The effects of chronic plasma cortisol elevation on the feeding behaviour,growth, competitive ability, and swimming performance of juvenile rainbow trout

Plasma cortisol elevation, a common consequence of stress, occurs in salmon ids of subordinate rank; these fish acquire a smaller share of available fo od and grow more slowly. This study examined the role of cortisol itself in these phenomena. Cortisol implants, with parallel sham and control treatme nts, were used to create a chronic threefold elevation in plasma cortisol l evels in juvenile rainbow trout, and the individual feeding patterns of the fish were evaluated using X-ray radiography. The three treatment groups we re (I) held alone and fed to satiation, thereby providing a measure of volu ntary appetite, or mixed together in equal proportions and fed to either (2 ) satiation or (3) half-satiation, thereby allowing assessment of the addit ional effects of competitive interaction and food limitation. Chronic plasm a cortisol elevation had significant negative effects on individual appetit e, growth rate, condition factor, and food conversion efficiency, independe nt of whether the fish were held under unmixed or mixed conditions. Under t he latter, mean share of meal was reduced and fin damage increased in corti sol-treated fish; negative growth effects were more severe with food limita tion, but the response patterns were otherwise unchanged. Even in the absen ce of other groups, cortisol-treated fish showed more variable feeding patt erns. When compared at the same individual ration levels, cortisol-treated fish had lower growth rates, reflecting a higher "cost of living." Cortisol treatment had no effect on aerobic swimming performance. These results sug gest that the structure of the feeding hierarchy may not be determined sole ly by competitive ability but may also be greatly influenced by differences in the Feeding behaviour of unstressed fish versus stressed fish caused by cortisol elevation in the latter.