powerful sperm whales may become entangled in nets and drown, or starve to death if gear becomes wrapped around or embedded in its mouth.
The UN has banned the use of large-scale (defined as greater than 2.5km) driftnets, and the EU has placed a ban on driftnets of any length. These are partial solutions to a problem that is the leading factor in the mortality of numerous cetacean species.