Influence of pH and Ammonia Salts on Ammonia Toxicity and Water Balance in Young Channel Catfish

Eight static 24-h median lethal tests were conducted with 16-g channel catfish Ictalurus punctatus at water temperatures of about 21°C. Tests were performed at four acidities with ammonium chloride and with ammonium sulfate. When compared on the basis of un-ionized ammonia nitrogen (NH₃-N), ammonium chloride solutions were more toxic than ammonium sulfate solutions. Median lethal concentrations (24-h LC50s) at pH 8.8, 8.0, 7.2, and 6.0 were, respectively, 1.91, 1.45, 1.04, and 0.74 mg NH₃-N/L for ammonium chloride, and 2.24, 1.75, 1.16, and 0.81 mg NH₃-N/L for ammonium sulfate. Ionized ammonia (NH 4) was not lethal at concentrations up to 1,787 mg NH 4-N/L. Slopes of the probit regressions of NH₃-N concentration versus mortality increased with decreases in pH and were nearly vertical at pH 6.0. Hematocrits of channel catfish exposed to lethal ammonia solutions also increased with decreases in pH, indicating an effect on osmoregulation. Plasma osmolality and red blood cell diameters did not change when channel catfish were exposed to NH₃-N LC50 solutions at low and high pH; increases in hematocrit were due to isotonic dehydration. Common physiological salts elicited similar blood and lethality responses from channel catfish at pH 6.0 when compared on an equimolar basis to ammonium salt solutions. Hemoconcentration seems to potentiate the toxicity of ammonia by increasing blood ammonia concentrations. If so, isotonic dehydration would account for the apparent increase in the toxicity of NH₃ at low pH. Based on acute toxicity tests that account for osmotic effects, NH₄ must be considered to be essentially nontoxic to channel catfish.