Chronic Toxicity of Ammonia to Early Life Stage Rainbow Trout

—A 90-d ammonia toxicity test for early life stage rainbow trout Oncorhynchus mykiss was conducted using newly fertilized eggs from a wild strain of fish. The toxicity test was conducted at a pH of 7.75 and temperature of 11.48C. Hatch success and survival of sac fry were not affected by ammonia exposure. Survival, growth, and biomass of swim-up fry were significantly reduced at an ammonia-nitrogen (NH 3 -N) concentration of 16.8 mg NH 3 -N/L of water but were unaffected by exposures to 7.44 mg NH 3 -N/L or lower concentrations. The chronic toxicity value was 11.2 mg NH 3 -N/L, and the EC20 (concentration estimated to cause a 20% reduction in organism performance compared with the control) based on biomass at test termination was 7.72 mg NH 3 -N/L. Development of sac fry to the swim-up stage was retarded by ammonia, but fry exposed to 7.44 mg NH 3 -N/L or lower concentrations appeared to recover by the end of the test. Histological analysis of the gills of exposed fry did not detect any gill pathology. U.S. Environmental Protection Agency chronic ammonia criteria are protective of early life stages of rainbow trout under test