Toxicity of environmental acid to the rainbow trout: interactions of water hardness, acid type, and exercise

Classical 7-day lethality tests were used to establish the influence of water hardness [Formula: see text], acid type (HCl versus H 2 SO 4 ) and activity level (rest versus exhaustive exercise) on acid toxicity to fingerling rainbow trout (Salmo gairdneri) at 15 °C. Seven-day mean lethal concentration (LC50) pH's ranged from 4.1 to 4.5. Hardness reduced H 2 SO 4 toxicity at all pH levels during both rest and exericise, but reduced HCl toxicity only at very low pH levels. Hardness increased HCl toxicity at pH's > 3.8. H 2 SO 4 was generally less toxic than HCl, except at pH's > 3.8 in soft water. Exchaustive exercise markedly potentiated H 2 SO 4 toxicity in both hard and soft water except at very low pH levels. Below pH = 4.4–4.6, critical swimming speed declined linearly by about 4% per 0.1 pH unit. Possible physiological mechanisms responsible for these modifying influences and their ecological significance are discussed.