Acute Toxicity of Ammonia to Fathead Minnows

The acute toxicity of ammonia to fathead minnows Pimephales promelas was measured in 35, 96-hour, flow-through tests. The fish were from both wild and hatchery-reared stocks, and ranged in size from 0.1 to 2.3 g. The 96-hour median lethal concentrations (LC50) ranged from 0.75 to 3.4 mg/liter un-ionized ammonia (34 to 108 mg/liter total ammonia-nitrogen). The toxicity of ammonia decreased as temperature increased over the range 12 to 22 C. There was no significant relationship between ammonia toxicity and dissolved oxygen concentration, over the range of the latter from 3 to 9 mg/liter. Toxicity was not related to the size or the source of test fish.