| Research Abstract |
Thyroid hormones have been implicated in migration of salmonids, particularly in their downstream migration associated with smoltification. In this study, a method has been established to extract thyroid hormones, thyroxine and triiodothyronine, from the.egg and whole embryo and to quantify by radioimmunoassays. Thyroid hormones were contained in the yolk of unfertilized eggs of not only salmonids but also of all teleost species so far examined. In chum salmon and rainbow trout, the hormone concentrations decreased gradually until hatching, and then rapidly during yolk absorption.
The tissue and plasma levels of the hormones increased during emergence of chum salmon fry from the gravel bed, just before downstream migration.
No such increase was seen in non-migratory rainbow trout. A significant increase in the plasma levels of thyroxine was observed in the chum salmon fry as well as in amago salmon during downstream migration. A marked increase in the tissue levels of the thyroid hormones was also observed during metamorphic climax of the flounder larvae. Corticosteroid, cortisol, was also contained ih the salmon eggs, and changes in its concentrations were more or less in parallel with those in the thyroid hormones. The hormones seem to play important roles during early development of fishes, especially in association with migration and metamorphosis.
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