Annual change in reproductive condition and plasma concentrations of sex steroids in black bream, Acanthopagrus butcheri (Munro) (Sparidae)

Abstract

Changes in gonadosomatic index (GSI), hepatosomatic index (HSI), gonad stage and plasma concentrations of sex steroids were studied over one year in black bream (Acanthopagrus butcheri). Black bream have an annual reproductive cycle with a 3-month spawning season in spring–early summer. GSI and HSI values were highest in October and May respectively. Plasma concentrations of oestradiol-17β (E₂), testosterone (T) and 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) were highest in females in October. Plasma concentrations of E₂ and T were highest in ovulated fish. Concentrations of 17,20βP were higher in fish undergoing final oocyte maturation (FOM) than in fish with regressed gonads. In males, plasma concentrations of T and 11-ketotestosterone (11KT) increased in September and remained elevated until January, but concentrations of 17,20βP did not change with season. However, 17,20βP concentrations in spermiated fish were higher than in non-spermiated fish. Daily changes in gonad condition indicated that females undergo daily cycles of ovarian maturation with ovulation occurring after midday. Plasma T and 17,20βP concentrations of females were elevated at midday in association with FOM, but E₂ showed no diel change. In males, partially spermiated fish were dominant in the early morning and fully spermiated fish at midday. Plasma T, 11KT and 17,20βP concentrations were low at midnight and reached maximum levels at 0600 hours.