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Gillnetting is a common fishing method used by commercial and artisanal fishermen of all the oceans and in some freshwater and estuary areas. “Gill nets are vertical panels of netting normally set in a straight line. Fish may be caught by gill nets in 3 ways : (1) wedged – held by the mesh around the body (2) gilled – held by mesh slipping behind the opercula, or (3) tangled – held by teeth, spines, maxillaries, or other protrusions without the body penetrating the mesh. Most often fish are gilled. A fish swims into a net and passes only part way through the mesh. When it struggles to free itself, the twine slips behind the gill cover and prevents escape.”〔Murphy, B. and Willis, D. (1996) ("Fisheries Techniques: Second edition." ) Bethesda, MD: American Fisheries Society.〕 Gillnets are so effective that their use is closely monitored and regulated by fisheries management and enforcement agencies. Mesh size, twine strength, as well as net length and depth are all closely regulated to reduce bycatch of non-target species. Gillnets have a high degree of size selectivity. Most salmon fisheries in particular have an extremely low incidence of catching non-target species.〔(Selective Fisheries ) ''Oregon Department of Fish and Wildlife''. Retrieved 26 September 2014.〕 ==History== Gillnets existed in ancient times as archaeological evidence from the Middle East demonstrates.〔Nun, Mendel (1989). ''The Sea of Galilee and Its Fishermen in the New Testament,'' pp. 28-44. Kibbutz Ein Gev, Kinnereth Sailing Co.〕 In North America, aboriginal fishermen used cedar canoes and natural fibre nets, e.g., made with nettles or the inner bark of cedar.〔Stewart, Hilary (1977). ''Indian Fishing: Early Methods on the Northwest Coast,'' p.79 in 1st paperback edition, 1982. Seattle, University of Washington Press.〕 They would attach stones to the bottom of the nets as weights, and pieces of wood to the top, to use as floats. This allowed the net to suspend straight up and down in the water. Each net would be suspended either from shore or between two boats. Native fishers in the Pacific Northwest, Canada, and Alaska still commonly use gillnets in their fisheries for salmon and steelhead. Both drift gillnets and setnets have long been used by cultures around the world. There is evidence of fisheries exploitation, including gillnetting, going far back in Japanese history, with many specific details available from the Edo period (1603–1868).〔Ruddle, Kenneth and Akimich, Tomoya. “Sea Tenure in Japan and the Southwestern Ryukyus,” in Cordell, John, Ed. (1989), ''A Sea of Small Boats'', pp. 337-370. Cambridge, Mass., Cultural Survival, Inc.〕 Fisheries in the Shetland Islands, which were settled by Norsemen during the Viking age, share cultural and technological similarities with Norwegian fisheries, including gillnet fisheries for herring.〔Goodlad, C.A. (1970). ''Shetland Fishing Saga'', pp. 59-60. The Shetland Times, Ltd.〕 Many of the Norwegian immigrant fishermen who came to fish in the great Columbia River salmon fishery during the second half of the 19th century did so because they had experience in the gillnet fishery for cod in the waters surrounding the Lofoten Islands of northern Norway.〔Martin, Irene (1994). ''Legacy and Testament: The Story of the Columbia River Gillnetter'', p. 38. Pullman, Washington State University Press.〕 Gillnets were used as part of the seasonal round by Swedish fishermen as well.〔Lofgen, Ovar. “Marine Ecotypes in Preindustrial Sweden: A Comparative Discussion of Swedish Peasant Fishermen,” in Andersen, Raoul, Ed. (1979), ''North Atlantic Maritime Cultures'', pp. 83-109. The Hague, Mouton.〕 Welsh and English fishermen gillnetted for Atlantic salmon in the rivers of Wales and England in coracles, using hand-made nets, for at least several centuries.〔Jenkins, J. Geraint (1974). ''Nets and Coracles'', p. 68. London, David and Charles.〕 These are but a few of the examples of historic gillnet fisheries around the world. Gillnetting was an early fishing technology in Colonial America, used for example, in fisheries for Atlantic salmon and shad.〔Netboy, Anthony (1973) ''The Salmon: Their Fight for Survival,'' pp. 181-182. Boston, Houghton Mifflin.〕 Immigrant fishermen from northern Europe and the Mediterranean brought a number of different adaptations of the technology from their respective homelands with them to the rapidly expanding salmon fisheries of the Columbia River from the 1860s onward.〔Martin, 1994, p. 44.〕 The boats used by these fisherman were typically around long and powered by oars. Many of these boats also had small sails and were called "row-sail" boats. At the beginning of the 1900s, steam powered ships would haul these smaller boats to their fishing grounds and retrieve them at the end of each day. However, at this time gas powered boats were beginning to make their appearance, and by the 1930s, the row-sail boat had virtually disappeared, except in Bristol Bay, Alaska, where motors were prohibited in the gillnet fishery by territorial law until 1951.〔Andrews, Ralph W. and Larsen, A.K. (1959). ''Fish and Ships'', p. 108. Seattle, Superior Publishing Co.〕 In 1931, the first powered drum was created by Laurie Jarelainen. The drum is a circular device that is set to the side of the boat and draws in the nets. The powered drum allowed the nets to be drawn in much faster and along with the faster gas powered boats, fisherman were able to fish in areas they had previously been unable to go into, thereby revolutionizing the fishing industry. During World War II, navigation and communication devices, as well as many other forms of maritime equipment (ex. depth-sounding and radar) were improved and made more compact. These devices became much more accessible to the average fisherman, thus making their range and mobility increasingly larger. It also served to make the industry much more competitive, as the fisherman were forced to invest more in boats and equipment to stay current with developing technology. The introduction of fine synthetic fibres such as nylon in the construction of fishing gear during the 1960s marked an expansion in the commercial use of gillnets. The new materials were cheaper and easier to handle, lasted longer and required less maintenance than natural fibres. In addition, multifilament nylon, monofilament or multimonofilament fibres become almost invisible in water, so nets made with synthetic twines generally caught greater numbers of fish than natural fibre nets used in comparable situations. Nylon is highly resistant to abrasion and degradation, hence the netting has the potential to last for many years if it is not recovered. This ghost fishing is of environmental concern. Attaching the gillnet floats with biodegradable material can reduce the problem.〔 However it is difficult to generalize about the longevity of ghost-fishing gillnets due to the varying environments in which they are used. Some researchers have found gill-nets still catching fish and crustaceans over a year after loss, while others have found lost nets destroyed by wave action within one month or overgrown with seaweeds, increasing their visibility and reducing their catching potential to such an extent that they became a microhabitat used by small fish. This type of net was heavily used by many Japanese, South Korean, and Taiwanese fishing fleets on the high seas in the 1980s to target tunas. Although highly selective with respect to size class of animals captured, gillnets are associated with high numbers of incidental captures of cetaceans (whales and dolphins). In the Sri Lankan gillnet fishery, one dolphin is caught for every 1.7-4.0 tonnes of tuna landed. This compares poorly with the rate of one dolphin per 70 tonnes of tuna landed in the eastern Pacific purse seine tuna fishery. Gillnets are sometimes used by fisheries scientists to monitor fish populations.〔Bruce Crawford. "Variable Mesh Gill Nets (in Lakes)." In Johnson, David, et al. ''Salmonid Field Protocols Handbook''. Bethesda, Maryland, American Fisheries Society, 2007, pp. 425-433.〕 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「gillnetting」の詳細全文を読む スポンサード リンク
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