Abiotic and biotic drivers shaping the schooling behavior of small pelagic fish in multispecies environments.

Small pelagic fish play a vital role in pelagic ecosystems and global fisheries, but their aggregative behavior poses challenges for accurate assessments. While the basic functions of schooling-defense, foraging, migrating and mating-are well-documented, the dynamics and variability of these aggregations are less well understood. This study analyzes data from the JUVENA survey (2013-2020) in the Bay of Biscay, examining 54,240 schools from 158 daytime fishing hauls. We analyzed the effect of local biomass fluctuations, dolphin density, planktivorous competition, euphotic zone depth, phytoplankton concentration, and geostrophic velocity on school size, packing density, and number for European anchovy (juvenile and adult), European sardine, and Muller's pearlside. Results indicated that changes in packing density and the number of schools are positively correlated with local biomass fluctuations, while school size is more significantly influenced by interspecies interactions and environmental conditions. The schooling behavior of juvenile anchovy is mainly density-dependent. In contrast, adult anchovy, sardine, and Muller's pearlside form smaller and more numerous schools under competitive or low food availability conditions offshore, and larger, denser, but fewer schools in low visibility coastal areas with high predator density as a defensive strategy. This study advances our understanding of the primary drivers of small pelagic fish schooling behavior in the Bay of Biscay, providing insights for effective management.