Seabird prey in space and time: combining multiple datasets to model the distribution of sandeel (Ammodytes marinus) and sprat (Sprattus sprattus) in the North Sea

Many seabird populations along the UK's East Coast have declined over the past three decades. A key concern is reduced availability of their primary breeding season prey, sandeel (Ammodytes marinus). While some seabirds have partially switched to sprat (Sprattus sprattus), this is not universal. Studying these predator–prey dynamics requires fine-scale estimates of prey distributions that are both spatially and temporally resolved—yet such data are limited, especially for sandeel due to sampling challenges. We evaluate the utility of large-scale ICES bottom trawl surveys to model sandeel distribution, combining them with winter dredge surveys designed specifically for this species. For sprat, we combine bottom trawl data with the Herring Acoustic Survey. Using hierarchical Bayesian spatiotemporal models (R-INLA), we estimate the biomass of small and medium sandeel (≤ 16 cm) and sprat across the entire North Sea from 2000 to 2020, accounting for spatial and temporal dependencies. We validate our sandeel models by comparison with other modelling approaches and ICES stock assessments. Results reveal marked regional variation: sandeel increased in Area 3 (Norwegian EEZ), declined in Areas 1 and 2 and fluctuated in Area 4. Sprat biomass increased over time, with a modest northward expansion. This study provides the first North Sea-wide, fine-scale, multi-decadal distribution models for these two key seabird prey species and identifies important regional drivers of their populations.