Abstract
The ovarian morphology, annual reproductive cycle and spawning characteristics fthe kichiji rockfish Sebastolobus macrochir collected from the Pacific coast of southeasternHokkaido, was examined on the basis of histological observation. The ovaries of this fish are categorized as cystovarian type. The ovarian parenchyma, stroma, and blood vessels run longitudinally through the center of each ovary, suggesting a specialized structure for formation of gelatinous egg masses.
The female kichiji rockfish begins vitellogenesis from August to October, and continues yolk accumulation until March. Post-ovulatory follicles or oocytes at final maturation stage were observed in March and April, suggesting that spawning was carried out mainly in this period.During the spawning season, there were two clutches of developing oocyte, or post-ovulatory follicleand one clutch of oocyte in the ovary. Therefore, females seem to spawn twice within a single spawning season. Fish which finished spawning rapidly form yolk vesicles for the next reproduction, and maintain a resting condition for several months.
Gonad somatic index (GSI) corresponds clearly to ovarian maturity, and could be divided into immature (GSI<2), spent (2<GSI<3), and vitellogenic or maturation (GSI>3) during the spawning season. This indicates that ovarian maturity can be evaluated on the basis of GSI. By this method, the minimum maturation size of the female kichiji was estimated to be 151 to 175mm in standard legth.
