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Cetacean bycatch is the incidental capture of non-target cetacean species such as dolphins, porpoises, and whales by fisheries.〔Alverson et al. (1994)〕 Bycatch can be caused by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets. Cetacean bycatch is increasing in intensity and frequency.〔Demaster et al. (2001)〕 This is a trend that is likely to continue because of increasing human population growth and demand for marine food sources, as well as industrialization of fisheries which are expanding into new areas. These fisheries come into direct and indirect contact with cetaceans. An example of direct contact is the physical contact of cetaceans with fishing nets. Indirect contact is through marine trophic pathways where fisheries are severely reducing fish stocks that cetaceans rely on for food. In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait.〔Read et al. (2006)〕 In this fashion, cetaceans can become a target of fisheries. ==Bycatch trends== Generally cetacean bycatch is on the increase. Most of the world’s cetacean bycatch occurs in gillnet fisheries.〔 The mean annual bycatch in the U.S. alone from 1990–1999 was 6,215 marine mammals, with dolphins and porpoises being the primary cetaceans caught in gillnets.〔 A study by Read et al.〔 estimated global bycatch through observation of U.S. fisheries and came to the conclusion that an annual estimate of 653,365 marine mammals, comprising 307,753 cetaceans and 345,611 pinnipeds were caught from 1990–1994. While gillnets are a principal concern, other types of nets also pose a problem: trawl nets, purse seines, beach seines, longline gear, and driftnets. Driftnets are known for high rates of bycatch and they affect all cetaceans and other marine species.〔Silvani et al. (1999)〕 They are fatal for small toothed whales (''Odontocetes'') and sperm whales, as well as other marine mammals and fish such as sharks, sea birds and sea turtles. Many fisheries routinely use driftnets exceeding the EU size limit of 2.5 km/boat.〔Miragliuolo et al. (2002)〕 This illegal drift-netting is a major issue, especially in important feeding and breeding grounds for cetaceans. However, the tuna industry has achieved successes in reversing cetacean bycatch trends. International recognition of the problem of cetacean bycatch in tuna fishing led to the Agreement on the International Dolphin Conservation Program in 1999 and overall there has been a dramatic reduction in death rates.〔http://www.nmfs.noaa.gov/ia/intlagree/aidcp.htm〕 In particular, dolphin bycatch in tuna fishing in the East Tropical Pacific has dropped from 500,000 per year in 1970 to 100,000 per year in 1990 to 3,000 per year in 1999 to 1,000 per year in 2006.〔(NOAA Fisheries Service - The Tuna-Dolphin Issue )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cetacean bycatch」の詳細全文を読む スポンサード リンク
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