An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

San Francisco Bay Delta

2020 Chinook Salmon Interpopulation Thermal Tolerance Investigation

EPA funded an investigation of Chinook salmon populations in Oregon and California to determine whether there is 1) a difference in peak performance temperature among the populations and 2) a trade-off between increased thermal performance at higher temperatures and remaining capacity to acclimate. The study considered multiple measures of physiological performance (aerobic scope, critical thermal maximum, and growth) over a range of environmentally relevant temperatures in laboratory conditions. The study examined performance only and did not consider how factors in the ecological setting (e.g., diet, competition, predators, disease, duration, and habitat quality) might reduce a salmon’s capacity to tolerate high temperatures in the natural setting.

The following list summarizes key observations in the final report, Interpopulation Variation among Juvenile Chinook Salmon from California and Oregon.

  • Peak performance temperatures were higher in the Central Valley fall-run population than in the two Oregon fall-run populations and in the winter-run Central Valley population.
  • The Central Valley fall-run population showed minimal capacity for further acclimation at the highest acclimation temperature, suggesting that it is susceptible to a further increase in the temperature of its habitat.
  • The Central Valley winter-run population showed a decrease in peak performance temperature with increased acclimation temperature, implying reduced warm-temperature performance in winter-run populations.
If you experience a problem reading a document with assistive technology, please contact us.
You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.