The Mechanism of Increased Tolerance to the Pesticide, 3-Trifluoromethyl-4-Nitrophenol (TFM), in Invasive sea Lamprey ( <i>Petromyzon marinus</i> ) at Warmer Water Temperatures

The phenolic compound 3-trifluoromethyl-4'-nitrophenol (TFM) is used as a lampricide, applied to rivers and streams to control populations of invasive sea lamprey (Petromyzon marinus) in the Great Lakes. 3-Trifluoromethyl-4'-nitrophenol is used to selectively target larval sea lamprey because of their limited capacity to detoxify it. The tissue TFM accumulation impairs mitochondrial adenosine triphosphate production by uncoupling oxidative phosphorylation, leading to energy depletion and death. Sea lamprey tolerance to TFM is greater in the summer, but the underlying mechanism(s) are unresolved. The present study tested the hypothesis that an increased capacity of sea lamprey to eliminate TFM at warmer temperatures increases their tolerance to TFM. Acute toxicity tests demonstrated that the 12-hr median lethal concentration (LC50) of TFM steadily increased by approximately 1.5-fold as water temperature rose from 7 to 28 °C. When lamprey were acclimated to one of three temperatures (6, 12, 24 °C) and exposed to an identical TFM concentration (i.e., 12-hr LC25 at 12 °C), muscle and liver TFM concentrations were approximately 30% and 36% lower in lamprey acclimated to 24 °C, suggesting more effective elimination of TFM at warmer temperatures. Calculations of the TFM steady state concentration and elimination half-life (t1/2) in the liver and muscle following TFM exposure suggested that they have a greater capacity to eliminate TFM in warmer water. We propose that the sea lamprey's capacity to detoxify TFM is greater at higher temperatures, preventing internal concentrations from reaching lethal levels during acute (9-12 hr) TFM exposure of similar duration to field applications. We also propose that water temperature, in addition to water pH and alkalinity, be considered when determining TFM application concentration used to optimize sea lamprey treatment effectiveness.