<p>Small pelagic fish are essential to marine food webs and human food security. These species typically aggregate in large schools comprising thousands of individuals. Given that the spatial distribution and intrinsic school characteristics (such as packing density and size) of small pelagic fish influence predation rates, fisheries catchability and biomass estimation, no consensus has yet been reached regarding density-dependent effects on school characteristics and spatial density. This study aims to investigate the influence of population density on school size, packing density, and school number in the East Greenland capelin (<i>Mallotus villosus</i>), a predominant small pelagic fish species in sub-arctic waters. We reanalyzed acoustic surveys conducted annually between 2018 and 2020 to monitor the capelin stock on the East Greenland shelf. We detected capelin schools in the echograms using a pixel clustering algorithm and computed cross-sectional school area, packing density, and school number per nautical mile. These metrics were then analyzed in relation to local acoustic density measurements using Generalized Additive Models (GAM). Our analysis revealed a statistically significant non-linear relationship between local population density and both school cross-sectional area and encounter rate, while no such link was found for average packing density. Additionally, an aggregation index derived from Lorenz curves of school sizes indicated increasing variability in school size with increasing local population density. At low to medium population densities, both the number and size of schools increased while, at further increasing population density, only the school size continued to grow with a greater concentration of biomass within fewer, larger schools.</p>