A COMPARATIVE STUDY OF THYROID FUNCTION IN MIGRANT JUVENILE SALMON

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Abstract
Seasonal thyroid change was assessed by histological (mean lowest epithelial height) and radiochemical (conversion ratio) techniques in four species of juvenile anadromous Pacific salmon in fresh water. Both sockeye and coho, migrating predominantly in their second or third spring, had, at migration, relatively much higher thyroid activities, which declined rapidly in postmigrants held in fresh water. Pink and chum, obligatory fry migrants, had moderate cell heights at migration. Migrant chum were too small for radioiodine examination but migrant pink showed very slow I131 turnover. Prolonged retention of pink and chum salmon in freshwater brought about a general decline in cell height, despite very high conversion ratios. It is concluded that, at migration, fry migrants (pink and chum) have relatively inactive thyroids while yearling migrants (sockeye and coho) have active thyroids.