Fast growth in rainbow trout is correlated with a rapid decrease in post-stress cortisol concentrations

Growth rates of rainbow trout (Oncorhynchus mykiss) were assessed with resp ect to serum concentrations of cortisol, glucose, and chloride following ex posure to a standardized handling stressor. Individually tagged rainbow tro ut (N = 400, 90-250 g) were reared for 10 months and the handling stressor was administered three times at approximately 3-month intervals. Fish were bled at either 1-, 3-, or 6-h post-stress for measurement of serum cortisol . glucose, and chloride, with individuals always being bled at the same tim e post-stress. For cortisol, 42% of the fish had a consistent response, def ined as being ranked in the upper or lower 50th percentile for all three sa mples collected, whereas for glucose and chloride, 35% and 37% had consiste nt responses. Among all the fish, growth was best correlated (P < 0.02) wit h 3-h serum cortisol concentrations, and to a lesser degree (P < 0.06) with 1-h serum cortisol concentrations. Among "consistent" fish. those with low 3-h cortisol concentrations grew significantly faster than those with high 3-h cortisol concentrations. These results suggest that the rate of return of cortisol concentration to baseline following a stressor is a more impor tant determinant of fish performance than the magnitude of the cortisol res ponse. (C) 2001 Elsevier Science B.V. All rights reserved.