Spatial distribution of thermal refuges analysed in relation to riverscape hydromorphology using airborne thermal infrared imagery

Abstract

Water temperature governs the distribution and behaviour of fish. Riverine salmonids use discrete cold water patches, termed thermal refuges, when stream temperature exceeds their thermal tolerances. Currently, little is known about the mechanisms driving the spatial distribution of thermal refuges, particularly at large scales. Given the threat posed by climate warming to salmon populations in Europe and North America, thermal refuges are increasingly important for salmonids during summer. In this study, we used airborne thermal infrared (TIR) imagery to characterise the spatial distribution of thermal refuges from ~700km of rivers in an eastern Canada salmon watershed. Thermal refuges were classified into a range of categories, and those identified as being driven by groundwater processes were observed to be both the most abundant and most spatially variable. Spatial analysis was used to assemble a range of landscape metrics that were tested for associations with the spatial distribution of groundwater-driven refuges. Associations between landscape metrics and the individual occurrence of thermal refuges were assessed using Jacobs' selectivity index. Results showed that the occurrence of groundwater thermal refuges was significantly associated with high values of channel curvature and close proximity of incoming tributary valleys (X2p<0.05, df=9). Regression was used to assess correlations between landscape metrics and the density of thermal refuges (number per river km). Channel confinement was found to correlate strongly with the density of groundwater thermal refuges using a quadratic model (R2=0.83, p<0.05). Our study is the first to quantify the relationship between the spatial distribution of groundwater thermal refuges and landscape hydromorphology throughout a riverscape. This information can be used to aid conservation efforts and manage critical thermal habitats in rivers and streams.