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 from vitellogenic coho salmon (Oncorhynchus kisutch) before and 32 days after injection of the aromatase inhibitor Fadrozole (AI). Plasma 17 beta-estradiol levels decreased significantly 6 h after injection in all Al treated fish. The higher the dose the longer the maintenance of low plasma 17 beta-estradiol levels. Inversely, plasma 17 alpha-20 beta-P increased significantly 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 after injection plasma testosterone levels were significantly higher in the Al treated groups. The oocyte maturation index showed that multiple injections with AI retarded oocyte development. Besides, oocyte diameter and GSI were lower in the same group, which presented high incidence of atresia of vitellogenic oocytes. The ovarian follicles and brain of the fish which received multiple injections secreted less 17 beta-estradiol, in vitro. These findings suggest that aromatase inhibitors such as Fadrozole may have a potential as a tool to regulate sexual development in salmon.