Spatial Distribution of European Grayling Reflects Longitudinal Temperature Patterns in a Swiss River

ABSTRACT Most salmonid populations are declining across their entire habitat range, partly because of large‐scale loss of crucial physical habitats. Alterations in river flow and temperature resulting from climate change are likely to further degrade habitat quality, particularly summer thermal conditions experienced by temperature‐sensitive fish species. Understanding how summer thermal conditions control the spatial distribution of ectotherms is thus central to helping project the consequences of climate change and develop management solutions. This study uses snorkelling fish surveys collected over 10 years and airborne thermal infrared (TIR) mapping of surface temperature acquired in 2022 to assess the relationship between European grayling distribution and thermal habitats along a 9‐km long reach of the Allondon River, Switzerland. Results show that all 3 grayling life stages (adults, sub‐adults and juveniles) respond negatively to elevated summer temperature, with distribution patterns highlighting thermal structuring effects on fish populations. The presence of two cooler reaches appears critical to the survival of the Allondon's declining grayling population, while the warmest reach that separates these habitats potentially acts as a thermal barrier during critical summer conditions. These results were used to guide local stakeholders towards short‐term and longer‐term actions to be taken on the river, which include: concerted trans‐national management to protect key upstream tributaries, tree planting to limit summer peak temperature and strategic protection of cold‐water patches that may act as thermal refuges during critical periods.