サケ成魚 (<i>Oncorhynchus</i> sp.) の自然死亡係数を推定する一つの試み,ならびにこれに基づく沖取漁業の合理性の検討-I

Abstract

In order to consider the gillent fisheries operated in the North Pacific from the view point of full utilization of salmon resources, it is necessary to estimate the difference between the increase in weight by the growth of salmon and the decrease in weight by the natural death in the term of migration from offshore to coast. As to the growth rate, it has been shown in the previous paper, thence an estimate of the instantaneous rate of natural mortality in chum salmon was tried in this paper. And the consideration for the rationality of offshore fishing for chum salmon has been given on the basis of above estimate. Adopting the symbols as below, the next formulas are given. R0……Size of mature salmon run in number of fish in offshore waters. R……Size of mature salmon run in number of fish in coastal areas. C……Catch of offshore fishing. C1……Catch of coastal fishing. M0……Instantaneous rate of natural mortality. M1……Instantaneous rate of mortality by net mark. F0……Instantaneous rate of fishing mortality. F1……Instantaneous rate of occurrence of net mark. t0……Term of offshore fishing (Number of day) t1……Term of migration from offshore to coast. R0e-(M0+M1)(1/2t0+t1)[1-e-(F0+F1)t0]F1/F0+F1=0.15R…………(1) 0.85R=R0e- ?? M0(1/2t0+t1)+(F0+F1)t0 ?? ……………………………(2) e-(F0+F1)t0=0.55…………………………………………(3) 1-e-(F0+F1)t0=C(F0+F1)/R0F0…………………………………(4) M0+M1=0.0197…………………………………………(5) R=C1/ƒ…………………………………………………(6) In above formulas next data were used. 1) Numbers of net marked fishes have occupied 15% of R. 2) The value of M0+M1 was given by the results of tagging experiments in which the fishes used for tagging were caught by gillnet. 3) Assuming that R1 is the size of mature salmon run in the coastal areas when offshore fishing has not exist, the size of R1 in the case of which the offshore fishing has exist is estimated as shown in Table 3 by the repreduction curve which drawn by the data obtained during 1940 to 1952. As the results of calculation, the value of R/R1 was estimated at 0.65. Applying t0=t1=50, and ƒ=0.5 or 0.6, the value of M0•M1•F0,