bycatch

bycatch, term for any fish, mammal, bird, or other animal or group of animals captured unintentionally by the commercial fishing industry. Bycatch is closely related to the term discard, which is the amount of haul that is discarded back into the ocean, alive or dead. Bycatch becomes discard when it is returned to the ocean; however, discard may also include commercially relevant species that are cast aside, for example, to make room for higher-quality catch or catch that falls within a regulated limit.

Fisheries regularly capture nontarget organisms from marine environments as part of the process of harvesting targeted species. Large fishing nets—such as trawls, drift nets, purse seines, gill nets, or longlines (heavy fishing lines with baited hooks that may be miles long)—do not discriminate and often capture all marine life in the path of the net. Sea turtles, seabirds, sharks, and marine mammals that are neither commercially profitable nor targeted are killed when caught as bycatch, and scientists consider bycatch to be a substantial factor in threatening many species with extinction. Most commercial fisheries have been unwilling to take time or invest in the equipment necessary to facilitate the safe release of the nontargeted marine life caught during their hauls. Moreover, even if fisheries did make this effort, the physical trauma and stresses associated with being dragged in a net packed with other living things often proves fatal, often by the time such marine life reaches the deck of a boat. Consequently, bycatch is regarded generally as an unavoidable consequence of large-scale fishing, and the carcasses of these nontargeted animals are cast back into the ocean.

Measurement challenges

Bycatch data is often unreliable and incomplete, because figures frequently are self-reported (and therefore difficult to verify) or they are collected only when industry or government observers and monitors are present (which may be the case during only a small portion of the overall fishing effort across a fishing season). According to some estimates, more than 40 percent of all fish harvested annually (some 38 million metric tons [nearly 84 billion pounds]) is bycatch, and hauls are responsible for the bycatch deaths of more than 650,000 marine mammals worldwide each year.

Discover how ordinary objects work, and expand your understanding of the familiar.

SIGN UP

" height="20" alt="HEW Logo">

Thank you for signing up!"

Back to homepage

By clicking "Sign Up", you consent to the Britannica Privacy Policy and Terms of Use.

Bycatch occurs in every fishery, but the fraction of the haul that is bycatch varies with the target species, geographical location, and type of net used for capture. For individual fisheries, statistics relating to discard may be used as a surrogate for understanding rates of bycatch. For example, one study found that fisheries targeting crustaceans (such as shrimp) discard more than 30 percent of the catch, while tuna fisheries discard just over 5 percent of the catch. Fisheries using otter trawls (a cone-shaped net that uses small boards to widen the net) along the seafloor discard just over 30 percent of their catch, whereas fisheries using pelagic gill nets (a type of open-ocean drift net) discard just over 10 percent of their catch.

Solutions

The negative attention generated by the accidental capture of sea turtles, sharks, birds, and cetaceans (that is, whales, dolphins, and porpoises) has fueled the motivation to find technical solutions to the problem of bycatch. For example, declines in sea turtle populations worldwide are blamed, in part, on their ensnarement in fishing nets, which causes them to drown. This problem has prompted the development of turtle excluder devices, which are doors or barriers constructed within nets that effectively allow turtles to exit a net when captured. Other technological advances to reduce bycatch have included adding weights to longlines to keep them at a depth that reduces risks to seabirds, adding bird-scaring lines that prevent birds from reaching baited hooks in the water, developing hooks that ensure that setting (piercing an individual fish with a hook) occurs only underwater, and attaching acoustic pingers to gill nets that warn away larger nontarget animals by making a sound.

Several international legal frameworks also contribute to the reduction of bycatch. Probably the most well-known of these includes the prohibition of using drift nets more than 2.5 km (1.6 miles) in length, which stemmed from United Nations resolutions passed during the early 1990s. Since that time, several countries and economic blocs, including the United States and the European Union, have turned the resolutions into legislation. For example, the U.S. High Seas Driftnet Fisheries Enforcement Act of 1992 formally prohibits 2.5 km-long drift nets, whereas the U.S. Driftnet Modernization and Bycatch Reduction Act of 2022 phases out or eliminates drift nets designed to harvest swordfish and other species. In addition, the European Union, which has similar regulations on drift nets, also has introduced regulations that set strategies to eliminate the practice of discard. Some of these regulations require boat operators to land the organisms they catch in nets and haul them aboard rather than discard them, and others require that bycatch be counted as part of a given operator’s haul limit.