Potential thermal refuge zones in Atlantic salmon rivers in Quebec (Canada) based on multi-site modelling

Thermal refuges are key protective habitats for some Atlantic salmon (Salmo salar) life stages during warm water events. This study describes models to estimate the area of cold tributary plumes located at the confluence of tributaries and main river stem that may serve as refuges for salmonids. Potential thermal refuge area (PTRA), i.e. the number of m2 downstream of the confluence with cooler temperature than the main river stem, was modelled at a regional (i.e. multiple sites in multiple rivers) scale for the first time using two statistical approaches: principal component regression (PCR) and a generalized additive model (GAM). PCR explained a relatively high percentage of PTRA variance (> 70%) when models were developed separately for two data subsets: PTRA < 200 m2 and PTRA ≥ 200 m2. However, PCR is constructed using a large combination of predictors (7) and may overfit the data. The GAM model was implemented for the entire database (i.e. all refuges, regardless of area) and explained 53% of the variance in PTRA. The width of the river main stem and drainage area of the tributary were selected as significant predictors in the GAM. The coefficient of determination between observed PTRAs and those estimated by the GAM model is > 50%. This study is a promising first step in the development of predictive PTRA models, and it is anticipated that model performance will improve as the sample size increases. A PTRA model will be instrumental to managers for the prioritization of larger, stable refuges for protection.