Increase of heart rate without elevation of cardiac output in adult Atlantic salmon (Salmo salar) exposed to acidic water and aluminium

Adult Atlantic salmon (Salmo salar) were exposed for 48 h to water from aci dified (pH 5.2) Fossbekk. River (Norway), with and without 94 mu g aluminiu m (Al)/L added as AlCl3, and to water from circumneutral (pH 6.7) Ims River (Norway) (controls). Cardiac output, heart rate, and stroke volume were mo nitored throughout the exposure period with Doppler flow probes placed arou nd the ventral aorta of the fish. Fish exposed to Fossbekk River water with out added Al showed few physiological disturbances. When 94 mu g Al/L was a dded to Fossbekk River water, most of the fish died before the end of the 4 8-h exposure period, and a large elevation in heart rate was observed toget her with a decrease in plasma chloride concentrations and an increase in ha ematocrit, plasma glucose and plasma cortisol levels. Cardiac output was ma intained at basal levels during the first 24 h of exposure because the tach ycardia was accompanied by a concomitant reduction of stroke volume. Signs of arrhythmia appeared after 32 h of exposure and were associated with a fu rther decrease in stroke volume that caused cardiac output to decrease belo w basal levels. The incapacity of the tachycardia to elevate cardiac output and the subsequent death of the fish suggest that this response to low pH and Al is more of a maladaptation reaction than a compensatory or adaptativ e reaction.