IONOREGULATORY AND RESPIRATORY RESPONSES OF BROWN TROUT, SALMO-TRUTTA, EXPOSED TO LETHAL AND SUBLETHAL ALUMINUM IN ACIDIC SOFT WATERS

Soft water acclimated (Ca2+ 0.02 mM; Na+ 0.03 mM; K+ 0.01 mM; pH 7.0), cannulated brown trout (Salmo trutta) were exposed to various pH and aluminium (Al) regimes (pH 7.0, pH 5.0, pH 5.0 plus Al: 50, 25, and 12 .5 mu g l(-1)) for up to 5 days in order to determine (i) the subletha l concentration of Al at pH 5.0 for this species (ii) their ionoregula tory and respiratory status. No mortality or physiological disturbance s were evident at pH 7.0 or pH 5.0. All trout died within 48 h at pH 5 .0 in the presence of Al at 50 mu g l(-1) and 67% died over the 5 day period at pH 5.0 in the presence of Al at 25 mu g l(-1). Fish at these lethal Al concentrations showed significant decreases in arterial blo od oxygen content (CaO2) but no changes in plasma osmolarity or the co ncentrations of plasma Na+, K+ and Cl-. Physiological disturbance was more marked at the 50 mu g l(-1) Al concentration. The surviving fish at 25 mu g l(-1) showed few signs of physiological recovery while cont inually exposed to this regime. No fish died during the exposure to wa ter of pH 5.0 containing 12.5 mu g l(-1) Al, but physiological disturb ance was still apparent. These sublethally-stressed trout showed a tra nsient decline in the plasma concentrations of Na+ and Cl-. Although C aO2 decreased, recovery was evident. The data suggest that in the brow n trout, environmental Al concentration is as important as pH and calc ium concentration in determining the physiological status of the fish.