The effect of feeding and fasting on ammonia toxicity in juvenile rainbow trout, Oncorhynchus mykiss

Present fresh water ammonia standards have been established using data collected from toxicity tests on unfed fish. Ammonia, however, is an unusual toxicant as it is produced as a metabolic waste following protein catabolism. The present research was conducted to investigate the relationship between feeding and ammonia toxicity in rainbow trout, Oncorhynchus mykiss. Results from these studies revealed that some fish fed to satiation have plasma ammonia levels greater than 30 μg/ml. This level was similar to the plasma ammonia levels in rainbow trout at the ammonia LC 50 value calculated in the present experiments. Even though plasma ammonia in fed fish was elevated there was no significant difference between the 96 h LC 50 values for fed and unfed fish (174 mg N per l) at pH 7.2. Feeding rates during these experiments decreased during the first 48 h of ammonia exposure, but increased again in the second 48 h at all but the highest ammonia level. Feeding rate never increased to the control level in ammonia exposed fish. In a second set of experiments feeding fish had a significantly higher 24 h LC 50 level, 177 mg N per l, than fish fasted for 5 or 10 days, 135–143 mg N per l. No significant difference was noted however, between the 48 h LC 50 values for fed and fasted fish. It was evident from these studies that feeding protects rainbow trout from ammonia toxicity during the first 24 h of exposure and that fasting exacerbates ammonia toxicity.