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To examine the initial hormonal and metabolic changes in exercising fish, rainbow trout (Oncorhynchus mykiss) were subjected to moderate (1.5 body lengths/s) or low (0.5 body lengths/s) water velocities for 24 h and compared with unexercised controls. Blood sampling occurred immediately before (0 h) and at 1, 2, 4, 8, and 24 h after the onset of exercise for determination of plasma concentrations of lactate, potassium, glucose, cortisol, growth hormone, and triiodo-L-thyronine (T3). There was no effect of either exercise or time on plasma lactate levels in any of the three groups, indicating that neither exercise regime induced anaerobic conditions. Within the first 2 h of exercise, plasma, K+, glucose, cortisol, and growth hormone levels were all significantly (P < 0.05) elevated over controls in the moderate-exercise group. By 4 h, none of these parameters were significantly different from controls except plasma K+ levels, which remained significantly higher throughout the experiment. There were no differences between 0 and 24 h of exercise in any parameters of any of the groups, again with the exception of plasma K+ levels in the moderate-exercise fish, which were significantly higher at 24 h. It is concluded that the effects of the onset of exercise resemble those described for acute stress in fish. Furthermore, short-term (24 h) exercise at moderate levels (1.5 body lengths/s) resulted in continuously elevated plasma K+ levels, but did not seem to cause similar increases in the other parameters.

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