Q&A: What happens when warming streams push young salmon beyond their limits

As global temperatures continue to rise, the impact on wildlife is becoming increasingly evident. In British Columbia, researchers have been investigating how rising stream temperatures affect young salmon, revealing critical insights into their vulnerability to heat stress. Two studies conducted by UBC's Pacific Salmon Ecology and Conservation Lab highlight the differing responses of young salmon to warming water, indicating that traditional methods of assessing heat tolerance might not fully capture the challenges these fish face in their natural environments.

The research team, led by Dr. Scott Hinch, examined the physiological responses of juvenile Chinook salmon from the Nicola River in B.C., focusing on two distinct life stages: fry, approximately two months old, and parr, around six months old. Their findings suggest that when monitored while swimming, fish exhibited reduced physiological capacity to cope with warm temperatures compared to stationary trials, implying that conventional assessments may overestimate their heat tolerance in real-world scenarios.

A crucial threshold identified in the study was 20°C (68°F), where fish acclimated to this temperature or warmer showed improved performance up to that point, but beyond it, their ability to cope significantly declined. Interestingly, fry demonstrated greater tolerance to warmer temperatures than parr, enduring about 1.8°C higher on average. Researchers hypothesize that older fish may struggle to supply sufficient oxygenated blood to their muscles as temperatures increase.

Moreover, fish acclimated to cooler temperatures, such as 12 to 15°C, who then swam vigorously at 24°C (75°F), succumbed shortly after the trials, highlighting the inability to recover from sudden temperature changes. This suggests that a juvenile salmon residing in a cool river encountering abrupt warmth may not survive such an event.

These findings underscore the importance of considering the entire freshwater life cycle and habitats for effective salmon conservation strategies. While some populations exhibit resilience against ongoing warming, supporting this resilience requires mitigating thermal stressors, especially during peak heat periods. Strategies could involve preserving and enhancing riparian trees and groundwater sources to maintain cooler waterways.

Looking ahead to 2026, forecasting salmon returns remains complex. Juveniles migrating to the ocean two years ago encountered favorable conditions, potentially leading to similar returns this year. However, current El Niño conditions could present challenges for returning adult salmon, as they may face warm and low water levels in migration and spawning rivers due to drought. For young salmon remaining in freshwater for another year or two before heading to the ocean, the outlook is more concerning, with projections of warm coastal waters, limited food availability, and increased predation.

While the 2025 Fraser River sockeye run exceeded expectations, a single year does not indicate a broader recovery trend. Expectations for 2026 include continued variability, with some runs near average and others falling below historical norms.

Meanwhile, in the United Kingdom, the effects of extreme heat have manifested in unexpected ways, impacting livestock transportation. During the record-hot summer of 2025, twice as many animals died due to heat stress en route to slaughterhouses compared to the previous year. Government data revealed nearly 6,600 animals—primarily chickens—perished during transport, a significant increase from 3,100 in 2024 and no official cases in 2023.

The data highlights the dire consequences of treating animals as mere cargo, with reports detailing thousands of deaths amidst heat stress, high humidity, and extended journeys. Notably, the summer of 2025 marked the warmest on record in the UK, with human-induced climate change making this heat 70 times more likely to occur. Specific incidents saw almost 1,000 chickens perish during a heatwave on 11 July 2025, with some exhibiting visible signs of distress, including panting and immobility. On 12 August, 2,154 chickens died due to heat stress, with body temperatures reaching as high as 46°C.

Efforts to prevent such tragedies include implementing measures outlined by UK government guidelines, such as ensuring proper water and ventilation systems on transport vehicles, avoiding travel during the hottest or coldest parts of the day, and recognizing signs of heat and cold stress in animals. Despite these efforts, the number of "dead on arrival" incidents caused by cold and heat stress increased by more than 50% between April 2024 and March 2025 compared to the same period the previous year.

Animal welfare advocates emphasize the urgent need to address the ethical implications of industrial farming practices, calling for a shift towards a plant-based food system to mitigate the suffering of animals subjected to extreme conditions. As climate change continues to intensify, both aquatic and terrestrial ecosystems face unprecedented challenges, necessitating comprehensive strategies to safeguard biodiversity and ensure sustainable practices.