Common agricultural pesticides that attack the nervoussystems of salmon can turn more deadly when they combine with otherpesticides, researchers have found.Scientists from the NOAA Fisheries Service and Washington StateUniversity were expecting that the harmful effects would add up asthey accumulated in the water.They were surprised to find a deadly synergy occurred with somecombinations, which made the mix more harmful and at lower levels ofexposure than the sum of the parts.The study looked at five common pesticides: diazinon, malathion,chlorpyrifos, carbaryl and carbofuran, all of which suppress an enzymenecessary 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 showthe true risks, especially for fish protected by the EndangeredSpecies Act, the authors concluded in the study published Monday inthe journal Environmental Health Perspectives."We need to design new research that takes into effect the real-worldsituation where pesticides almost always coincide with otherpesticides," co-author Nathaniel Scholz, a research zoologist at theNOAA Fisheries Service Northwest Fisheries Science Center, said fromSeattle.Inge Werner, director of the aquatic toxicology laboratory at theUniversity of California at Davis, was not involved in the study. Shesaid while the idea was not new, the findings were definitive, even atlevels 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 thatbasically render them unfit for survival in the wild are much moreimportant. In certain areas, pesticides really are a very importantfactor" in salmon survival.Jeffrey Jenkins, professor of environmental and molecular toxicologyat Oregon State University, was not part of the study. He said thestudy was well done, but it would take more research to push the U.S.Environmental Protection Agency to change its pesticide testingstandards as they relate to fish, which are defined by law.Last year, NOAA Fisheries issued findings under the Endangered SpeciesAct that diazinon, malathion and chlorpyrifos jeopardize the survivalof all 28 species of Pacific salmon listed as threatened or endangeredin the West.The three chemicals, found by the U.S. Geological survey tocontaminate rivers throughout the West, interfere with salmon's senseof smell, making it harder to avoid predators, locate food and evenfind their native spawning streams and reproduce. At higherconcentrations, they kill fish outright.NOAA Fisheries and EPA must evaluate 34 more pesticides by 2012 underterms of a settlement reached in a lawsuit brought by NorthwestCoalition for Alternatives to Pesticides and others.In the study, scientists combined the pesticides two at a time atvarious concentrations, then exposed juvenile coho salmon in tanks forfour days. Many of the fish died outright.Fish that survived were killed, and their brains analyzed for thelevels of the enzyme acetylcholinesterase, which allows impulses tomove between neurons in the brain. In every fish, the levels of theenzyme were below the level considered healthy.Earlier research found that lower levels of the enzyme affected theability of fish to feed and swim, which would affect their ability tosurvive, Scholz said.The researchers suggested that the reason harmful affects ofcombinations of chemicals were greater was that they also suppressedanother enzyme, which helps the body rid itself of toxins.The amounts of the individual pesticides were calculated to have astandard effect on the fish nervous systems, and in some cases werehigher than would be expected to be seen in the environment, Scholzsaid. Some combinations produced effects that added up to the sum ofthe parts. But as the doses of the individual pesticides increased,the effects became more synergistic — in effect multiplying ratherthan just adding.The results indicated that similar effects would occur at much lowerlevels, and future research will consider just how little exposure isneeded to harm fish, he added.Another new avenue for research will be how pesticides combine withother water quality problems, such as warm water, to harm salmon,Scholz said.