Rising summer temperatures undermine threatened charr populations in the Shiretoko Peninsula

Climate change poses increasing risks to cold-adapted species, particularly through rising summer temperatures. However, the demographic impacts of thermal stress on these species remain poorly quantified. To address this, we estimated three population indices of a threatened charr species, Southern Asian Dolly Varden (SADV, Salvelinus curilus ), including fish density, biomass, and environmental DNA (eDNA) concentration, across 8-24 streams in the Shiretoko Peninsula, a UNESCO World Natural Heritage site in Japan. By integrating these indices with water temperature records from the preceding summer, we identified significant effects of summer water temperatures on SADV populations in the following spring, with particularly strong relationship detected using eDNA. In addition, comparisons of summer water temperature metrics indicated that streams containing several artificial in-stream barriers upstream of the survey sites exhibited, on average, 2°C higher summer temperatures than streams with fewer barriers ( p < 0.01). These streams also exhibited 15-28% higher proportions of time above 16°C (>16°C(%), p < 0.05), a threshold known to suppress foraging activity in this species. Regression analyses further suggest that a 2°C rise in mean summer temperature or a 20% increase in >16°C(%) could lead to population declines exceeding 40%. These findings highlight the high climate sensitivity of the cold-adapted species and underscore the urgency of incorporating thermal vulnerability into river management and conservation planning under future climate scenarios.