Thermal exposure risk in different life stages of Chinook salmon in the Nechako River system, British Columbia

Climate change is affecting freshwater systems, leading to increased water temperatures, which is posing a threat to freshwater ecological communities. In the Nechako River, British Columbia, a water management program has been in place since the 1980s to maintain water temperatures at 20 °C during the migration of adult Sockeye salmon. However, the program's effectiveness in mitigating the impacts of climate change on resident species like Chinook salmon's thermal exposure is uncertain. In this study, we utilised the CEQUEAU hydrological model and life stage-specific physiological data to evaluate the consequences of the current program on Chinook salmon's thermal exposure under two contrasting climate change and socio-economic scenarios (SSP2-4.5 and SSP5-8.5). The results indicate that the thermal exposure risk is projected to be above the optimal threshold for parr (intermediate juvenile) and adult life stages under both scenarios relative to the 1980s. Under the SSP5-8.5 scenario, these life stages could experience an increase in thermal exposure ranging from two to five times higher by the 2090s compared to the 1980s. This exposure is projected to occur during the months in which these life stages emerge, including the period when the program is active (July 20th to August 20th). Additionally, our study shows that climate change will result in a substantial rise in cumulative heat degree days, ranging from 1.9 to 5.8 times (2050s) and 2.9 to 12.9 times (2090s) in comparison to the 1980s under SSP5-8.5. Our study highlights the need for a holistic approach to reviewing the current Nechako management plan, ensuring that all species in the Nechako River system are considered especially in the face of climate change.