Abstract
Gonad and blood samples were taken from the cultured female Korean spotted sea bass Lateolabrax maculatus from August to November in 1998 and 1999. Under net cage culture, the female fish did not mature and spawn, but spermiation in males occurred spontaneously in the natural spawning season. Gonadosomatic index (GSI) began to increase in September and reached the highest value (10.1±1.8) in mid November, and then decreased sharply in late November. The cultured fish had been in the tertiary yolk stage oocyte longer then the wild fish. After the breeding period, the cultured fish still had tertiary yolk stage oocytes in mid November. Changes in serum levels of gonadal steroids were correlated with ovarian development. Serum estradiol-17β (E 2) level increased in September, reached their highest levels in October and early November (394.6±75.3 and 380.2±96.6 pg/ml, respectively), and then decreased in mid November and in late November ( P<0.05). Serum 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) maintained low levels (38.5±5.5-48.8±8.9 pg/ml) and was not significantly changed during the experimental period ( P>0.05). The effects of several hormones on the maturation of follicle-enclosed oocytes were examined in vitro. 17,20β-P and human chorionic gonadotropin (hCG) were found to have high potency to induce maturation. When cultured fish with fully grown oocytes in the ovary were injected with hCG at a dose of 1000 or 2000 IU/kg, all the fish matured and ovulated. However, the number of eggs per fish was not sufficient for mass seedling production. Considering all these results, we could infer that the ovarian oocytes of the cultured female Korean spotted sea bass were not matured and ovulated because of the lack of a gonadotropin surge. Moreover, hCG treatment could induce oocyte maturation and ovulation in the cultured fish.
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