Locomotor Response by Arctic Char (Salvelinus alpinus) to Gradients of H⁺ and CO₂

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Abstract
Evidence from the literature suggests that fish are more responsive to CO₂ than H⁺ when these two stimuli appear together. Here Arctic char were presented with plumes of H⁺ and CO₂ and their preference/avoidance responses compared with those obtained with acidified, moderately hard water. Hydrochloric acid presented in pH 7.0 decarbonated water was attractive when plume pH exceeded 5.5 but avoided at higher concentrations down to pH 3.3. The avoidance of plumes less than pH 5.5 was not diminished by lowering ambient pH to 5.5. Carbon dioxide plumes were avoided at plume/ambient gradients greater than 50 μmol/liter but was attractive at lower concentrations. The response to acidified natural water closely followed the concentration-response curve for CO₂ but not H⁺. The results, therefore, demonstrate that fish behaviorally discriminate between H⁺ and CO₂, relying principally on the latter for orientation within H⁺/CO₂ gradients. However, fish can use H⁺ alone to avoid lethal levels of pH.