Effects of continuous administration of human chorionic gonadotropin, salmon pituitary extract, and gonadotropin-releasing hormone using osmotic pumps on induction of sexual maturation in male Japanese eel, Anguilla japonica

The purpose of this study was to examine the effects of continuous administration of human chorionic gonadotropin (HCG), salmon pituitary extract (SPE), and gonadotropin-releasing hormone analogue (GnRHa) via an osmotic pump with a long-term sustained hormone-release system on the induction of sexual maturation in male Japanese eels (Anguilla japonica). Implantation of a single HCG-loaded osmotic pump induced spermatogenesis and increased the gonadosomatic index (GSI) values at 35–42 days postimplantation; GSI values were higher (10.7 ± 1.0) than those of the fish implanted with a cholesterol pellet containing HCG (2.4 ± 0.6). Implantation of an SPE-loaded osmotic pump also stimulated spermatogenesis, although the GSI values were lower than those of the fish implanted with the HCG-loaded osmotic pump. Implantation of a single osmotic pump loaded with various doses of GnRHa (0.94, 1.86, or 3.75 µg/day) did not stimulate spermatogenesis. The GSI values and milt weight of the HCG-administered eels significantly increased at a dosage of 5 IU/day in a dose-dependent manner up to 50 IU/day. Sperm motility was not significantly different among the eels treated with different doses of HCG. Therefore, implantation of a single osmotic pump loaded with HCG (50 IU/day) is a reliable method for inducing spermatogenesis and spermiation in sexually immature Japanese eels.