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-II

Abstract

As to the consideration about the rationality of offshore fishing for chum salmon, it has been tried in the previous paper (1) of this report on the basis of the estimation of the instantaneous rate of natural mortality. Also in red salmon, since the same kinds of data have been given, the same estimation and consideration for red salmon were tried with the same purpose and method. Adopting the symbols as bellow equally in the case of chum salmon, 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 (Number of day). R0[1-e-(F0+F1)t0]F1/F0+F1e-(M0+M1)(1/2t0+t1)=0.15R…………(1) R0e- ?? M0(1/2t0+t1)+(F0+F1)t0 ?? =0.85……………………………(2) e-(F0+F1)t0=0.395…………………………………………(3) 1-e-(F0+F1)t0=C(F0+F1)/R0F0…………………………………(4) M0+M1=0.0374…………………………………………(5) In above formulas next data were used. 1) Numbers of net-marked fishes have occupied 15% of R. 2) Assuming R1 to be the size of mature salmon run in the coastal areas when the offshore fishing does not exist, R/R1=e-(F0+F1)t0/0.85=0.465 as shown in the appendix (1). 3) R and C are 1, 920, 000 and 6, 600, 000 respectively as shown in appendix (2) and (3). 4) The value of M0+M1 is 0.0374. This was calculated with the results of tagging experiment shown in appendix (4).