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A selected review of the biological, chemical and environmental factors that affect the acute and chronic, lethal and sublethal toxicity of pyrethroid insecticides to fish is presented. Pyrethroid insecticides are extremely toxic to fish; for example, 96—h LC50 values are in the nmol/L range. The symptoms associated with acute lethality suggest that effects on the nervous system, respiratory surfaces and renal ion regulation are associated with the mechanism of lethal action in fish. Qualitative structure—activity relationships indicate that the structural features required for good insecticidal activity and for lethality to fish are the same. Lethality also varies with biological (species, size) and environmental (temperature, sediment) factors. Some of these effects may be related to bioavailability and rates of pyrethroid biotransformation. Chronic exposure studies indicate that newly hatched larvae or early juveniles are the life stages most sensitive to pyrethroids. Exposures of fish to sublethal concentrations of pyrethroids have resulted in decreased growth and impaired swimming performance. The effects on bioenergetics and energy metabolism are variable.

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