1998 Volume 64 Issue 4 Pages 531-537
Induction of the ovarian development was investigated in three groups of eels with respect to the importance of the pretreatment reproductive stage. Initially, the developmental stages of oocytes varied widely among these groups; silver eels had the most advanced oocytes which were in the early stage of vitellogenic growth, whereas oocytes from feminized eels were in the oil droplet or early vitellogenic stage and those from cultivated eels were in the oil droplet stage.
Eels received weekly intramuscular injections of salmon pituitary homogenate (SPH) at 20μg per g body weight (low-dose treatment). The ovarian follicles of silver eels developed the fastest and reached the final maturational stage after, on average, 10 weeks of treatment. The oocytes of feminized eels and cultivated eels developed more slowly, requiring approximetely 17 and 19 weeks of treatment, respectively. All silver eels were induced to the final maturational phase (100%), but lower ratios were seen for feminized and cultivated eels (64 and 29%, respectively).
Feminized eels were also subjected to a high-dose treatment of SPH (40μg per g body weight). High-dose treated feminized eels were induced to the final maturational phase in less time (11 weeks) and with greater efficiency (71 %) than low dose-treated feminized eels. It was shown that eels whose follicle diameter was over about 180μm could be stimulated to develop through the final maturational phase. This result indicates that the majority of feminized eels may be expected to reach the final maturational phase after SPH treatment. However, a 100% rate to reach the final maturational phase could not be attained, possibly due to immunoreaction to SPH, the stress of injections, or other yet unidentified factors.
We conclude that silver eels are the best candidates for artificial maturation. Even though feminized eels were not the most responsive group, their abundance makes them an important resource.Moreover, we recommend that eels should be induced to the final maturational phase during a relatively short period of SPH treatment (about two to three months) for artificial maturation to be successful.
