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Examining the Problem of Bycatch

A topic of increasing relevance to the conservation of marine life is bycatch-fish and other animals that are unintentionally caught in the process of fishing for a targeted population of fish. Bycatch is a common occurrence in longline fishing, which utilizes a long heavy fishing line with baited hooks placed at intervals, and in trawling, which utilizes a fishing net (trawl) that is dragged along the ocean floor or through the mid-ocean waters. Few fisheries employ gear that can catch one species to the exclusion of all others. Dolphins, whales, and turtles are frequently captured in nets set for tunas and billfishes, and seabirds and turtles are caught in longline sets. Because bycatch often goes unreported, it is difficult to accurately estimate its extent. Available data indicate that discarded biomass (organic matter from living things) amounts to 25-30 percent of official catch, or about 30 million metric tons.

The bycatch problem is particularly acute when trawl nets with small mesh sizes (smaller-than-average holes in the net material) are dragged along the bottom of the ocean in pursuit of groundfish or shrimp. Because of the small mesh size of the shrimp trawl nets, most of the fishes captured are either juveniles (young), smaller than legal size limits, or undesirable small species. Even larger mesh sizes do not prevent bycatch because once the net begins to fill with fish or shrimp, small individuals caught subsequently are trapped without ever encountering the mesh. In any case, these incidental captures are unmarketable and are usually shoveled back over the side of the vessel dead or dying.

The bycatch problem is complicated economically and ecologically. Bycatch is a liability to shrimp fishers, clogging the nets and increasing fuel costs because of increased drag (resistance) on the vessel. Sorting the catch requires time, leading to spoilage of harvested shrimp and reduced time for fishing. Ecologically, high mortality rates among juvenile fishes could contribute to population declines of recreational and commercial species. Evidence to this effect exists for Gulf of Mexico red snapper and Atlantic Coast weakfish. Because the near-shore areas where shrimp concentrate are also important nursery grounds for many fish species, shrimp trawling could have a profound impact on stock size.

Once the dead or dying bycatch is returned to the ecosystem, it is consumed by predators, detritivores (organisms that eat dead plant and animal matter), and decomposers (organisms that break down dead or decaying organic matter), which could have a positive effect on sport fish, seabird, crab, and even shrimp populations. Available evidence indicates that 40-60 percent of the 30 metric tons of catch discarded annually by commercial fishing vessels, and even more of the noncatch waste (organisms killed but never brought to the surface), does not lie unused on the bottom of the sea. It becomes available to midwater and ocean-bottom scavengers, transferring material into their food web and making energy available to foragers (organisms that search for food) that is normally tied up in ocean-bottom, deep-ocean, midwater, and open-ocean species.

Overfishing and overdiscarding may thus contribute to a syndrome known as “fishing down of food webs,” whereby we eliminate apex (top) predators and large species while transforming the ocean into a simplified system increasingly dominated by microbes, jellyfish, ocean-bottom invertebrates, plankton, and planktivores. The strongest evidence for the fishing down phenomenon exists in global catch statistics that show alarming shifts in species composition from high-value, near-bottom species to lower-value, open-ocean species. In the last three decades of the twentieth century, the global fishing fleet doubled in size and technology advanced immeasurably. Despite increased effort and technology, total catch stabilized, but landing rates (rates at which species are caught) of the most valuable species fell by 25 percent.

Conservation organizations have condemned the obvious and extreme waste associated with bycatch. Public concern over high mortality rates of endangered marine turtles captured in shrimp trawls led to the development of turtle exclusion devices (TEDs) in the 1980s. TEDs were incorporated into the shrimp net design with the purpose of directing turtles out of nets without unacceptably reducing shrimp catches. Marine engineers and fishers also developed shrimp net designs that incorporate bycatch reduction devices (BRDs), taking advantage of behavioral differences between shrimp and fish, or between different fishes, in order to separate fishes. 

1.A topic of increasing relevance to the conservation of marine life is bycatch-fish and other animals that are unintentionally caught in the process of fishing for a targeted population of fish. Bycatch is a common occurrence in longline fishing, which utilizes a long heavy fishing line with baited hooks placed at intervals, and in trawling, which utilizes a fishing net (trawl) that is dragged along the ocean floor or through the mid-ocean waters. Few fisheries employ gear that can catch one species to the exclusion of all others. Dolphins, whales, and turtles are frequently captured in nets set for tunas and billfishes, and seabirds and turtles are caught in longline sets. Because bycatch often goes unreported, it is difficult to accurately estimate its extent. Available data indicate that discarded biomass (organic matter from living things) amounts to 25-30 percent of official catch, or about 30 million metric tons.