熊野灘におけるイサキの性比と生残率

抄録

The present report deals with the fishery biology of Parapristipoma trilineatum, Poma-dasyidae, with particular reference to sex ratio and survival rate, on 1242 individuals taken from coastal areas of Kumano-nada (Kii Peninsula, southern Honshu, Japan) from March 1978 to August 1979.
Monthly changes in the sex ratio showed that both sexes are equal in number or males slightly dominate over females all year round except for September.We presume that the predominance of females in September results from increased catches from hand line fish-ing in September when females feed more voraciously than males to compensate for the energy expenditure of spawning.It seems probable that the sex ratio in the population does not change throughout the year.There is little difference in the sex ratio of males to females in 0-3-year age groups, whereas in 4-8-year age groups the ratio evidently decreases.The relationships between the age (x) and the sex ratio (s= _??_/_??_) were expressed by the following equations: for 0-3-year age groups, s=1.21;for 4-8-year age groups, log s= -0.112x+0.423.From the regression of age composition, the survival rates (S) were estimated as follows: for females of 1 3-year age groups and males of 1-4-year age groups, S=0.774;for females of 4-8-year age groups, S=0.490;for males of the same age groups, S=0.387.Changes in the survival rate at 3- or 4-age groups in both sexes are likely to be attributable to the increase of both fishing and natural mortality.