Unraveling fishing and upwelling impacts on Sardinella aurita and Sardina pilchardus in the Canary Current Upwelling

Abstract In the Canary Current Large Marine Ecosystem (CCLME), significant variations in upwelling dynamics have been observed in the last decades, but their impact on small pelagic fish biomass remains unclear. Identifying the factors driving these stock variations is challenging, particularly for heavily overexploited species. In this study, we attempt to disentangle the influence of fishing and environmental factors on the biomass of Sardinella aurita and Sardina pilchardus using structural causal models. Fisheries data alone cannot explain the year-to-year variations in biomass, indicating that other factors are at play. Furthermore, unrealistic results using fishing data point to high levels of illegal, unreported, and unregulated catches, leading to an underestimation of S. aurita biomass. In the tropical subregion (southernmost CCLME), fluctuations in the biomass of S. aurita are primarily driven by upwelling dynamics rather than temperature or salinity. A similar pattern is observed in the Western Sahara subregion, where primary productivity, along with the previous year’s biomass of S. aurita, is the main driver of biomass fluctuations. Additionally, a negative relationship between S. aurita biomass in the two subregions suggests that the Western Sahara may serve as a refuge during periods of declining suitable habitat in the tropical subregion. In the Western Sahara subregion, fluctuations in S. pilchardus biomass are influenced by primary productivity, while in the South Morocco subregion the driving factor is likely temperature. The results highlight the significant influence of environmental variables on pelagic fish biomass, underscoring the potential impacts of climate change on the distribution and abundance of these species. Such shifts could profoundly affect local economies and food security, while also heightening geopolitical tensions among nations competing for shared marine resources. Comprehensive evaluation and effective management strategies that account for environmental changes are urgently required.