Effects of Rearing Conditions on the Developmental Physiology of Smolting Coho Salmon

We determined the effects of rearing conditions on the physiological development of coho salmon Oncorhynchus kisutch during smoltification. Combinations of various levels of both rearing density and water inflow rate were used in this study. The experiments were performed at a production hatchery and at our laboratory. High rearing density resulted in reduced levels of plasma thyroid hormones in the fish, but no effect of water inflow rate on these hormones was recognized. Although rearing density and water inflow rate affected plasma cortisol and gill Na,K-ATPase activity, their effects on these physiological variables seemed to vary through time producing different developmental patterns of physiology during smoltification. Measurements of plasma corticosteroid clearance and in vitro activity of interrenal cells suggested that rearing density affected plasma cortisol levels primarily by altering the submaximal activity of the interrenal cells. Lowering the rearing density of fish from high to low 2 weeks before sampling resulted in cortisol levels similar to those of fish reared at low density throughout the experiment; however, plasma thyroxine and gill Na,K-ATPase activity were not affected. The relationships observed among experimental rearing conditions, water quality, and physiological status of the fish suggested that crowding stress itself may be an important factor by which high rearing density affects the physiology of coho salmon.