Sprat availability for seabirds in marine bird directive areas of the Dutch North Sea

Small pelagic fish are a key food resource for seabirds, which are protected under the EU Birds Directive, yet pelagic fish availability for seabirds is difficult to quantify at ecologically relevant spatial and temporal scales. In this study, we assessed sprat density for seabirds in the Dutch North Sea using acoustic survey data, with a focus on designated marine Birds Directive areas. We analysed data from the Acoustic Survey for Herring and Sprat (HERAS), which provides high-resolution, spatially extensive observations of pelagic fish and a consistent long-term time series for June-July (2016-2024). To present contrasting foraging strategies, while ensuring ecological and spatial alignment with the available fish data, we focusing on two seabird species, the common guillemot (Uria aalge) and the northern gannet (Morus bassanus), that occur within Natura 2000 areas and rely on pelagic fish. Based on their dietary overlap, we selected sprat (Sprattus sprattus) as a shared and ecologically relevant prey species. We then examined sprat distribution within two contrasting Natura 2000 sites, the Brown Ridge and the Frisian Front, which coincide with the core ranges of these seabirds. Sprat availability inside and outside the bird areas was compared using an Enrichment Index (EI), defined as the ratio of mean sprat density within a certain bird area to that in the total Dutch Exclusive Economic Zone. Spatial and interannual variability were characterised using measures of central tendency and dispersion. Sprat density was found to be sparse and highly patchy, with water depth emerging as the only environmental covariate showing a clear ecological signal. Interannual variability was pronounced. Relative Sprat abundance was consistently higher in the Frisian Front than in surrounding waters, suggesting that this frontal system functions as a persistent high-prey -density area for sprat-consuming seabirds. In contrast, sprat abundance at the Brown Ridge was consistently lower, indicating comparatively poorer sprat conditions during the survey period. These findings support previous evidence that frontal systems act as predictable prey concentration zones, whereas sandbank regions such as the Brown Ridge may not reliably sustain comparable densities of pelagic fish. These results highlight the importance of bathymetry and hydrographic structure in shaping sprat distribution and demonstrate how acoustic survey data can support assessments of typical prey availability and its variability within Birds Directive areas. Future work may extend this framework by incorporating additional surveys, seasonal data, additional prey species, and explicit propagation of model uncertainty, and by linking prey abundance and accessibility more directly to seabird abundance.