Treatment of tributaries to the Great Lakes with the lampricide 3-trifluoromethyl-4-nitrophenol (TFM) occasionally results in incomplete kills of sea lamprey larvae (Petromyzon marinus ) or excessive mortality of nontarget fish. In continuous-flow toxicity tests conducted on the Millecoquins River, Michigan, TFM remained selective for sea lamprey at the ambient stream pH and at an increased pH. At all but one concentration, TFM killed all sea lampreys and none of the target fish. Selectivity decreased when the pH was lowered by approximately 1 unit. TFM at the lowest tested concentration (2.3 mg/L) killed 100% of the sea lampreys, 50% of the rainbow trout (Oncorhynchus mykiss ), and 40% of the fathead minnows (Pimephales promelas ). When the Millecoquins River was treated at a concentration of 4.2 mg/L of TFM, all the caged sea lampreys were killed at the ambient stream pH (8.35). Treated stream water that was diverted through stainless steel tanks killed only 55% of the sea lampreys and none of the nontarget organisms when the pH was raised to 9.23. All of the sea lampreys and nontarget organisms were killed when the pH of the treated water was lowered to 7.25. These results indicate that diurnal changes in stream pH of approximately 1 pH unit can either cause TFM to become toxic to nontarget organisms or render the treatment ineffective for killing sea lampreys.
