Changes in size‐at‐age of juvenile Atlantic salmon cohorts over the past 50 years and linkages to environmental factors

Quantifying the potential effects of climate change on juvenile salmonid body length at a large spatiotemporal scale is challenging given the multitude of biological processes and methodological constraints. In the present work, we analysed potential changes in size‐at‐age of juvenile Atlantic salmon cohorts over the past 50 years in the Miramichi and Restigouche rivers (Canada), and how they relate to environmental factors. To do so, we built a novel age‐structured hierarchical model expanding on the von Bertalanffy growth function (VBGF) framework to incorporate environmental parameters and the cessation of growth in winter independently among age classes. This model supports the prediction that size at the end of the growing season of all life stages increased from 1970 to 2021 in the Restigouche River and decreased (marginally) over the same period in the Miramichi River. This difference may be due to the Miramichi River being warmer and less suitable for growth than the Restigouche River. However, the inclusion of temperature metrics, density and discharge to growth parameter K did not improve the VBGF models. Similarly, size at the end of the growing season was related negatively to fish density, but no clear relation with temperature metrics or discharge was identified, despite important differences in temperature regimes among rivers. Thus, although trends in size at the end of the growing season followed our predictions given global warming, linking them explicitly to the expected growth–temperature relationship was challenging given the spatial and temporal scales of the data, and perhaps due to the influence of alternative factors.