Effects of Environmental pH and Calcium on Ammonia Toxicity in Channel Catfish

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Abstract
The 24-h LC50 of total ammonia N (TA-N) to channel catfish (I. punctatus) at pH 7, 8, and 9 (total hardness, 40 mg/l, temperature, 21-25.degree. C) were 263.6 .+-. 11.3 (SE), 38.8 .+-. 1.8, and 4.5 .+-. 0.2 mg/l, respectively. The 24-h LC50 of un-ionized ammonia N (UIA-N) concentration at pH 8 was significantly higher (1.82 .+-. 0.06 mg/l) than at pH 7 or 9 (1.39 .+-. 0.06 and 1.49 .+-. 0.12 mg/l). Enrichment of the water to 440 mg/l total hardness at pH 7 significantly increased the 24-h LC50 of TA-N and UIA-N (356.3 .+-. 16.4 and 1.79 .+-. 0.07). Fish exposed to 25 mg/l TA-N for 12 h at pH 7 and 8 showed no differences from control fish in hematocrit, percent total plasma protein, or plasma and muscle chloride. Plasma Na showed no difference between control and experimental groups at pH 7; a significant decrease occurred in fish exposed to 25 mg/l TA-N at pH 8. No differences in blood pH were found between the control groups and fish exposed to 100 and 200 mg/l TA-N at pH 7, and to 10 and 25 mg/l TA-N at pH 8. Plasma Na depletion is suggested as a contributing mechanism of ammonia toxicity.

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