Why do fish die after severe exercise?

Abstract

Trout fitted with dorsal aortic cannulae were subjected to 6 min of intensive exercise and monitored over the following 12 h recovery period. Delayed mortality was 40%; the majority of deaths occurred 4–8 h post‐exercise. Surviving fish exhibited a short‐lived haemoconcentration reflected in increased haematocrit, haemoglobin, plasma protein, Na+ and Ch‐ levels; an extended rise in plasma [K+]; a quickly corrected respiratory acidosis; and a more prolonged metabolic acidosis in concert with a rise in blood lactate. Dying fish exhibited very similar trends except for a significantly greater metabolic acidosis, lower plasma [Cl‐], and the apparent accumulation of an unknown anion in the blood prior to death. Cardiac failure did not occur. Blood metabolic acid levels, while elevated, were only ∼ 50% of peak lactate anion levels and well within the normal range of tolerance, as were all other changes observed in the blood of non‐survivors. The hypothesis that post‐exercise mortality is due to excessive ‘lactic acid’ accumulation in the blood is discounted. It is suggested that intracellular acidosis may be the proximate cause of death.