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

Ž . Growth rates of rainbow trout Oncorhynchus mykiss were assessed with respect to serum concentrations of cortisol, glucose, and chloride following exposure to a standardized handling Ž . stressor. Individually tagged rainbow trout Ns400, 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 time post-stress. For cortisol, 42% of the fish had a consistent response, defined as being ranked in the upper or lower 50th percentile for all three samples collected, whereas for glucose and chloride, 35% and 37% had consistent responses. Ž . Among all the fish, growth was best correlated P-0.02 with 3-h serum cortisol concentrations, Ž . and to a lesser degree P-0.06 with 1-h serum cortisol concentrations. Among AconsistentB 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 important determinant of fish performance than the magnitude of the cortisol response. q 2001 Elsevier Science B.V. All rights reserved.