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.

• Interpopulation Variation among Juvenile Chinook Salmon from California and Oregon (PDF)(68 pp, 2 MB, July 2020)
  This study uses physiological performance testing to explore whether Chinook salmon populations near the species’ southernmost distribution (i.e., California’s Central Valley) may have a higher thermal tolerance than more northerly populations.