Joint microstructure analysis and isotopic composition of archived scales suggest non-stationary correlations between Atlantic salmon growth and oceanic conditions over the past 50 years

In marine ecosystems, plankton abundance and composition can vary drastically over space and time, influencing fish population resilience. Atlantic salmon abundance has declined sharply since the 1970s, partly due to lower growth and survival rates at sea, despite bans of most marine salmon fisheries. Trophic limitation in the North Atlantic is suspected as a main driver, but no clear causal link between food resources and individual growth has been demonstrated. Archived scales have the potential to relate trophic conditions to salmon growth, as their isotopic composition reflects that of their environment and prey. Analysis of 4,722 salmon scales from southern European populations revealed common trends between a reduced growth and changes in the stable-isotopic composition (δ15N and δ13C) over the past five decades, supporting the hypothesis that trophic limitation at sea has contributed to the widespread decrease in salmon abundance. Non-stationary correlations between morphological traits and isotope ratios highlighted a ubiquitous yet non-linear response, which further complicate the interpretation of temporal signals and require carefully interpreting the isotopic composition of fish scales in multi-decadal studies.