by-catch

Shrimp 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

Shrimp trawling

Double-rigged shrimp trawler hauling in the nets

Shrimp bycatch

The highest rates of incidental catch of non-target species are associated with shrimp trawling. In 1997, the FAO documented the estimated bycatch and discard levels from shrimp fisheries around the world. They found discard rates as high as 20 pounds for every pound of shrimp, with a world average of 5.7 pounds for every pound of shrimp.^[1]

Shrimp trawl fisheries catch two percent of the world total catch of all fish by weight, but produce over one third of the world total bycatch. Shrimp trawls in the US produce bycatch to catch ratios from 3:1 to 15:1.^[2]

Trawl nets in general, and shrimp trawls in particular, have been identified as sources of mortality for cetacean and finfish species.^[3] Bycatch is often discarded dead or dying by the time it is returned to the sea.^[4]

Recent sampling in the South Atlantic rock shrimp fishery found 166 species of finfish, 37 crustacean species, and 29 other species of invertebrate among the bycatch in the trawls.^[3] Another sampling of the same fishery found, over a two year period, that rock shrimp amounted to only 10 percent of the total catch weight. Iridescent swimming crabs, dusky flounders, inshore lizardfish, spot, brown shrimp, longspine swimming crabs, and other bycatch made up the rest.^[3]

Despite the use of bycatch reduction devices, the shrimp fishery in the Gulf of Mexico removes about 25-45 million red snapper annually as bycatch, nearly one half the amount taken in directed recreational and commercial snapper fisheries.^[5][6]

Cetacean

Group of Fraser's dolphins.

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.^[7] In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait.^[8] In this fashion, cetaceans can become a target of fisheries.

A Dall's Porpoise caught in a fishing net

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. However, "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.^{[citation needed]}

Albatross

This Black-browed Albatross has been hooked on a long-line.

Of the 21 albatross species recognised by IUCN on their Red List, 19 are threatened, and the other two are near threatened.^[9] 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,^[10] 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.

Sea turtles

Loggerhead Sea Turtle

Sea turtles, already critically endangered, have been killed in large numbers in shrimp trawl nets. Estimates indicate that thousands of Kemp's Ridley, loggerhead, green and leatherback sea turtles are caught in shrimp trawl fisheries in the Gulf of Mexico and the US Atlantic annually^[11]

Sea turtle interactions with shrimp trawls can involve both being captured and escaping from the trawls. In the Gulf of Mexico, the Kemp’s Ridley turtles recorded most interactions, followed in order by loggerhead, green, and leatherback sea turtles. In the US Atlantic, the interactions were greatest for loggerheads, followed in order by Kemp’s Ridley, leatherback, and green sea turtles.^[11]

Bycatch mitigation

A turtle excluder device.

Concern about bycatch has led fishermen and scientists to find ways of reducing unwanted catch. There are two main approaches.

First, fishing may be banned in areas where bycatch is unacceptably high. Such area closures could be permanent, seasonal, or issued for a shorter period when a bycatch problem is registered. Temporary area closures are common in some bottom-trawl fisheries where catch of under-sized fish or non-target species occur unpredictably. In some cases fishermen are expected to automatically relocate when certain bycatch problem occurs.

The other approach is modifying fishing gear such that the tendency of gear to capture bycatch species is reduced. A technically simple solution is increasing mesh size, such that smaller species and smaller individuals of target species slip through the meshes more easily. However, this usually requires replacing the existing gear. In other cases, it is possible to modify existing gear to reduce bycatch. The "bycatch reduction device" (BRD) and the Nordmore grate are net modifications that help fish escape from shrimp nets.

BRDs allow many commercial finfish species to escape. Federal approved BRDs reduce the bycatch of finfish by 30 percent. Spanish mackerel and weakfish bycatch in the South Atlantic was reduced 40 percent.^[3] However, recent surveys suggest BRDs may be reduce bycatch less effectively than thought previously.^[5] A rock shrimp fishery off Florida found the devices did not exclude 166 species of fish, 37 crustacean species, and 29 species of other invertebrates.^[3]

In 1978, the National Marine Fisheries Service started to develop the turtle excluder device (TEDs). A TED uses a grid which deflect turtles, along with other big animals, so they exit from the trawl net through an opening above the grid. US shrimp trawlers and foreign fleets which want to market shrimp in the US are required to use TEDs. Not all nations enforce the use of TEDs

For the most part, when they are used, TEDs have been successful reducing sea turtle bycatch.^[3][12] Crowder 2001). However, they are not completely effective, and some turtles are still captured^[3][11] The National Marine Fisheries Service certifies TED designs if they are 97 percent effective. In heavily trawled areas, the same sea turtle may pass repeatedly through TEDs.^[11] Recent studies indicate recapture rates of twenty percent or more, but it is not clear how many turtles survive the escape process.^[11]

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."

Seabirds with longline fishing vessel.

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 excessive 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.

See also

• Discards
• Cetacean bycatch
• List of environmental issues
• Incidental mortality
• Fish hydrolysate
• Shrimp turtle case

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.
• Epperly, S; Avens, L; Garrison, L; Henwood, T; Hoggard, W; Mitchell, J; Nance, J; Poffenberger, J; Sasso, C; Scott-Denton, E and Young, C (2002) Analysis of Sea Turtle Bycatch in the Commercial Shrimp Fisheries of Southeast US Waters and the Gulf of Mexico NOAA Technical Memorandum NMFS-SEFSC-490.
• Gulf of Mexico Fishery Management Council (GMFMC) (2006) Scoping Document for Amendment 15 to the Shrimp FMP
• GMFMC(2005) Final Amendment Number 13 to the Fishery Management Plan for the Shrimp Fishery of the Gulf of Mexico US Waters with Environmental Assessment Regulatory Impact Review, and Regulatory Flexibility Act Analysis.
• Hall M A (1996) On bycatches. Reviews in Fish Biology and Fisheries vol. 6 (3) pp 319 – 352 (1996)
• Hall, M; D.L. Alverson, DL and Metuzals, KI (2000) By-Catch: Problems and Solutions. Marine Pollution Bulletin. Vol. 41, Nos. 1-6: 204-219
• Morgan, LE and Chuenpagdee, R (2003) Shifting Gears. Addressing the Collateral Impacts of Fishing Methods in U.S. Waters.
• 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.
• SAFMC (1998) Final Habitat Plan for the South Atlantic Region Essential Fish Habitat Requirements for Fishery. Management Plans of the South Atlantic Fishery Management Council. The Shrimp Fishery Management Plan (FMP), the Red Drum FMP, the Snapper Grouper FMP, the Coastal Migratory Pelagics FMP, the Golden Crab FMP, the Spiny Lobster FMP, the Coral, Coral Reefs, and Live/Hard Bottom Habitat FMP, the Sargassum Habitat FMP, and the Calico Scallop FMP.
• Southeast Data, Assessment, and Review (SEDAR) (2005) Stock Assessment Report of SEDAR 7 Gulf of Mexico Red Snapper: Assessment Summary Report.
• U.S. Shrimp (South Atlantic and Gulf of Mexico) – Blue ocean institute. Retrieved 22 March 2008.

External links

• Project GLOBAL: Global Bycatch Assessment of Long-Lived Species project
• Oceana facts about bycatch/dirty fishing
• FAO document on bycatch and discard
• Greenpeace facts about bycatch
• Alaska Marine Conservation Council

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