Effect of Prior Acid Exposure on Physiological Responses of Juvenile Rainbow Trout (Salmo gairdneri) to Acute Handling Stress

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Abstract
Acid-stressed fish appear to be more sensitive to additional stressors than unstressed fish. When juvenile rainbow trout, S. gairdneri, were exposed to acid conditions (pH 5.7-4.7) for 5 days, plasma cortisol was affected only slightly during the initial hours of exposure, but plasma glucose and hematocrit increased, and plasma Na decreased. When fish held at pH 4.7 were subsequently subjected to a 30s handling stress, poststress plasma cortisol rose to a peak level of more than twice that in handled fish held at ambient pH (6.6). Effects of handling on plasma glucose of Na were not apparent against levels already altered by the chronic acid exposure. Judging by the corticosteroid response, the acid-stressed fish were more sensitive to additional handling, even though they appeared to be physiologically normal after 5 days. As a management consideration, when fish are stocked in acidified waters, care should be taken to avoid situations where the fish may encounter additional disturbances in the new environment. Plasma glucose and Na were better indicators of chronic acid stress alone than plasma cortisol, but the greater cortisol response to handling at low pH may be a useful method of detecting increased interrenal activity during early stages of environmental acidification.

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