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Study Looks at Impacts of Warmer Stream Temperatures/Pesticide Combination on Subyearling Chinook by Staff Columbia Basin Bulletin, April 18, 2014

Higher stream temperatures increase the sensitivity of subyearling chinook salmon in the Columbia River Basin to pesticides and the combined exposure to temperature and pesticides, such as malathion, may also make the fish more susceptible to disease.

According to a recent article in an online journal, the interaction of stressors occurring at the same time, such as rising temperatures and greater concentrations of malathion -- stressors that already result in higher mortality -- reduce the ability of juvenile salmon to ward off disease, more so than if each stressor occurs in isolation.

This has the potential for population level effects in salmon, according to the article.

"The implications of these preliminary observations are that the susceptibility of salmon to certain diseases will increase as river temperatures increase, and that previous pesticide exposure may exacerbate their susceptibility," said Joseph Dietrich, natural resource specialist, NOAA Fisheries in Newport, Ore.

"The impact of temperature stress and pesticide exposure on mortality and diseases susceptibility of endangered Pacific salmon," the article outlining these findings, appeared in the latest edition of Chemosphere Its authors are Joseph Dietrich; Ahna Van Gaest, research assistant II, Knight Cancer Institute, OHSU; Stacy Strickland, fisheries biologist, former NOAA contractor currently at the Oregon Department of Fish & Wildlife, and Mary Arkoosh, supervisory research microbiologist, NOAA Fisheries.

Malathion is an organophosphate pesticide that has been detected at acceptable concentrations in water typically used by salmon, but at much higher concentrations in water that includes runoff and overspray during pest control activities. It is neurotoxic to salmon and can potentially alter their immune system. Its toxicity increases with higher water temperatures and its application is generally during the summer when those higher temperatures are present. So malathion exposure is greater when fish are already stressed by high water temperatures.

Elevated temperatures can result in temperature stress and is considered a pollutant under the Clean Water Act of 1972. Increasing stream temperatures can cause a decrease in food supply for fish, altered smolt and spawning timing, increased disease and increased toxicity of contaminants.

"The NOAA Fisheries Service's Office of Protected Resources had recently completed a biological opinion on the registration of pesticides containing OPs: chlorpyrifos, diazinon, and malathion," said Dietrich. "The conceptual framework for characterizing the risks of the OPs to ESA-listed species includes the concept of multiple stressors, such as temperature and environmental mixtures of pesticides."

This study, he said, was initiated to fill a gap in knowledge associated with the effects of multiple stressors on chinook salmon, namely the interaction of temperature and organophosphate (OP) pesticide exposure on chinook salmon health. In the study, fish were exposed to one of two temperatures - 11degrees Celsius or 20 degrees C -- and five levels of malathion.

NOAA's Ecotoxicology Program in Newport began investigating the interaction of pesticide exposure and temperature on acute mortality and disease susceptibility, while the Ecotoxicology Program in Seattle continued their investigations into the interaction of environmental mixtures, he added.

In addition to this study of the interaction of temperature and malathion, after exposure to malathion, the authors tested the susceptibility of juvenile salmon exposed to bacterial pathogen, Aeromonas salmonicida, that causes furunculosis in salmon.

"This was the first preliminary investigation into the effects of malathion and temperature on the immunity of chinook salmon," Dietrich said. "Although we did not observe any statistically significant effects, we did observe an increase in disease associated mortality due to temperature alone and a further increase in disease associated mortality when the salmon had been previously exposed to malathion and the elevated temperature."

The implications of these preliminary observations are that the susceptibility of salmon to certain diseases will increase as river temperatures increase, and that previous pesticide exposure, even exposure at sub-lethal levels, may exacerbate their susceptibility, he added.

Subyearling chinook salmon previously exposed to malathion at an elevated temperature had the highest mortality during the subsequent disease challenge (exposing the fish to Aeromonas salmonicida). Fish exposed to malathion at 20 degrees C had an 11.2 percent greater reduction in survival than fish exposed at 11 degrees C.

The threat to salmon is pervasive. According to the article, over 90 percent of water samples from rivers and lakes in developed urban and agricultural areas contain a mixture of more than one pesticide. The authors suggest more studies that include mixtures of varying temperature and pesticides, including tests with a disease challenge used in this study.