HEAT AND COLD TOLERANCE OF THE FATHEAD MINNOW, PIMEPHALES-PROMELAS, EXPOSED TO THE SYNTHETIC PYRETHROID CYFLUTHRIN

Critical thermal maximum (CTMax) and minimum (CTMin) techniques were u sed to bioassay the effects of sublethal exposures of a synthetic pyre throid, cyfluthrin, to 240 larval fathead minnows, Pimephales promelas . Fathead minnows were exposed for 24 h to nominal concentrations of c yfluthrin with acetone as a carrier equal to approximately 20 and 70% of the 96-h LC50. Exposure to cyfluthrin caused maximum decreases of 3 .3-degrees-C below median heat tolerance of controls (34.4-degrees-C) and a 5.6-degrees-C increase in median cold tolerance above controls ( 5.9-degrees-C). At the highest concentrations of cyfluthrin tested, th e zone of temperature tolerance was reduced by 30%. Concentrations of cyfluthrin that caused these effects were as low as 170 parts per tril lion. We believe that this is the first use of CTMin as a bioassay. Al though the physical and chemical properties of cyfluthrin may preclude its presence in the environment in large amounts, extremely small con centrations can compromise the ability of fathead minnows to resist ex treme temperatures. Cyfluthrin use may have greater adverse effects on nontarget species in northern climates or during colder seasons in so uthern climates.