The relationship between thermal physiology and lampricide sensitivity in larval sea lamprey (Petromyzon marinus)

The sensitivity of larval sea lamprey (Petromyzon marinus) to the lampricide 3-trifluoromethyl-4-nitrophenol (TFM) varies with season, with highest sensitivity in spring and tolerance increasing by 2- to 3-fold in the mid-late summer. Until recently, the physiological basis for these differences was unresolved. Using previously published and unpublished findings, we illustrate how the acute toxicity of TFM (12-h LC50, 12-h LC99.9) changes with season in two populations of larval sea lamprey collected through the spring, summer and fall from Deer Creek and the Au Sable River, Michigan, U.S.A. Our findings reveal that the greater TFM tolerance of larval sea lamprey in the summer is most closely related to increases in water temperature. Although the energy reserves (glycogen, lipid) and body condition of larval sea lamprey may be lower in the spring after overwintering, these physiological indices have little impact on TFM sensitivity. We therefore conclude that water temperature, rather than energy stores or body condition, explains the greater tolerance of sea lamprey to TFM in the summer. We propose that as water temperature increases through the spring and summer, and approaches the thermal optima of larval sea lamprey, their metabolic rate and capacity to detoxify TFM increases, which slows the rate at which TFM accumulates in the body, despite concurrent increases in TFM uptake rate. We therefore recommend that water temperature be considered when planning and executing lampricide applications to mitigate temperature-induced increases in sea lamprey tolerance to TFM that could undermine sea lamprey control efforts in the Great Lakes.