Abstract
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., δ 15 N and δ 13 C) 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.