Influence of Photoperiod on Smolt Development and Growth of Atlantic Salmon (Salmo solar)

Abstract

Atlantic salmon (Salmo salar) subjected in the spring to a constant photoperiod of 13 hr light/day or to simulated natural photoperiod (increasing day length) smoltified and grew rapidly before and after entering sea water. Others subjected to the reciprocal (complement) of natural photoperiod (decreasing day length in the spring) had the silvery color of smolts and grew rapidly but, unlike smolts, developed high condition factors while in fresh water. After they were transferred to sea water these fish ate less, grew more slowly, and had lower efficiencies of food conversion than those subjected to natural and constant photoperiod.Salmon experiencing reciprocal photoperiod had decreased sensitivity to external stimuli, suggesting reduced thyroid activity. Histological studies in June showed no differences in the appearance of the thyroid glands, suggesting no difference in thyroid activity between groups in natural and reciprocal photoperiod. Nor was there any difference in plasma osmotic and chloride levels between natural and reciprocal photoperiod groups, either in routinely active or fatigued fish.It is suggested that photoperiod acts through the endocrine system to affect growth patterns during and after smoking and possibly certain aspects of the smolting process itself. Effects of photoperiod on smolting and growth of salmon are discussed in connection with other cyclical changes in salmonid physiology known to be influenced by photoperiod.