Inhibitory effects of 17α-methyltestosterone and estradiol-17β on smoltification of sterilized amago salmon ( Oncorhynchus rhodurus)

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Underyearling amago salmon parr, previously sterilized by 17α-methyltestosterone (MT) treatment, were fed either MT (1 or 10 μg/g diet), estradiol-17β (E 2; 1 or 10 μg/g diet), or no hormone (control), in late autumn for 60 days. The occurrence of smolts in the different treatment groups, as judged by body silvering and dorsal fin-margin darkening at the end of the experiment, was as follows: control, 80%; MT − 1 ppm, 0%; MT − 10 ppm, 0%; E 2 − 1 ppm, 33%; E 2 − 10 ppm, 2%. A seawater challenge test indicated a lower hypoosmoregulatory ability of fish treated with both doses of MT and with the higher dose of E 2. The histological activity of the gill chloride cells was lower in the hormone-treated fish than in the control animals. MT and E 2 stimulated pituitary gonadotrophs. The pituitary somatotrophs of the control fish showed the most active features among the experimental groups, while the cells of the fish treated with the high dose of E 2 showed pathological changes. These results indicate that both MT and E 2 inhibit smoltification of amago salmon, morphologically and physiologically. The inhibitory effect of MT on smoltification was greater than that of E 2. Thus the present experiment suggests that androgens and estrogens, which are synthesized in the gonad of sexually maturing amago salmon parr, prevent parr-smolt transformation of the fish.