by-catch

A Dall's Porpoise caught in a fishing net

In fisheries science, by-catch includes species caught in a fishery intended to target another species, as well as reproductively-immature juveniles of the target species. OECD (1997) defines bycatch as Total fishing mortality excluding that accounted directly by the retained catch of target species. According to Alverson et al. (1994) there are at least four different uses of the word by-catch in fisheries.

• By-catch may refer to catch which is retained and sold but which is not the target species for the fishery.
• Particularly the Northeast and Western Pacific and in American legislation bycatch means species/sizes/sexes of fish which are discarded.
• By-catch is used on all non-target fish whether retained and sold or discarded (Hall, 1996)
• Unwanted invertebrate species such as echinoderms and non-commercial crustaceans.

Overview

Shrimp by-catch

One example of by-catch is dolphins caught in tuna nets. As dolphins are mammals and do not have gills they may drown while stuck in nets underwater. This by-catch issue has been one of the reasons of the growing ecolabelling industry, where fish producers mark their packagings with something like "Dolphin Friendly" to reassure buyers. Unfortunately for the dolphins, "dolphin friendly" does not mean that dolphins were not killed in the production of a particular tin of tuna, but that the fleet which caught the tuna did not specifically target a feeding pod of dolphins, but relied on other methods to spot tuna schools.

According to the U.S. National Marine Fisheries Service, in the Gulf of Mexico, three pounds of bycatch are caught for every pound of shrimp that goes to market. According to the Worldwide Fund for Nature, in the Gulf of Thailand it can be 14 pounds of bycatch per pound of shrimp. Bycatch is often discarded dead or dying by the time it is returned to the sea. Trawl nets in general, and shrimp trawls in particular, have been identified as sources of mortality for species of concern, including cetaceans. Sea turtles, already critically endangered, have been killed by the thousands in shrimp trawl nets.

A turtle excluder device.

Concerns about bycatch have led fishermen and scientists to develop devices they can put on their nets to reduce unwanted catch. The "bycatch reduction device" (BRD) and the Nordmore grate are net modifications that help fish escape from shrimp nets. All U.S. shrimp trawlers—and all foreign fleets selling shrimp in the U.S—are supposed to outfit their nets with trap-door "Turtle excluder device," or TEDs, to let sea turtles escape. However, not every nation enforces TED use with equal vigor. The size selectivity of trawl nets is often controlled by the size of the openings in the net, especially in the "cod end". The larger the size of the openings, the more easily small fish can escape. The development and testing of modifications to fishing gear to improve selectivity and decrease impact is called "conservation engineering."

References

Alverson D L, Freeberg M K, Murawski S A and Pope J G. (1994). A global assessment of fisheries bycatch and discards. FAO Fisheries Technical Paper No 339 Rome.

Hall M A (1996) On bycatches. Reviews in Fish Biology and Fisheries vol. 6 (3) pp 319 - 352 (1996)

OECD (1997) Towards sustainable fisheries: economic aspects of the management of living marine resources. OECD Paris.

See also

• Cetacean bycatch
• Shrimp farm
• List of environment topics
• Incidental mortality

External links

• Project GLOBAL: Global Bycatch Assessment of Long-Lived Species project
• Oceana facts about by-catch/dirty fishing
• FAO document on by-catch and discard
• Greenpeace facts about by-catch

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