The ovarian development, and plasma levels of gonadotropin II (GtH II) and sex-steroid hormones at the end of vitellogenesis were examined in captive white bass Morone chrysops. The changes in plasma hormone levels and oocyte morphology associated with gonadotropinreleasing hormone agonist (GnRHa)-induced final oocyte maturation (FOM) were studied. Although plasma 17 -oestradiol (E 2 ) and oocyte diameter increased, there were no changes in GtH II, testosterone (T), 17,20 -dihydroxy-4-pregnen-3-one (17,20 -P) or 17,20 ,21dihydroxy-4-pregnen-3-one (17,20 ,21-P) in non-hormone-treated females, and no FOM was observed. Treatment with a sustained-release GnRHa delivery system (GnRHa implant) induced two FOM cycles separated by about 24 h, with the release of approximately equal numbers of eggs in each spawn. Plasma GtH II levels were elevated significantly throughout FOM, reaching a maximum of 9·07 1·55 ng ml 1 in ovulated fish. Both plasma E 2 and T increased soon after the GnRHa treatment, but E 2 declined in fish undergoing germinal vesicle (GV) migration. Plasma T increased further during FOM (7·55 2·87 ng ml 1 ), but declined precipitously at ovulation. A surge in plasma 17,20 -P and 17,20 ,21-P (4·11 0·97 ng ml 1 and 3·10 0·77 ng ml 1 , respectively) was observed in females undergoing GV breakdown (GVBD). Based on the involvement of different sex-steroid hormones, FOM was separated into two stages. Early FOM included lipid-droplet coalescence and GV migration, and was associated with elevations in plasma GtH II and T. Late FOM included GVBD and yolk-globule coalescence, and was associated with elevations in plasma GtH II, 17,20 -P and 17,20 ,21-P. The results of this study point to the absence of a surge in plasma GtH II as the missing link in the reproductive axis responsible for the failure of captive white bass to undergo FOM at the end of vitellogenesis. Sustained elevation of plasma GtH II via treatment with a GnRHa implant induced two consecutive spawns with an overall egg production two-to eightfold higher than previously obtained from captive broodstocks, and similar to annual egg production values reported for wild fish. 1997 The Fisheries Society of the British Isles
