FACTORS INFLUENCING SEASONAL CHANGES IN THYROID ACTIVITY IN JUVENILE STEELHEAD TROUT, SALMO GAIRDNERI

Abstract

Seasonal changes in thyroid cell height and radioiodide indices of thyroid activity (CR and T/S) were measured in yearling and 2-year-old steelhead trout held in freshwater. Cell height was high in yearlings in mid-June and in 2-year-olds in late April and May at parr–smolt transformation; in both it was low in late summer and moderately high in winter. CR and T/S were high both at parr–smolt transformation and in late summer, but were low in winter. Only at parr–smolt transformation did cell height and radioiodide indices agree.Two-year-old potential smolts were held from January to May under four controlled temperature and photoperiod regimes. These experiments revealed that at parr–smolt transformation, both rising temperature and lengthening photoperiod induced the characteristically high CR and T/S values, but that the tall cell height depended on the lengthening photoperiod. Cell height and T/S were measured in yearlings (non-migrants) subjected to similar regimes at the same season. T/S was uninfluenced by photoperiod but responded to rising temperature. Cell height responses were more complex and indicated involvement of other variables.In September, small yearlings possessed significantly higher T/S and CR values than did large yearlings. A relationship between thyroid function and body weight could explain seasonal changes not attributable to temperature or photoperiod.