This study provides a regional picture of long‐term change in Atlantic salmon growth at the southern edge of their distribution, using a multi‐population approach spanning 49 years and 5 populations. We provide empirical evidence of salmon life history being influenced by a combination of common signals in the marine environment, as well as population‐specific signals. We identified an abrupt decline in growth from 1976 and a more recent decline after 2005. As these declines have also been recorded in northern European populations, our study significantly expands a pattern of declining marine growth to include southern European populations, thereby revealing a large‐scale synchrony in marine growth patterns for almost 5 decades. Growth increments during their sea sojourn were characterised by distinct temporal dynamics. At a coarse temporal resolution, growth during the first winter at sea seemed to gradually improve over the study period. However, analysis of finer seasonal growth patterns revealed ecological bottlenecks of salmon life histories at sea in time and space. Our study reinforces existing evidence of an impact of early marine growth on maturation decision, with individuals of small size at the end of the first summer at sea being more likely to delay maturation. However, each population was characterised by a specific probabilistic maturation reaction norm, and a local component of growth at sea in which some populations have better growth in some years, might further amplify differences in maturation rate. Differences between populations were smaller than the differences between sexes, suggesting that sex‐specific growth threshold for maturation is a well‐conserved evolutionary phenomenon in salmon. Finally, our results illustrate that although most of the gain in length occurs during the first summer at sea, the temporal variability in body length at return is buffered against the decrease in post‐smolt growth conditions. The intricate combination of growth over successive seasons, and its interplay with the maturation decision, could be regulating body length by maintaining diversity in early growth trajectories, life histories, and the composition of salmon populations. This article is protected by copyright. All rights reserved.