Warm River Temperatures Make Pesticides More Harmful to Salmon


Low snowpack and an early hot summer have led to warmer temperatures for Pacific Northwest rivers. Temperatures have risen into the 70s in the majority of rivers with monitoring stations in Oregon, Washington and California. These high temperatures have been blamed for thousands of recent adult and juvenile salmon deaths of both wild and hatchery fish. Salmon need cold water for optimal health, preferring temperatures in the 50s.

Did you know that three recent scientific studies have shown that pesticide toxicity to fish rises with increasing water temperatures? That means it takes less pesticides in warmer rivers to cause adverse effects to salmon, including death. (See Below.)

Our Clean Water for Salmon campaign works hard to monitor Environmental Protection Agency (EPA) actions on pesticides that affect salmon. Help us keep the pressure on EPA over this next year when some key decisions will be made on three organophosphates: chlorpyrifos, malathion, and diazinon. Follow us on Facebook or Twitter to get the timeliest information on how you can take action, and be sure to read future newsletters. We’ll keep you informed when your voice needs to be heard.

We’re also working to reduce the amount of malathion entering the environment by bringing effective ways to manage pests like spotted wing drosophila to grower audiences. You can register for our August 4 workshop for soft fruit growers here.

Studies showing pesticide toxicity to fish rises with increasing water temperatures:

Laetz et al. (2014) studied the toxicity of malathion (an insecticide used in fruits, vegetables, and mosquito control) and ethoprop (an insecticide and nematacide used in various vegetables) on juvenile coho salmon in mixture at temperatures between 12-21°C (54-70°F).  Toxicity effects of these two pesticides in mixture doubled when fish were exposed to similar concentrations at 64°F, compared with exposures at 54°F.


A study by Dietrich et al. (2014) examined the effect of acute and sublethal exposures to malathion on ocean-type subyearling Chinook salmon at optimal 11 °C (52°F) and elevated 19°C  (66°F) temperatures.  The amount of malathion necessary to kill 50% of the salmon test subjects (LC-50) was significantly less (274.1 μg/liter) at 66°F than at 52°F (364.2 μg/liter).  Researchers also noted that mortality increased 11.2% in Chinook salmon exposed to malathion at the elevated temperature and challenged with the disease microbe Aeromonas salmonicida compared to fish held at the optimal temperature. 

Patra et al. (2015) tested the impact of endosulfan (no longer legal for use in the U.S), chlorpyrifos (an insecticide used on food, feed, turf, and wood products), and phenol (used in disinfectants and a precursor for some herbicides) to rainbow trout and other fish species at temperature intervals ranging from 15-35°C (59-95°F).  Acute effects (death) of rainbow trout with exposures to each pesticide increased with increasing temperatures.

Laetz C.A., Baldwin D.H., Hebert V.R., Stark J.D., Scholz N.L. 2014. Elevated temperatures increase the toxicity of pesticide mixtures to juvenile coho salmon. Aquat Toxicol 146:38–44.

Patra R.W., Chapman J.C., Lim R.P., Gehrke P.C. and Sunderam R.M. 2015. Interactions between water temperature and contaminant toxicity to freshwater fish.   Environmental Toxicology and Chemistry 34:1809–1817.

Dietrich, J.P., Van Gaest, A.L., Strickland, S.A., and Arkoosh, M.R.  2014.  The impact of temperature stress and pesticide exposure on mortality and disease susceptibility of endangered Pacific salmon.  Chemosphere: 108:353-359.

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