Study: Combining pesticides makes them more deadly for fish

Common agricultural pesticides that attack the nervous systems of salmon can turn more deadly when they combine with other pesticides, researchers have found.

Scientists from the NOAA Fisheries Service and Washington State University were expecting that the harmful effects would add up as they accumulated in the water. They were surprised to find a deadly synergy occurred with some combinations, which made the mix more harmful and at lower levels of exposure than the sum of the parts.

The study looked at five common pesticides: diazinon, malathion, chlorpyrifos, carbaryl and carbofuran, all of which suppress an enzyme necessary for nerves to function properly.

The findings suggest that the current practice of testing pesticides - one at a time to see how much is needed to kill a fish - fails to show the true risks, especially for fish protected by the Endangered Species Act, the authors concluded in the study published Monday in the journal Environmental Health Perspectives.

"We need to design new research that takes into effect the real-world situation where pesticides almost always coincide with other pesticides," co-author Nathaniel Scholz, a research zoologist at the NOAA Fisheries Service Northwest Fisheries Science Center, said from Seattle.

Inge Werner, director of the aquatic toxicology laboratory at the University of California at Davis, was not involved in the study. She said while the idea was not new, the findings were definitive, even at levels that don't kill fish outright.

"We may not see the big fish kills out there anymore like we used to," she said from Davis, Calif. "But the subtle, sublethal effects that basically render them unfit for survival in the wild are much more important. In certain areas, pesticides really are a very important factor" in salmon survival.

Jeffrey Jenkins, professor of environmental and molecular toxicology at Oregon State University, was not part of the study. He said the study was well done, but it would take more research to push the U.S. Environmental Protection Agency to change its pesticide testing standards as they relate to fish, which are defined by law.

Last year, NOAA Fisheries issued findings under the Endangered Species Act that diazinon, malathion and chlorpyrifos jeopardize the survival of all 28 species of Pacific salmon listed as threatened or endangered in the West.

The three chemicals, found by the U.S. Geological survey to contaminate rivers throughout the West, interfere with salmon's sense of smell, making it harder to avoid predators, locate food and even find their native spawning streams and reproduce. At higher concentrations, they kill fish outright.

NOAA Fisheries and EPA must evaluate 34 more pesticides by 2012 under terms of a settlement reached in a lawsuit brought by Northwest Coalition for Alternatives to Pesticides and others.

In the study, scientists combined the pesticides two at a time at various concentrations, then exposed juvenile coho salmon in tanks for four days. Many of the fish died outright.

Fish that survived were killed, and their brains analyzed for the levels of the enzyme acetylcholinesterase, which allows impulses to move between neurons in the brain. In every fish, the levels of the enzyme were below the level considered healthy.

Earlier research found that lower levels of the enzyme affected the ability of fish to feed and swim, which would affect their ability to survive, Scholz said.

The researchers suggested that the reason harmful affects of combinations of chemicals were greater was that they also suppressed another enzyme, which helps the body rid itself of toxins.

The amounts of the individual pesticides were calculated to have a standard effect on the fish nervous systems, and in some cases were higher than would be expected to be seen in the environment, Scholz said. Some combinations produced effects that added up to the sum of the parts. But as the doses of the individual pesticides increased, the effects became more synergistic - in effect multiplying rather than just adding.

The results indicated that similar effects would occur at much lower levels, and future research will consider just how little exposure is needed to harm fish, he added.

Another new avenue for research will be how pesticides combine with other water quality problems, such as warm water, to harm salmon, Scholz said.