Pesticide Mixtures Have Greater Effect on Salmon

(Beyond Pesticides, February 20, 2008)

Pesticides that run off agricultural land and mix in rivers and streams combine to have a greater than expected toxic effect on the salmon nervous system, according to researcher Nathaniel Scholz, PhD, a zoologist with the National Oceanic and Atmospheric Administration (NOAA) in Seattle.

Dr. Scholz, who presented his findings at the symposium entitled From Kitchen Sinks to Ocean Basins: Emerging Chemical Contaminants and Human Health, which was organized by NOAA and hosted at the American Association for the Advancement of Science (AAAS) Annual Meeting, had previously looked at the effects of individual pesticides. However, to get a more realistic idea of exposure, combinations of several pesticides were used and juvenile salmon exposed to them two at a time. The results surprised Dr. Scholz and his team. The total impact observed from combined pesticides was greater than the sum of the individual pesticides, demonstrating a synergistic effect. Some pesticides that were not deadly when tested in individual trials killed all salmon exposed to combinations. A mixture of the pesticides diazinon and malathion, exhibited the greatest synergistic effect and killed all the salmon exposed to them, even at the lowest concentrations.

“It was eye opening,” Dr. Scholz said. “We’re seeing relatively dramatic departures” from what happens with each pesticide by itself. “The real world is usually more complex, and exposures to mixtures of chemicals can be more of the rule than the exception.”

Dr. Scholz said the findings, which are in review for publication, mean that the Environmental Protection Agency (EPA) may be underestimating the hazards pesticides pose to salmon. It also indicates that combinations of pesticides found on fruits and vegetables may also pose an unknown and unexpected risk for people.

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 Daily News Blog of November 7, 2007).