In fisheries science, bycatch (or less commonly "by-catch") refers to species caught in a fishery intended to target another species, as well as reproductively-immature juveniles of the target species. Bycatch is a serious issue that can contribute to species endangerment.
One example of bycatch is dolphins caught in tuna nets. As dolphins are mammals and do not have gills they may drown while stuck in nets under water. This bycatch issue has been one of the reasons of the growing ecolabelling industry, where fish producers mark their packaging with something like "Dolphin Friendly" to reassure buyers. Unfortunately for the dolpins, "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 World Wide 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.
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 vigour. 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".
This extract from a recent court judgment summarises some of the New Zealand way of dealing with bycatch.
"When fishing for species for which it holds quota, a fisher frequently catches other quota species for which it does not hold quota. These fish are called by-catch. The Fisheries Act 1996 and its predecessor, the Fisheries Act 1983, both recognise that by-catch is inevitable, and seek to create incentives to minimise it and avoid wastage. That is done by requiring the fisher to land by-catch and allowing it to be processed and sold, but requiring the fisher to buy quota or make a ‘deemed value payment’ to the Ministry. Deemed values are set from time to time for each quota species. The Ministry seeks to set them at a level that makes by-catch unprofitable but creates an incentive, having caught by-catch, to land, process, and sell it. Adverse consequences follow should the fisher not make the deemed value payments, although the consequences differ under the 1983 and 1996 Acts."
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