ブリ太平洋系群成魚の長期減少傾向について
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概要
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The abundance of adult yellowtail Seriola quinqueradiata TEMMINCK et SCHLEGEL in its pacific sub-population has been decreasig sice 1956 until now. In order to grasp the char-acteristics of fluctuations of catches on a long-term basis and to estimate the level of catches in future, the long-term catch records after 1897 by large type set nets in the sea off three prefectures, Kanagawa, Mie and Krichi which are major fishing grounds for adult yellowtail in the Pacific coast of Japan have been investigated. The results are summarized as follows; 1) There are two types of long-term fluctuations of the stock. During 1897-1955, it is the pertod of stable high-level stock. During 1956-1984, it is the other period of long-term decreasing stock. 2) On the long-term fluctuations, there are short-term fluctuations of a cycle of 9-10years. 3) It is estimated that the present stock level being on the decrease by yearly ratio of 0.918, has decreased by 14% in comparison with the level at 1956. Also present levels of the catches have decreased 6% at Kanagawa, 21% at Mie and 16% at Kôchi in com-parison with each levels in 1956, due to variation of space by oceanographic conditions. 4) Main factors of fluctuations of the stock in the period of stable high-level stock were divided into two natural factors, such as variabilities of oceanographic conditions and natural fluctuations of stock. But the main factor of long-term decreasing stock was estimated only as increased fishing intensity for immature yellowtail.
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