The Neurological and Behavioral Effects of Pesticide Mixtures to Juvenile Coho Salmon

Pesticides are commonly detected in freshwater habitats that support threatened and endangered species of Pacific salmon (Oncorhynchus sp.) in the Pacific Northwest.  These chemicals can enter salmon habitat from urban, residential, and agricultural areas, usually as complex mixtures of multiple chemicals.  One class of commonly detected pesticides, the organophosphates, is known to inhibit the activity of the neurological enzyme acetylcholinesterase (AChE).  Enzyme inhibition disrupts normal nerve transmission as well as behaviors that may be essential for salmon survival.  Preliminary exposures in larval zebrafish (Danio rerio) indicated that binary mixtures of the pesticides malathion and diazinon, as well as binary mixtures of malathion and ethoprop, produced synergistic toxicity.  This was confirmed in coho salmon (Oncorhynchus kisutch), where a malathion and diazinon mixture at 1.1 and 2.2 ug/l (respectively), as well as a malathion and ethoprop mixture at 1.1 and 1.4 ug/l (respectively), produced AChE inhibitions greater that 90% relative to unexposed controls.  Spontaneous swimming speed at these same exposure concentrations were also significantly reduced relative to controls to less than 1 cm/s.  Preliminary data indicate that mixture toxicity might be even greater at modest increases in water temperatures above 14 °C.  We observed that juvenile salmon exposed to pesticide mixtures showed both synergistic neurotoxicity and altered behavior.  Therefore, pesticide mixtures at environmentally relevant concentrations may pose a significantly greater threat to salmon health than predicted by conventional single-chemical risk assessments.  These chemicals may represent a more important recovery challenge for declining salmon populations that previously anticipated.