Spatial Variability of Thermal Refuges Analysed in Relation to Landscape Hydromorphology in the Restigouche River Watershed

English Abstract:

Water temperature is an important variable governing the distribution and behaviour of fish. Salmonids are particularly intolerant of high temperatures and survive heat waves using discrete units of cold water, known as ‘thermal refuges’. Currently, patterns of thermal refuge distribution and the mechanisms governing it are poorly understood, especially at large scales. However, given that climate change poses a threat to salmon populations in Europe and North America, these cold water units are increasingly important for salmonids in the summer. We used airborne thermal infrared thermal imaging (TIR) to characterise the spatial distribution of thermal refuges in nearly 700 km of rivers in the Restigouche River watershed. Thermal refuges were classified into different categories, with those identified as groundwater-driven proving to be the most abundant. GIS analysis was used to assemble a series of landscape metrics that were tested for links with the spatial distribution of groundwater thermal refuges. Jacobs’ selectivity index was used to investigate the relationship between landscape metrics and the local-scale occurrence of thermal refuges. Results showed that the presence of thermal refuges was significantly associated with high values of channel curvature (X2 p <0.05, df = 9). Regression analysis was used to assess correlations between landscape metrics and the larger-scale density of thermal refuges (no. refuges per river km). Using a quadratic model (R² = 0.83, p <0.05), we observed a strong correlation between the river’s entrenchment ratio (a measure of channel confinement) and the density of thermal refuges. Our results indicate that the spatial distribution of groundwater thermal refuges is strongly connected to landscape hydromorphology. This is the first study to examine the spatial distribution of thermal refuges at riverscape scales and it is hoped that such data will help conservation efforts to preserve critical thermal habitats in rivers threatened by climate change.