Heat and Cold Tolerance of the Fathead Minnow, Pimephales promelas, Exposed to the Synthetic Pyrethroid Cyfluthrin

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Abstract
Critical thermal maximum (CTMax) and minimum (CTMin) techniques were used to bioassay the effects of sublethal exposures of a synthetic pyrethroid, cyfluthrin, to 240 larval fathead minnows, Pimephales promelas. Fathead minnows were exposed for 24 h to nominal concentrations of cyfluthrin 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 °C below median heat tolerance of controls (34.4 °C) and a 5.6 °C increase in median cold tolerance above controls (5.9 °C). At the highest concentrations of cyfluthrin tested, the zone of temperature tolerance was reduced by 30%. Concentrations of cyfluthrin that caused these effects were as low as 170 parts per trillion. We believe that this is the first use of CTMin as a bioassay. Although the physical and chemical properties of cyfluthrin may preclude its presence in the environment in large amounts, extremely small concentrations can compromise the ability of fathead minnows to resist extreme temperatures. Cyfluthrin use may have greater adverse effects on nontarget species in northern climates or during colder seasons in southern climates.