Larval fathead minnows (Pimephales promelas), <48 hours old, were exposed to water from the Apalachicola, Choctawhatchee, and Ochlockonee rivers in northern Florida during field toxicity tests in November 1985 and April 1986. The fish were exposed for 6 or 7 days in a flow-through system to control water; full-strength river water; and 50%, 25%, and 12.5% dilutions of river water. Mortality in full-strength Choctawhatchee River water was significantly higher (40% mortality) than in control water or in the 3 other dilutions of the river water (<15% mortality). Mortalities of fathead minnows in the different dilutions of the Apalachicola and Ochlockonee rivers did not differ significantly from those in the controls. Further study is needed to determine the cause of the toxicity of Choctawhatchee River water to larval fish.
