Effects of thyroxine and methyltestosterone on smoltification of masu salmon ( Oncorhynchus masou)

Abstract

Administration of thyroxine (T 4; 250 μg/g diet) from 16 July to 24 December induced silvery skin in underyearling masu salmon, but failed to induce seawater adaptability, darkening of dorsal and caudal fin margins and slimness of the body. There was no difference in the percentage of precocious males in autumn between T 4-treated and untreated groups. Administration of methyltestosterone (MT; 2.5 μg or 25 μg/g diet) from 10 February to 27 April inhibited natural smoltification both in T 4-pre-treated and untreated yearling masu salmon. Fish treated with MT resembled parr or dark parr, and seawater tolerance did not develop. In contrast, approximately 90% of the fish in the control group became smolts. Increases in body length and weight were greatest in the control group and MT 2.5 μg group, respectively. Condition factor decreased in control fish but remained unchanged in the groups treated with MT. The MT 25 μg group showed the slowest growth. A dose-dependent decrease in plasma T 4 level was observed in the MT-treated groups. On the other hand, a dose-dependent increase was found in plasma and pituitary GtH content.