Elevated temperatures increase the toxicity of pesticide mixtures to juvenile coho salmon.

Pesticide mixtures and elevated temperatures are parallel freshwater habitat stressors for Pacific salmon in the western United States. Certain combinations of organophosphate (OP) insecticides are known to synergistically increase neurotoxicity in juvenile salmon. The chemicals interact to potentiate the inhibition of brain acetylcholinesterase (AChE) and disrupt swimming behavior. The metabolic activation and detoxification of OPs involve temperature-sensitive enzymatic processes. Salmon are ectothermic, and thus the degree of synergism may vary with ambient temperature in streams, rivers, and lakes. Here we assess the influence of water temperature (12-21°C) on the toxicity of ethoprop and malathion, alone and in combination, to juvenile coho salmon (Oncorhynchus kisutch). A mixture of ethoprop (0.9 μg/L) and malathion (0.75 μg/L) produced synergistic AChE inhibition at 12°C, and the degree of neurotoxicity approximately doubled with a modest temperature increase to 18°C. Slightly lower concentrations of ethoprop (0.5 μg/L) combined with malathion (0.4 μg/L) did not inhibit brain AChE activity but did produce a temperature-dependent reduction in liver carboxylesterase (CaE). The activity of CaE was very sensitive to the inhibitory effects of ethoprop alone and both ethoprop-malathion combinations across all temperatures. Our findings are an example of how non-chemical habitat attributes can increase the relative toxicity of OP mixtures. Surface temperatures currently exceed water quality criteria in many western river segments, and summer thermal extremes are expected to become more frequent in a changing climate. These trends reinforce the importance of pollution reduction strategies to enhance ongoing salmon conservation and recovery efforts.