Ionoregulatory and respiratory responses of brown trout, Salmo trutta, exposed to lethal and sublethal aluminium in acidic soft waters.

Soft water acclimated (Ca(2+) 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 μg l(-1)) for up to 5 days in order to determine (i) the sublethal concentration of Al at pH 5.0 for this species (ii) their ionoregulatory and respiratory status. No mortality or physiological disturbances 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 μg l(-1) and 67% died over the 5 day period at pH 5.0 in the presence of Al at 25 μg l(-1). Fish at these lethal Al concentrations showed significant decreases in arterial blood oxygen content (CaO2) but no changes in plasma osmolarity or the concentrations of plasma Na(+), K(+) and Cl(-). Physiological disturbance was more marked at the 50 μg l(-1) Al concentration. The surviving fish at 25 μg l(-1) showed few signs of physiological recovery while continually exposed to this regime. No fish died during the exposure to water of pH 5.0 containing 12.5 μg l(-1) Al, but physiological disturbance was still apparent. These sublethally-stressed trout showed a transient decline in the plasma concentrations of Na(+) and Cl(-1). Although CaO2 decreased, recovery was evident. The data suggest that in the brown trout, environmental Al concentration is as important as pH and calcium concentration in determining the physiological status of the fish.