The physiological responses of white sucker (Catostomus commersoni Lacépede) and rainbow trout (Salmo gairdneri Richardson), both reared in natural soft water, to a reduction in ambient pH were compared by simultaneous analyses of ion levels in various body compartments (plasma, muscle, whole fish) and net ion transfer rates. Following 24 h of exposure to acidified (H2SO4) natural soft-water, both species displayed a net influx of protons (or loss of base) and net losses of body Na+, Cl, Ca2+, Mg2+, K+ and phosphate. The magnitude of ion loss from plasma was twice as large in the trout as in the sucker. Shifts of fluid from the extracellular to the intracellular fluid occurred in both species. Losses of ions from epaxial white muscle were small relative to intracellular ion losses from the rest of the body in both species. The most notable finding was the entry of sulphate into the body fluids of both species, accumulating primarily in plasma and in the intracellular compartment of sucker and trout, respectively. The possible mechanism(s) and implications of sulphate influx into fish are discussed.

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