Study of the sexual maturity of female bluefin tuna: purification and partial characterization of vitellogenin and its use in an enzyme-linked immunosorbent assay

Plasma steroid levels of female bluefin tuna (BFT) Thunnus thynnus rose from c. 1·5 ng ml^?1 during the quiescent period (March) to c. 7 ng ml^?1 during the ripening period (May). Testosterone (T) increased further to c. 8 ng ml^?1 during the pre-spawning period (June) while 17β-oestradiol (E₂) began to decrease. In the post-spawning period (August) steroid levels decreased to < 1 ng ml^?1. Vitellogenin (Vtg) plasma levels seemed to follow changes in E₂, showing an increase from the quiescent period to the ripening period of c. 18 mg ml^?1, decreasing slightly before spawning, and then decreasing after spawning. The Vtg content in plasma showed a good correlation both with the plasma levels of E₂ and T and with the percentage of vitellogenic oocytes at different periods of the reproductive cycle. Thus the ELISA could be taken as validated. Immunohistochemical staining of ovaries with anti BFT-Vtg serum demonstrated a high cross-reactivity with yolk proteins allowing the identification of vitellogenic oocytes.