Spatial and temporal niche differentiation of two sympatric sandeel (Ammodytes) species

Sandeels (family Ammodytidae) are semi-pelagic fish that play a key role in marine food webs as prey for seabirds, marine mammals, and piscivorous fish. Despite their ecological importance, species-specific spatial and temporal patterns, particularly in areas of geographic overlap, remain poorly understood. We analysed the seabed occurrence of two sandeel species (Ammodytes marinus and A. tobianus) collected during the annual Dutch shellfish benthic survey in spring (2016–2018, 2020–2022). Niche differentiation was examined in relation to depth, sediment characteristics, bivalve biomass, salinity, and temporal variables (day of year, relative time-of-day). Both species showed an offshore–nearshore segregation, with A. marinus dominating offshore and A. tobianus in nearshore areas. While depth largely explained this pattern, substantial unexplained variation remained, suggesting that additional drivers influence their spatial segregation. Furthermore, concurrent measurements of bivalve biomass were negatively associated with the abundance of both sandeel species, suggesting that dense bivalve aggregations may hinder burrowing behaviour, or that sandeel and bivalve differ in sediment preference. Areas of co-occurrence were also observed, and both species of sandeel showed similar diurnal patterns, with higher densities in the evening, as well as comparable interannual trends. In conclusion, although generally segregated across the study area, their overlapping distributions, lack of diurnal differentiation, and similar annual trends suggest that these morphologically similar species can coexist locally without complete competitive exclusion. The resulting distribution maps and temporal insights, including diurnal, seasonal, and long-term abundance trends, could contribute to studies of sandeel–predator interactions and improve our understanding of the species-specific roles that sandeel play as forage fish in coastal ecosystems.