Regional synchrony in weight-at-age variability among pelagic fish stocks linked to competition and hydroclimatic factors

Synchrony in population dynamics is often linked to spatially correlated environmental factors. However, synchrony in biological traits, such as weight-at-age (WAA), remains rarely investigated and may also result from competitive trophic interactions. In this study, we analyzed temporal changes in herring (Clupea harengus) and sprat (Sprattus sprattus) WAA in the Baltic Sea. We hypothesized that the variability in WAA is synchronized and linked to competition and large-scale environmental changes. To test this, we analyzed data from 1974 to 2023 from three herring stocks (Central Baltic, Gulf of Bothnia, and Gulf of Riga) and one sprat stock (entire Baltic Sea). Using linear mixed models, we found that year effects—systematic interannual deviations observed across all age groups—accounted for 44–81% of the variance from random effects on mean WAA across four stocks, while cohort effects accounted for 5–48% across these stocks. The dynamic factor analysis (1980–2023) indicated a high level of synchrony and identified common trends in the year effect of mean WAA among stocks despite contrasting trends in stock size and fishing mortality. The first common trend was significantly correlated with sprat stock size and surface water salinity, while the second common trend correlated with temperature records. Our results highlight the predominant influence of competition and environmental factors on the year effect of mean WAA in the herring and sprat stocks, demonstrating that these factors can cause strong synchrony in the investigated trait among stocks at the regional level.