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Discards
Discards are the portion of a catch of fish which is not retained on board during commercial fishing operations and is returned, often dead or dying, to the sea. The practice of discarding is driven by economic and political factors; fish which are discarded are often unmarketable species, individuals which are below minimum landing sizes and catches of species which fishermen are not allowed to land, for instance due to quota restrictions. Discards form part of the bycatch of a fishing operation, although bycatch includes marketable species caught unintentionally. Discarding can be highly variable in time and space as a consequence of changing economic, sociological, environmental and biological factors.〔name=Catchpole>〕
Discarding patterns are influenced by catch compositions, which in turn are determined by environmental factors, such as recruitment of small fish into the fishery, and social factors, such as quota regulation, choice of fishing gear and fishermen's behaviour. There have been numerous studies on the scale of discarding. In the North Sea the total annual quantity of discards has been estimated at 800,000–950,000 tonnes, or the equivalent of one-third of the total weight landed annually and one-tenth of the estimated total biomass of fish in the North Sea.
== Impacts ==
Discarding impacts on the environment in two ways; firstly, through increased mortality to target and non-target species, particularly at juvenile life-history stages, and secondly, through alteration of food webs by supplying increased levels of food to scavenging organisms on the sea floor, and to sea birds. The survival of discarded fish and invertebrates is variable and depends on species and fishing gear used. For example, species such as the Pacific halibut caught in long-line fisheries in the Bering Sea, or lesser spotted dogfish in beam trawls in the English Channel have survival rates of between 88-98%. Survival rates of roundfish discards are significantly lower, and this mortality is included in most International Council for the Exploration of the Sea (ICES) stock assessments. Crustaceans are thought to be more hardy, though survival of crustaceans is variable;〔 for instance, a survival rate of 25% is assumed for ''Nephrops norvegicus'', the Norway Lobster, whilst survival rates of almost 100% have been demonstrated for the hermit crab, ''Pagurus bernhardus''.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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