Using osmotic pumps to deliver hormones to induce sexual maturation of female Japanese eels, Anguilla japonica

Abstract

This study examines how the continuous administration of various hormones using an osmotic pump, a long-term sustained hormone release system, influences the induction of sexual maturation in females of the Japanese eel, Anguilla japonica. Human chorionic gonadotropin (hCG), salmon pituitary extract (SPE), and gonadotropin-releasing hormone analogue (GnRHa) were tested, and the consequent egg quality was evaluated. The implantation of osmotic pumps loaded with SPE induced vitellogenesis and increased the gonadosomatic index (GSI) at 39–110days. In comparison, pumps loaded with hCG inconsistently induced early vitellogenesis, while those loaded with GnRHa did not exhibit any stimulatory effect. Fewer eels attained full maturity when using the osmotic pump system compared with SPE-injected female eels. However, after the final treatment, the number of eels that ovulated and the time required for ovulation were similar for both the osmotic pump and injection groups. Moreover, more eggs were spawned in the SPE-loaded osmotic pump group than in the SPE-injected group. Egg quality was similar for both experimental groups. Therefore, the implantation of an osmotic pump loaded with SPE represents a reliable method for inducing vitellogenesis and obtaining ovulated eggs from sexually immature Japanese eels.