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Bycatch

Bycatch are species caught in a fishery while it is intended to catch another species or reproductively-immature juveniles of the target species.

The 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 bycatch in fisheries.

Bycatch 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.
Bycatch 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.

Examples

Cetacean

Cetaceans, such as dolphins, porpoises, and whales, can be seriously affected by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets. Cetacean bycatch is increasing in intensity and frequency. In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait. In this fashion, cetaceans can become a target of fisheries.

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 underwater. This bycatch 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.

Albatross

Of the 21 albatross species recognised by IUCN on their Red List, 19 are threatened, and the other two are near threatened. Two species (as recognised by the IUCN) are considered critically endangered: the Amsterdam Albatross and the Chatham Albatross. One of the main threats is commercial long-line fishing, as the albatrosses and other seabirds which will readily feed on offal are attracted to the set bait become hooked on the lines and drown. An estimated 100,000 albatross per year are killed in this fashion. Unregulated pirate fisheries exacerbate the problem.

Shrimp

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 neta 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.

Bycatch mitigation

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."

Longline fishing is controversial in some areas because of by-catch. Methods to mitigate such incidental mortality have been developed and successfully implemented in some fisheries. These include the use of weights to ensure the lines sink quickly, the deployment of streamer lines to scare birds away from the baited hooks as they are deployed, setting lines only at night with ship lighting kept low (to avoid attracting birds), limiting fishing seasons to the southern winter (when most seabirds are not feeding young), and not discharging offal while setting lines. However, gear modifications do not eliminate by-catch of many species and the controversy continues. In March 2006, the Hawaii longline swordfish fishing season was closed due to excess ive loggerhead sea turtle by-catch after being open only a few months, despite using modified circle hooks which attempt to reduce by-catch.

In addition to efforts to reduce the amount of bycatch caught in nets, some fisheries are starting to implement programs to effectively utilize bycatch species, rather than throwing the fish back into the ocean. One such use of bycatch is the formulation of fish hydrolysate that can be used as a soil amendment in organic agriculture.

Notes

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.
Demaster, DJ, Fowler, CW, Perry, SL, and ME Richlen (2001). Predation and competition: the impact of fisheries on marine mammal populations over the next one hundred years. Journal of Mammology. 82: 641-651.
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.
Read, AJ, Drinker, P, and S Northridge (2006). Bycatch of marine mammals in the U.S. and Global Fisheries. Conservation Biology. 20(1): 163-169.