Summary

Adult brown trout were acclimated for 2–4 weeks to artificial, soft water (Ca²⁺ 25μmoll⁻¹) at neutral pH and at summer (15°C) or winter (5°C) temperatures. During this period they swam against a current of approximately 0.25 ms⁻¹. They were then exposed to neutral or sublethal pH for 4 days in still water. For fish with their dorsal aorta catheterized, sublethal pH was 4 at 5°C and 4.5 at 15°C.

After 4 days of exposure to sublethal pH, resting oxygen uptake (M_o2 w a s 40 % higher than that at neutral pH for fish held at 15°C and 38 % higher for fish held at 5°C. Critical swimming speeds (U_crit), in contrast, were 35% and 31% lower, respectively. These two phenomena may be related in as much as the ‘metabolic cost’ of exposure to low pH may increase as swimming speed increases, thus reducing the scope for activity. Another important factor could be an impairment of oxygen delivery to the red muscle fibres. Although arterial O₂ concentrations and heart rate are both similar for fish at U_crit in neutral and acid water, there are signs of haemoconcentration in fish exposed to low pH, and the consequent increase in blood viscosity could disrupt the local circulation in the red fibres. Whatever its causes, an impairment of swimming activity resulting from exposure to acid water may have severe consequences for active fish such as salmonids.

Exposure to sublethal pH caused significant reductions in plasma Na⁺ and Cl⁻ concentrations at both temperatures, although these were more substantial at 5°C than at 15°C. Swimming at U_crit had no significant effect on plasma concentrations of Na⁺, K⁺ and Cl⁻ except at sublethal pH at 5°C, when there were significant reductions in all three.

Seasonal temperature had significant but small effects on resting M_O2 and U_crit, and these are discussed in terms of the possible effects of low temperature and continued swimming activity (training) on hypertrophy of skeletal and cardiac muscles and on the aerobic capacity of the ‘red’ muscle fibres.