Ambient ammonia, diet and growth in lake trout

Juvenile lake trout were exposed to ambient free (un-ionized) ammonia concentrations of 0, 99, 198 and 297 μg NH₃N 1^-1 for 60 days and fed one of two diets which were similar in energy concentration. Diet did not influence food intake at ammonia concentrations of 0, 99 and 198 μg NH₃N 1^-1. Food intake was unaffected by ammonia concentrations of 0 and 99 μg NH₃N 1^-1 and was only temporarily reduced when ammonia was 198 μg NH₃N 1^-1. Trout exposed to 297 μg NH₃N 1^-1 consumed significantly less food than fish exposed to the lower concentrations of ammonia. Food intake did not differ with diet during the first 30 days of exposure to 297 μg NH₃N 1^-1 but during the final 30 days, it was higher for trout fed the low protein diet. Growth, measured as a change in live body weight was not influenced by ammonia concentrations of 0, 99 and 198 μg NH₃N 1^-1 but declined significantly at 297 μg NH₃N 1^-1. Weight gain tended to be larger for trout fed the high protein diet. Efficiency of protein-N gain was greater for trout fed the low protein diet, presumably as a consequence of a sparing effect afforded by high dietary lipid. Efficiency of protein-N gain was significantly reduced among lake trout exposed to the highest concentration of ammonia. Mortalities were observed only among trout exposed to the highest concentration of ammonia.