Brown trout kept under chronically crowded conditions grew more slowly over a 110 day period than did uncrowded fish given similar rations. Crowding resulted in a significant elevation of plasma cortisol for at least 25 days but by day 39 the cortisol levels in both crowded and uncrowded fish were similar. After 110 days, plasma cortisol levels in both groups were low ( < 2 ng ml 1) and no significant differences in interrenal histology were detected. It is concluded that prolonged growth rate suppression as a result of crowding is not mediated by a chronic elevation of plasma corticosteroids. Under the conditions of the present study, water quality deterioration due to crowding was not sufficient to account for any of the changes in interrenal activity of the fish. These findings are discussed in relation to social interactions between saimonid fish maintained at unnaturally high stocking densities.

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