Effects of the aromatase inhibitor Fadrozole on plasma sex steroid secretion and oocyte maturation in female coho salmon (Oncorhynchus kisutch) during vitellogenesis

17 beta-estradiol, 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (17 alpha-20 beta-P), and testosterone levels were measured in plasma samples obtained f rom vitellogenic coho salmon (Oncorhynchus kisutch) before and 32 days afte r injection of the aromatase inhibitor Fadrozole (AI). Plasma 17 beta-estra diol levels decreased significantly 6 h after injection in all Al treated f ish. The higher the dose the longer the maintenance of low plasma 17 beta-e stradiol levels. Inversely, plasma 17 alpha-20 beta-P increased significant ly 6 h after injection in all AI treated fish, and the higher the dose the longer the maintenance of high plasma 17 alpha-20 beta-P levels. At 48 h af ter injection plasma testosterone levels were significantly higher in the A l treated groups. The oocyte maturation index showed that multiple injectio ns with AI retarded oocyte development. Besides, oocyte diameter and GSI we re lower in the same group, which presented high incidence of atresia of vi tellogenic oocytes. The ovarian follicles and brain of the fish which recei ved multiple injections secreted less 17 beta-estradiol, in vitro. These fi ndings suggest that aromatase inhibitors such as Fadrozole may have a poten tial as a tool to regulate sexual development in salmon.