日本海西部の流刺網漁業の対象となるマイワシ資源の変動の解析と機構について
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概要
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In determining a population size in the initial period of fishing or the size of recruit, the availability will be considered as a factor playing a greater role than mortality. Unless we take the availability into consideration in our treatment, therefore, it would be sometimes difficult for us to explain the fluctuations in fish catch by combination of any other factors. In this report, we, comparing the variations in actual catches and in theoretical catches calculated from the assumed population models, have proved that the availability can be computed with the aid of an analogue computer designed for analysing population problems. In order to postulate the mathematical models of a population, the problems concerning the sardine, Sardinops melanosticta, exploited by the gill nets in the west Japan Sea, have been treated as examples, because the data such as fishing effort and age composition by gear are better available for the catch in this region than in other parts of our country. Indicated in Fig. 1 are the comparisons between the actual catches and theoretical ones that are assumed in equations (1) to (9) for various combinations of gear efficiency and natural mortality; in Fig. 3 are the comparisons for another combination assumed after the previous treatment in equation (10). Except for 1953 in Fig. 3, the patterns of theoretical catches in these figures do not fit to the actual ones in any combination of gear efficiency and natural mortality. Therefore, we have to compute the availability by assuming the most probable values of efficiency of gear (1/100, 000) and coefficient of natural mortality (0.5). In Table 1 showing the results obtained, the column A indicates the availability compared with the preceding year, and the column B series of relative availability compared on the basis of 1951. The reasons making the availability vary from year to year and the questions concerning the structures of mechanisms by which the conditions of the initial abundance of a population or recruit will be established are important problems to be solved in the future.
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