Plasma sex steroid and vitellogenin profiles during gonad development in wild Mediterranean amberjack (Seriola dumerilii)

Mediterranean amberjacks, Seriola dumerilii Risso, were caught off the Pelagie Islands, in the south Mediterranean Sea, between May 1997 and June 1999. Fish blood was sampled, and gonads were collected at 10-day intervals throughout the spawning period and at monthly intervals during the resting period. Concentrations of plasma estradiol-17ß (E₂), testosterone (T), 17,20ß-dihydroxy-4-pregnen-3-one (DHP) and vitellogenin (Vtg) in females; and plasma T and 11-ketoT (11-KT) in males were correlated with changes in gonadal development. The first females that had already ovulated (F5) were found in late May. Most mature females (F4) were caught in June. Post-spawned females (F6) were found from late July until September. Estradiol-17ß was at baseline levels (<0.02 ng ml⁻¹) during autumn/winter and rapidly peaked (6.29±0.68 ng ml⁻¹) from May to early July. Plasma T levels showed a similar profile and were positively correlated to E₂ (r ²=0.668) during the spawning period. Continuously elevated levels of E₂ and T were observed during the spawning season in vitellogenic females (stages F3, F4 and F5). Resting females were found during the autumn/winter months. Vitellogenin levels increased during May and peaked in June, reverting to undetectable levels in August, in parallel with sex steroid changes. Plasma DHP levels peaked in June (283.45±97.3 pg ml⁻¹), falling to basal values (<5 pg ml⁻¹) in August. DHP levels were higher in mature females (F4) than in maturing (F3) and in partially ovulated (F5) females. DHP values increased during germinal vesicle migration, peaked during germinal vesicle breakdown and decreased again during complete oocyte hydration. In males, changes in T and 11-KT plasma levels were related to testis development. The highest levels of T (5.76±2.64 ng ml⁻¹) were measured during spermatogenesis and highest 11-KT (5.28±3.6 ng ml⁻¹) in males with milt, from May to June. This study provides information, for the first time, on the relation between plasma sex steroid profiles and gonad development in wild Mediterranean amberjack, a useful benchmark for broodstock monitoring under controlled conditions.