Summary:
Young Chinook salmon (Oncorhynchus tshawytscha) rely on different physiological mechanisms to withstand rising water temperatures as they develop, according to a study published in Journal of Experimental Biology. The research shows that the physiological basis of heat tolerance changes across early life stages, offering new insight into how climate-driven warming may affect juvenile salmon in freshwater.
Researchers from the University of British Columbia (UBC), Fisheries and Oceans Canada, and the National Institute of Aquatic Resources in Denmark investigated the physiological factors associated with fatigue during swimming under acute warming. The team compared fry, about two months old, with older parr, around six months old, after acclimating the fish to different water temperatures to examine how and why thermal tolerance differs between life stages.
In fry, the ability to remain aerobic for longer while swimming and maintain lower lactate levels appeared to be an important factor in delaying fatigue during acute warming. In older parr, however, these factors no longer explained differences in heat tolerance, indicating that other physiological processes become more important as the fish mature.
The findings show that juvenile salmon should not be treated as a single group when assessing their vulnerability to warming rivers. Recognizing these life stage differences could improve predictions of how salmon populations respond to climate-driven warming and may help inform conservation planning for freshwater salmon.
— Press Release —
Warming streams are pushing young salmon beyond their limits: new research
As climate change warms rivers across B.C., young salmon are facing increasing heat stress at vulnerable stages of their lives.
Two new studies from UBC’s Pacific Salmon Ecology and Conservation Lab found that younger fish cope with heat differently than older fish, and that current methods may be underestimating the risks salmon face in warming waterways.
We spoke with Dr. Scott Hinch, professor in UBC’s faculty of forestry and environmental stewardship, about what these findings mean for salmon conservation.
What did you investigate?
We examined how young salmon respond to warming water and whether current methods accurately measure heat tolerance. Most studies assess fish at rest, but juvenile salmon are constantly moving to find food and avoid predators, so we need to mimic real-world conditions. We also considered how age and prior temperature exposures influence responses.
Collaborating with researchers from Fisheries and Oceans Canada, and the National Institute of Aquatic Resources in Denmark, we studied juvenile Chinook salmon from the Nicola River in B.C. We compared two life stages: fry, about two months old, and parr, around six months old. One study, published in the Journal of Experimental Biology, explored the biological mechanisms that influence heat tolerance, while a second study, accepted in Conservation Physiology, examined swimming performance and survival at different temperatures.
What did you find?
When monitored while swimming, fish were less able to physiologically cope with warm temperatures like they could in stationary trials, suggesting that traditional tests may overestimate their heat tolerance in natural conditions.
We identified 20 °C as an important threshold. Fish acclimated to this temperature or warmer performed better up to that point, but beyond it their ability to cope declined.
Surprisingly, fry handled warmer temperatures better than parr, tolerating about 1.8 °C higher on average. We think older fish may struggle to pump enough oxygen-rich blood to their muscles as temperatures rise.
Fish that were acclimated to cooler temperatures like 12 to 15 °C, but swam vigorously at 24 °C died shortly after the trials, suggesting an inability to recover from a single rapid heat event. A juvenile salmon living in a cool river that encounters really warm temperatures may not survive such a rapid temperature change.
Read also: Climate change stunts Chinook salmon growth in Alaska
What do these findings mean for salmon conservation?
Heat stress affects salmon differently at different life stages, so conservation and management strategies need to consider the full freshwater life cycle and habitats. Some populations do show resilience despite continued warming, but supporting that resilience means reducing thermal stressors, especially during the warmest periods. This may include maintaining and protecting riparian trees and groundwater sources to help keep waterways cool.
What does 2026 look like for B.C. salmon in fresh and salt water?
Forecasting salmon returns is complex. When juveniles migrated to the ocean two years ago, they encountered cool, food-rich coastal waters linked to La Niña conditions, so returns this year of some populations may be similar to last year.
However, current El Niño conditions could pose challenges particularly for returning adult salmon, which may encounter migration and spawning rivers that could be very warm and low due to drought.
For young salmon that will remain in freshwater for another year or two before heading to the ocean, the situation is more concerning. A “super” El Niño is projected to bring warm coastal waters, poor food availability and abundant predators, and a large blob of warm ocean water is also approaching our coast, reflecting cumulative climate change impacts.
The 2025 Fraser River sockeye run exceeded expectations, but a single year does not signal recovery. This year, we should expect continued variability with some runs near average, with others below historical norms.
Journal Reference:
Kim Birnie-Gauvin, Natalie Butler, Scott G. Hinch, Erika J. Eliason, ‘Individual and life stage variation in exercise fatigue under acute warming’, Journal of Experimental Biology 229 (7): jeb251892 (2026). DOI: 10.1242/jeb.251892
Article Source:
Press Release/Material by Charlotte Fisher | University of British Columbia (UBC)
Featured image credit: Vlad Vasnetsov | Pixabay
