Reduction of female proportion in lower growing fish separated from normal and feminized seedlings of hirame Paralichthys olivaceus

Each group of normal males and sex-reversed gynogenetic females (phenotypical male) of hirame Paralichthys olivaceus were crossed with normal females, and the obtained larvae and juveniles were reared at two different water temperatures of 18 degree C and 23-25 degree C (only normal generation) during the sex determination period. When lower growing fish (about 20 mm in total length) appeared in their original populations, these were separated and reared in other tanks. The populations reared at 18 degree C without growth retention indicated the primary genetic sex ratio, while the populations reared at high temperatures showed clearly lower proportions of females. The lower growing fish showed lower proportions than those in the original group, at both water temperatures. These results show that the sex ratio of a pond population is greatly affected not only by rearing water temperature, but also by the existence of lower growing fish.