Disparate size and stock status between two Sardina pilchardus stocks in northwest African waters

Small pelagic species like the European sardine (Sardina pilchardus) are highly sensitive to fishing pressure and environmental variability. This study evaluates the impact of oceanographic changes on sardine growth and stock status in western African waters. Using length-at-age data from commercial fisheries spanning Morocco to Mauritania, we estimated growth parameters through a Bayesian Monte Carlo Markov Chain approach based on the von Bertalanffy model. Results reveal faster growth in the southern stock compared to the central stock. We applied Generalised Additive Models (GAM) to assess environmental influences, finding positive effects of sea surface temperature (SST) and chlorophyll-a (Chl-a) on sardine size, with additional influence from primary production in the southern stock. Negative impacts were linked to coastal upwelling, atmospheric oscillations, and spatial factors. Incorporating these environmental variables into a length-based spawning potential ratio (LBSPR) model enhanced by machine learning, we estimated fishing mortality (F) and spawning potential ratio (SPR), revealing higher F and lower SPR in the southern stock under warmer, more productive conditions. The central stock showed greater resilience with lower F and higher SPR. These findings highlight the necessity of integrating environmental variability and advanced modelling techniques for sustainable fisheries management in dynamic ecosystems.