The effects of acid and acid/aluminum exposure on circulating plasma cortisol levels and other blood parameters in the rainbow trout, Salmo gairdneri

Softwater (Ca²⁺ = 50, Na⁺ = 50 μequiv. l^-1) acclimated rainbow trout were fitted with chronic arterial catheters to allow for repetitive blood sampling. After 48 h recovery they were then exposed to either control (pH 6·5, Al = 0 μg l^-1), acid (pH 4·8, Al = 0 μg l^-1) or acid plus aluminum (pH 4·8, Al = 112 μg l^-1) conditions for 72 h. Parameters measured included blood glucose, lactate, haemoglobin, haematocrit and plasma Na⁺, Cl^-, protein and cortisol. Exposure to pH 4·8 alone caused no mortality, a moderate ionoregulatory disturbance and a transient elevation in plasma cortisol. All other parameters were not significantly different from controls. Addition of aluminum to this exposure caused 100% mortality with a mean survival time of only 27·0 h. There was a marked decrease in plasma ions, hyperglycemia, lactate accumulation, haemoconcentration, red cell swelling, and a sharp rise in plasma cortisol becoming greatly increased as the fish neared death. The mechanism of toxicity of acute acid/aluminum exposure, the role for cortisol under such conditions, and the validity of cortisol and glucose as indicators of stress in fish are discussed.