Avoidance behaviour of Atlantic salmon (Salmo salar L) fry in waters of low pH and elevated aluminum concentration: Laboratory experiments

The avoidance behaviour of Atlantic salmon (Salmo salar) fry (5-7 cm) was tested in waters with different pH levels (4.0, 4.5, 5.0, and 6.9) and aluminum concentrations (70 and 150 mu g labile Al . L(-1); pH 5.0) in a steep-gradient choice rank: control (no Al addition; pH 6.9) and acidic water could be separated and fish could swim freely between the two halves. Each test lasted for 1 h, including 5 min of acclimation to the tank in control water, a 15-min mixing period, and a 40-min test period. With no Al additions, significant avoidance of the acid side occurred at pH 4.0 and 4.5 but not at pH 5.0. At pH 4.0 and 4.5 the fish spent about 40% of the time in the acid side versus 60% in the control side. Additions of 70 and 150 mu g labile Al . L(-1) at pH 5.0 caused no significant effects on avoidance but increased the number of disrupted entries to the acid side. Although threshold concentrations for avoidance in natural habitats cannot be directly inferred from laboratory studies, our findings clearly indicate that avoidance is a behavioural response that can enhance survival in acidic waters; however, environmentally realistic Al concentrations may not be avoided unless avoidance is triggered by low pH.