Thyroid Hormones in Migrating Atlantic Salmon.
Field studies and experimental work suggest that the response capacity of the thyroid system becomes increased in smolts but that this is expressed only intermittently. Thyroid hormone levels become raised in response to increases in water velocity and also vary in phase with the lunar cycle. Parallel changes in migratory activity are associated with the same environmental variables. It is suggested that increases in thyroid hormone levels follow increased activity. Maturing adults also develop an increased thyroid response capacity. As in smolts, this is exercised when they are most active. It is suggested that the development of increased thyroid response capacity and its expression during activity is a general phenomenon in salmon undertaking sustained migrations.