Multiple exposures to common farm pesticides can be lethal to salmon, according to a new federal study.
The study by the National Oceanic and Atmospheric Administration and Washington State University found that five of the most common pesticides used in California and the Pacific Northwest -- diazinon, malathion, chlorpyrifos, carbaryl and carbofuran -- acted in deadly synergy by suppressing an enzyme that affects the nervous system of salmon.
In lab tests, researchers exposed juvenile coho salmon to the five pesticides in various combinations, killing some fish immediately.
Exposures to a single chemical did no harm, but pairing chemicals lowered enzyme activity, sometimes fatally. Moreover, scientists noticed effects at lower pesticide levels when chemicals were applied in combinations.
The results challenge regulators' test methods, which rely on single-chemical risk assessments, researchers said.
The researchers say further study is necessary. There are still significant data gaps for some chemicals, they say, and creating a design protocol for testing multiple exposures poses a challenge.
But the findings add to a growing body of evidence that suggests chemical risk assessments should take into account multiple chemical exposures of animals and people. Salmon and steelhead are commonly exposed to many pesticides from agricultural runoff and other pollution.
The National Marine Fisheries Service said last year three pesticides -- diazinon, malathion and chlorpyrifos -- can be lethal to salmon at certain concentrations and can inhibit their growth by impairing their ability to smell prey and by reducing the availability of small fish and insects. The chemicals have also been found to impair salmon's swimming, making it difficult for them to spawn and avoid predators, the agency said.
NMFS also found that accepted uses of chlorpyrifos, diazinon and malathion are likely to jeopardize the continued existence of 27 species of endangered or threatened salmon and steelhead. All three pesticides are organophosphates, a class of neurotoxic chemicals used in agricultural and urban insect control.
Industry groups criticized the finding, saying the agency relied on old and outdated labels that do not represent the way the pesticides are used today (E&ENews PM, Nov. 18, 2008).
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