Ishikawa Prefectural Fisheries Research Center, Noto, Ishikawa 927-0435, Japan
The age of sex change from male to female is estimated by two methods: One is a theoretical calculation based on the results obtained so far, or a practical observation of samples caught by a research vessel. The obtained results are as follows. The optimum age of sex change was calculated to be around 5 years old. The actual age of sex change in the waters off Noto Peninsula were 4, 5, and 6 years old. According to the year class, the first age of sex change varied annually with 4 or 5 years old. Namely, in copulation and spawning areas, the size of sex change tended to be small as the mean carapace length of the breeding population was small. The breeding population consisting of small individuals was most conspicuous when the number of females was fewer than that of males due to the sex change from male to female. So in this species, the size of sex change may vary owing to the population structure influenced by fishing pressure or recruitment of year class strength. At that time the small size of sex change is supposed to make the age of sex change lower.