漁獲努力量制限の効果について
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概要
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When a fish population has been depleted by a heavy exploitation with the yield from the population maintaining an unfavourable level, one likely expects that the situation may be improved by reduction of fishing effort. Upon a sudden reduction of fishing mortality, p, from p1 to p2 at time ?? =0, the yield once decreases and then increases gradually until it reaches another steady level higher than the former level. The present paper deals, along the course of BARANOVs method, with an transition stage of the population following the sudden change of p, as well as a steady state before and long after the change. Relations between the steady yield and the steady fishing mortality (effort-yield curves) are calculated for various values of parameters, λ0 (=l0/u, where l0 is the length of recruit and u is yearly increase of the length), q (natural mortality rate) and b (remainder of life of fish at the time of recruitment) (Fig. 2). It is noteworthy that in case of a species growing so slowly after the recruitment, then λ0 is large, the fishing restriction could sc ?? ?? cely be effective on the yield (Fig. 4). Unsteady yield curve is calculated for the case in which λ0=4, q=0.15, b=10 and p1=1.35, representing the parameters in the present state of “sohachi-garei” Cleisthenes herzensteini (SCHMIDT) in the south-western area of the Japan Sea (Fig. 5). Density effects on the growth and the natural mortality, which are briefly discussed, appear to diminish the practicability of the restriction considerably (Fig. 6).