It it often said that the fishing grounds have been laid waste all over the Inland Sea of Japan because of overfishing …… especially in the situation of Porgy
Pagrosomus major ( T. & S.) fishery which is one of the most important in her waters. The writer attempted to argue and analyse the Porgy population, from the past records of catch and the fishing vessels with reference to the survival rates measured in 1937 by Y. WANG and in 1949 by the writer and his assistants, so as to make the nature of its population clear.
The trends of yearly yield in each prefecture facing the Inland Sea, are claseified into two types (Fig. 1) by analysing the secular variations, in regarding them as vectors and time series.
The survival rates are estimated to decrease from 0.82 in 1936 to 0.64 in 1949 (Tables 4, 6 and Fig. 8) for about ten years. It seems that the decrease has been. caused by strengthening fishing effort, represented in the number of fishing vessels.
In Tables 7 and 8, are shown the rates of natural mortality, fishing mortality, survival and exploitation of three prefectures respectively, calculated from competitive relation between the fishing mortality rates and the fishing efforts.
The thick full lines, as shown in Figs. 11, 12 and 13, represent the actual fluctuations of catch. The other four lines indicate the relative variato ?? of reconstructed catch, computed from the estimated values of the above-described population parameters under four trials. Considering the lines of recontructed catch and the actual one, the writer concluded that the number of 5-10 tons vessels is reasonable to show the fishing efforts.
Because the catch has not grown larger in spite of the annual increase of exploitation rate, the abundance of population is considered to be reduced by half for about past ten years : evidently it is a dangerous state of overfishing. Thus the optimalized fisheries or the regulation of fishing are inferred in Tables 9, 10, 11, and 12, concluded from
C-f diagram (Figs. 14 and 15).
The prediction of catch has been able to practise in three ways: namely ……the correlation (Tables 13, 14 and 15) between the catches and the environmental factors, the reconstruction (Figs. 11, 12 and 13) from dynamical relation among the population parameters, and the extrapolation (Figs. 16 and 17) d ?? d ?? ced statistically from the correlograms (Figs. 6 and 7) of the secular variations of catch.
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