PRETREATMENT REPRODUCTIVE STAGE AND OOCYTE DEVELOPMENT INDUCED BY SALMON PITUITARY HOMOGENATE IN THE JAPANESE EEL ANGUILLA-JAPONICA

Induction of the ovarian development was investigated in three groups of eels with respect to the importance of the pretreatment reproductiv e stage. Initially, the developmental stages of oocytes varied widely among these groups; silver eels had the most advanced oocytes which we re in the early stage of vitellogenic growth, whereas oocytes from fem inized eels were in the oil droplet or early vitellogenic stage and th ose from cultivated eels were in the oil droplet stage. Eels received weekly intramuscular injections of salmon pituitary homogenate (SPH) a t 20 mu g per g body weight (low-dose treatment). The ovarian follicle s of silver eels developed the fastest and reached the final maturatio nal stage after, on average, 10 weeks of treatment. The oocytes of fem inized eels and cultivated eels developed more slowly, requiring appro ximately 17 and 19 weeks of treatment, respectively. All silver eels w ere induced to the final maturational phase (100%), but lower ratios w ere seen for feminized and cultivated eels (64 and 29%, respectively). Feminized eels were also subjected to a high-dose treatment of SPH (2 0 mu g per g body weight). High-dose treated feminized eels were induc ed to the final maturational phase in less time (11 weeks) and with gr eater efficiency (71%) than low dose-treated feminized eels. It was sh own that eels whose follicle diameter was over about 180 mu m could be stimulated to develop through the final maturational phase. This resu lt indicates that the majority of feminized eels may be expected to re ach the final maturational phase after SPH treatment. However, a 100% rate to reach the final maturational phase could not be attained, poss ibly due to immunoreaction to SPH, the stress of injections, or other yet unidentified factors. We conclude that silver eels are the best ca ndidates for artificial maturation. Even though feminized eels were no t the most responsive group, their abundance makes them an important r esource. Moreover, we recommend that eels should be induced to the fin al maturational phase during a relatively short period of SPH treatmen t (about two to three months) for artificial maturation to be successf ul.