Slight stress does not lower critical thermal maximums in hatchery-reared rainbow trout

This study evaluated the physiological response of rainbow trout to various initial stressors, either confinement or electroshock, followed by a heat challenge in order to determine how the critical thermal maximum of the fish was affected by the intensity of the initial stressor. Plasma cortisol and glucose levels were determined following the initial stressor and the challenge stressor. The initial stressor induced a typical, but mild, response by increasing cortisol levels to 68 ng/mL for the electroshocked group and 112 ng/mL for the 60-minute confinement group. Although these increases are significantly different (P<.05) from the baseline group (9 ng/mL) they are lower than values observed in the literature for similar stressors. The challenge stressor did not induce any further cortisol increase in fish receiving an initial stress, and in fact decreased significantly (P<.05). The electroshock-challenged group had a significantly higher critical thermal maximum (30 C), but this was only 0.9 C higher than the lowest critical thermal maximum.