Dolphin killed by pair-trawling. Thousands of porpoises and dolphins die every year as accidental bycatch.

Bycatch

Many fisheries catch fish other than the ones that they target and in many cases these are simply thrown dead or dying back into the sea. In some trawl fisheries for shrimp, the discard may be 90 percent of the catch. Other fisheries kill seabirds, turtles and dolphins, sometimes in huge numbers.

Estimates vary as to how serious a problem bycatch is. Latest reports suggest that around eight percent of the total global catch is discarded, but previous estimates indicated that around a quarter of might be thrown overboard. Simply no-one knows how much of a problem this really is.

The incidental capture, or bycatch, of mammals, sea-birds, turtles, sharks and numerous other species is recognised to be a major problem in many parts of the world. This figure includes non-target species as well as targeted fish species that cannot be landed because they are, for instance, undersized. In short, anywhere between 6.8 million and 27 million tonnes of fish could be being discarded each year, reflecting the huge uncertainties in the data on this important issue.

The scale of this mortality is such that bycatch in some fisheries may affect the structure and function of marine systems at the population, community and ecosystem levels. Bycatch is widely recognised as one ofthe most serious environmental impacts of modern commercial fisheries.

The victims

Different types of fishing practices result in different animal/species being killed as bycatch: nets kill dolphins, porpoises and whales, longline fishing kills birds, and bottom trawling devastates marine ecosystems.

It has been estimated that a staggering 100 million sharks andrays are caught and discarded each year. Tuna fisheries, which in thepast had high dolphin bycatch levels, are still responsible for the death of many  sharks. An estimated 300,000 cetaceans (whales, dolphins and porpoises) also die as bycatch each year, because they are unable to escape when caught in nets.

Birds dive for the bait planted on long fishing lines, swallow it (hook included) and are pulled underwater and drowned. Around 100,000 albatrosses are killed by longline fisheries every year and because of this, many species are facing extinction.

Bottom trawling is a destructive way of 'strip-mining' the ocean floor, harvesting the species that live there. As well as the target fish species, this also results in bycatch of commercially unattractive animals like starfish and sponges. A single pass of a trawl removes up to 20 percent of the seafloor fauna and flora. The fisheries with the highest levels of bycatch are shrimp fisheries: over 80 percent of a catch may consist of  marine species other than the shrimp being targeted.

Technology

Many technical fixes exist to reduce bycatch. Turtle exclusion devices are used in some shrimp fisheries to avoid killing turtle species. In the case of longline fisheries, the process of setting the hooks can be changed and bird-scaring devices employed which radically cut the numbers of birds killed. To avoid dolphins being caught in nets other devices can be used. Pingers are small sound-emitting and dolphin-deterring devices that are attached to nets, but they are not always effective. Escape hatches (consisting of a widely spaced metalgrid, which force the cetacean up and out of the net) have also been used.

Although these devices may have a role to play, they cannot address the whole problem. Such devices need continual monitoring to check how well they work and assess any potential negative effects they may have. Realistically they will probably only be used in areas with well-developed fishery management and enforcement agencies.

On a global level, probably the only effective way to address the problems of bycatch is to control fishing effort. This will be best achieved through the creation of marine reserves. Nonetheless, in the case of highly mobile species such as seabirds and cetaceans, the only effective way of preventing bycatch is to discontinue the use of particularly damaging fishing methods.

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