Annual Variations of Gonadotropin Content and Ovarian Development of Feral Female Catfish, Silurus asotus, in Central China

The Japanese catfish Silurus asotus is widely distributed throughout the freshwaters reservoirs, lakes and rivers of China, Korea and Japan, and is a commercially valuable aquaculture fish in some regions of China and Japan. We studied seasonal variations of gonadotropin (GtH) content of the plasma and pituitary as well as ovarian development. Plasma GtH titres increase in April in the Pearl River and in May in the Liao River in central China. Annual cycles of plasma GtH levels in both rivers are the same, with a gradual decrease after ovulation with the lowest values observed in September (in the Liao River) or October (in the Pearl River). Plasma GtH levels increase gradually in the early stages of ovarian development, with the highest values observed during the late stages of development, and with the lowest GtH titres observed during ovarian regression in both rivers. GtH content of the pituitary remained very high through the spawning period, with the highest titres observed in March in the Liao River, and in July in the Pearl River. As with plasma GtH, the lowest concentrations of pituitary GtH were observed during stages when the ovary was regressed. We analyzed the relationships between plasma and pituitary GtH and reproductive conditions, such as gonadosomatic index (GSI), hepatosomatic index (HSI), oocyte diameter (OD) and oocyte weight (OW). The results indicate that annual changes of GtH content of the plasma and pituitary paralleled GSI, OD and OW, but were negatively correlated with HSI in feral female catfish.