Gonadal differentiation in coho salmon, Oncorhynchus kisutch, after a single treatment with androgen or estrogen at different stages during ontogenesis

Abstract

The labile period for hormonal treatment for purposes of controlling sexual differentiation in coho salmon was deternined by administration of estradiol-17β (E2) and 17α-methyltestosterone (MT) through immersion for 2 h at a concentration of 400 μg/l, to groups of coho salmon sampled weekly from the late eyed-egg stage until the third week of feeding. Histological samples of the gonadal region were prepared from a control group at each weekly treatment interval using a plastic embedding process. It was found that a single estrogen treatment significantly increased the proportion of phenotypic females when administered between 8 days pre-hatch and 13 days post-hatch, while a single androgen treatment significantly increased the proportion of phenotypic males when administered between 6 and 13 days post-hatch. The maximum response for E2-treated groups (84% females) occurred as a result of treatment 1 day before 50% hatch while the maximum response for MT-treated groups (73.1% males) occurred as a result of treatment 1 week later. Thus, the labile period during which the fish were able to respond to a single administration of exogenous steroid was determined to last 3 weeks, and coincided with the time of hatching and the first days of the alevin stage. Sexual differentiation was first observed at 27 days post-hatching, 21–28 days after the time of maximum steroid sensitivity and coinciding approximately with the time of first feeding.