White sucker (Catostomus commersoni) exposed to soft water ([Ca2+] = 0.207 mequiv./L) below pH 5 showed [Na+] and [Cl−] losses, that were approximately the same in plasma and whole body. At pH 4.5 (6- to 19-day exposure), body and plasma [Na+] were approximately 17% lower than in white sucker held at pH 6.6, and 42% lower in fish at pH 4 (<2 days). Since plasma ions are known to be affected by handling stress, whole-body [Na+] and [Cl−] in fishes may be a more useful indicator of ionoregulation status under field conditions. Pumpkinseed (Lepomis gibbosus) were clearly more acid tolerant, as indicated by greater survival and reduced ion loss in the same acid environments as white sucker. After 19 days exposure at pH 4.5, body Na+ was reduced by 8%; at pH 4 body Na+ was reduced by 38%. Hence, at pH 4, the net Na+ loss experienced by this acid-tolerant fish was similar to that observed in white sucker; however, the rate of loss in white sucker was 11-fold greater. White sucker in acidified, decarbonated hard water ([Ca2+] = 2.110 mequiv./L) at pH 4 (6 or 19 days), showed a 35% lower whole-body [Na+] than in fish held at pH levels of 4.5, 5, and 6.3; [Cl−] was 39% less. However, at pH 4 the ratio of whole-body Na+:Cl− losses was 1.4:1 in hard water and 1.08:1 in soft water. Total Na+ loss at pH 4 was similar to that in white sucker held at the same pH in soft water; however, the rate of loss in soft water was 15-fold greater.
