Assessing resource use as a predictor for Atlantic salmon ( <i>Salmo salar</i> ) smolt body size

Atlantic salmon (Salmo salar) have experienced significant population declines in eastern Canada for the past 30 years, primarily attributed to at-sea mortality. To identify factors contributing to smolt body size, which has been associated with variations in survival, we examined various predictors in linear mixed-effect models using data from out-migrating smolts collected from 2000 to 2016 at three rivers: Conne River (NFLD), de la Trinité River (QC) and St. Jean River (QC). Our analysis revealed that resource use (i.e., δ¹⁵N and δ¹³C) was a significant predictor for smolt length but not condition, explaining 66.2% of the variation in fork length and 19.1% of variation in condition factor. Furthermore, we observed no significant increasing or decreasing trends in body size across the 17-year period [Correction added on 4 September 2025, after first online publication: The time period in the preceding text has been corrected in this version.] for any rivers studied. Notably, rivers located in the boreal ecoregion did have the largest smolt sizes. This study identifies key predictors of increased smolt body size using a comprehensive long-term dataset, providing valuable insights into the dynamics of three major salmon rivers in eastern Canada, and indicates stable trends in smolt body size over time.