Annual Reproductive Cycle of the Honmoroko Gnathopogon elongatus caerulescens

Abstract

Honmoroko Gnathopogon elongatus caerulescens were captured from Lake Biwa in October and reared under natural conditions for up to 18 months. Fish were sampled every one or two months and seasonal changes of gonadsomatic indices (GSI), plasma levels and pituitary contentof gonadotropin (GtH) of both sexes, plasma levels of estradiol-17β (E₂) of females and plasma levels of testosterone of males were examined. Histological changes of gonads were also examined, Gametogenesis of honmoroko started in October and spawning was observed from mid Aprilto late July. GSI gradually increased from October and reached a maximum in April. Changes in the plasma steroid hormones correlated well with those of GSI. These steroids are considered to be involved in vitellogenesis and spermatogenesis in this subspecies. Plasma GtH levels in both sexes showed correlation with water temperature being elevated at high water temperatures. Pituitary GtH content was high in winter, and inverse correlation between pituitary content and plasma levels of GtH was indicated.