Compound thermal indices for two species of salmonids

ABSTRACT Water temperature is a determinant variable for the overall health of the river ecosystem and aquatic biota, particularly for cold-water fish. Therefore, the characterization of river temperature is essential for the management of thermal habitats. However, currently, river thermal regime characterization is often achieved by calculating numerous thermal indices that are not often related to cold-water fish physiological requirements and thermal preferences. In this study, we developed a compound thermal index (CTI) based on a methodology used to calculate the water quality index (WQI) in Canada. CTI is composed of specific indicators related to the thermal tolerance thresholds for different life stages for two cold-water species (Atlantic salmon and brook trout), providing a simplified measure of the quality of the thermal habitat for these species. CTI was determined in two salmon/trout rivers in Québec, Canada (Ouelle and Ste-Marguerite). The results showed that (i) CTI allowed the characterization and classification of thermal habitat quality; (ii) the thermal habitat degradation was primarily influenced by climate conditions, particularly during warm and dry years with high temperatures and low precipitations; (iii) the improved thermal habitat quality was associated with air temperature and precipitation values close to seasonal normals; (iv) cold tributaries provided excellent thermal habitats.