東北・北海道沖の南下サンマの資源動態について-II : 漁場移動を利用した漁具能率qの推定

概要

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Incorporating the effect of catch, immigration, emigration and stay of fish shoals into the abundance index in number of the saury population by size category and fishing ground, a simple dynamics model was developed. From this model, a linear relation is obtained between the rate of immigration or emigration and the catchability coefficient q. Possible minimum rate is estimated from the shift of the fishing ground, assuming that the fish shoals move only in the south-west direction. Possible ranges of q are obtained from these minimum rates. It was found that q=0.00028 meets all 24 requirements except one, and hence was adopted as the estimate of q. From various statistics and the value of q, the following quantities were calculated for every year and size category: total catch in number, total available population in number, number of the immigrants, rate of exploitation, mean of effective overall fishing intensity index in a 10-day period and mean of the number of 10-day periods of stay. The yearly variation of the dynamics of the saury population was examined. The poor catch in recent years seems to be caused, firstly by the decrease of fishing intensity, and secondly, by the decline of the abundance of the population. Reduced fishing intensity suggests that the cause of the population decline is not due to overfishing.
公益社団法人日本水産学会の論文