東北・北海道沖の南下サンマの資源動態について-III

大型・中型等の再生産関係

抄録

1) The reproductive relations of the large and medium sized fish were examined to investigate a general tendency of population and influence of catch, using total catch in number N₁ and number of the immigrants N₂ by size category and year given in the previous paper.
2) Relatively independent two cycles (even year cycle and odd year cycle) are considered to exist, as N₂ varies every two years. E-R relations by size category and cycle were examined on the basis of the widely recognized subpopulation theory. Here escapement E and return R are represented by the adult spawner (N₂-N₁) and the recruitment (N₂), respectively.
3) Proportional relation between E and R is not significant for each size category and cycle. Reproduction curve of RICKER'S type did not fit significantly to the large sized fish. For the medium sized fish, this application was significant and dependency of R/E upon density was shown.
4) Population for each size category and cycle is reduced by failure of reproduction. In the reproductive aspect, no positive evidence has been given that population is reduced by overfishing. Fluctuations in population are considered to be seriously affected by the reproductive success or failure.