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Study Shows Real-World Pesticide Mixtures Harm Salmon

(Beyond Pesticides, June 10, 2008) According to scientists at Simon Fraser University in British Columbia, Canada, real-world pesticide combinations, such as those found in Canada’s Nicomekl River, may contribute to latest decline in the region’s endangered salmon populations. The study, published in Environmental Science and Technology (ES&T), a publication of the American Chemical Society, examines the impact by simulating the river’s low-level pesticide mixture to examine its effects on fish. The study used steelhead trout, a member of the salmon family, and found that the pesticide mix can deaden the trout’s sense of smell. This could harm the fish’s ability to avoid predators, find mates, and migrate back to sea, the researchers say, and could contribute to the threatened and endangered status of salmon species.

“Most laboratory studies examine the effects of a single chemical, often at high concentrations, but real-world streams contain a mixture of chemicals at very low concentrations,” Keith Tierney, Ph.D., the study’s coauthor, told ES&T. Dr. Tierney and his colleagues re-created river water in the laboratory under controlled conditions with carefully measured levels of the 10 most frequently occurring pesticides in British Columbia’s Nicomekl River. The mixture contained four major classes of pesticides, including the commonly used agricultural pesticides atrazine and diazinon. The researchers exposed the trout to the mixture for 4 days instead of the more typical 30 minutes used in other studies. “Fish live in rivers [for long periods], so a longer exposure is more realistic,” Dr. Tierney said.

The research team also tested a pesticide mixture both at one-fifth and at double the concentrations found in the river. At the lowest pesticide levels, the fish’s response to odor was not impaired measurably, but they had higher levels of enzymes that detoxify contaminants. In some cases, animals can build up these enzymes and potentially gain protection from later pulses of the contaminant at higher concentrations, which occur commonly in streams during runoff events. However, that did not occur in this case. Regardless of the initial exposure level, the fish did not produce enough detoxifying enzymes to prevent damage from a later large pulse of pesticides.

“It’s the norm to find pesticides occurring as mixtures,” Nathaniel Scholz, Ph.D., ecotoxicology program manager at the U.S. National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center told ES&T. These mixtures can combine in ways that produce effects not caused by the individual chemicals. In his research, Dr. Scholz found that very low levels of copper impair salmon’s olfaction and thereby the fish’s ability to detect and avoid predators. Testing individual pesticides offers the advantage of ascribing effects to one chemical, he says, but testing all of the possible combinations in a stream quickly becomes “insurmountable.” The new study’s approach is more realistic, but “you don’t know which component drove the effect,” he says, so further tests are required.

Pesticides sprayed on agricultural crops are widespread in streams in the Northwest and half of the waters sampled by the U.S. Geological Survey contain six or more pesticides. Pesticides found in salmon watersheds were at concentrations at or above levels set to protect fish and other aquatic life. The pesticides used in Dr. Scholz’s study include the organophosphates malathion, chlopyrifos and diazion, among others. Organophosphates are neurotoxic and kill cells by inhibiting acetylcholinesterase, an enzyme that helps neurons communicate. This in turn harms the ability of salmon to feed. Diazinon also reduces sperm production in male salmon even at extremely low levels.

In a 2002 order, a U.S. District Court in Seattle found that the federal government had failed to protect threatened species of salmon and 26 other endangered species from toxic pesticides. The judge ordered EPA to consult with the National Marine Fisheries Service (NMFS) to identify permanent measures needed to protect the salmon and others from pesticides. Despite the ruling, federal agencies have been negligent in their efforts to protect declining salmon populations. In November 2007, fishing and environmental groups seeking to force the federal government to uphold the five-year-old rule once again filed a lawsuit (See the November 7, 2007 Daily News Blog).

For more information on endangered salmon see articles from the Spring 2002 and Summer 1999 issues of Pesticides and You.


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