Spatio-temporal patterns of parturition of the black rockfish Sebastes inermis in Sendai Bay, northern Japan

The reproductive performance of 941 female specimens of black rockfish Sebastes inermis collected from four sites (from north to south, Ayukawa, Shichigahama, Yuriage and Haragama) along Sendai Bay, northern Japan from October 2000 to March 2001, was analyzed. Mature female S. inermis ranged from 2 to 9 years in age, from 15 to 29 cm in total length and from 80 to 496 g in total weight. Mean gonadosomatic index ranged from 0.42 to 14.6, showing a similar tendency in all locations, increasing from October, peaking around early December, and decreasing thereafter. The same pattern was observed in ovary weight, ranging from 0.72 to 33.74 g. Seasonal fluctuation in the percentage of occurrence of four development stages of ovaries (ripe, fertilized, eyed and spent) indicated that parturition season extended from late December to early March in Sendai Bay. However, parturition shifted on both a temporal and spatial scale, that is, the extrusion began and finished later, the more northerly was the site. There were significant differences in the age of spawners as extrusion season progressed, with older female S. inermis (approx. 5 years old) occurring at the beginning and younger ones (approx. 2 years old) occurring at the end of the season. A similar tendency was also observed in size, weight and ovary weight in most ovarian developmental stages.