The small pelagic fish food landscape and its use by common terns Sterna hirundo in the Wadden Sea

Abstract

Small pelagic fish (SPF) play a vital role in marine ecosystems, transferring energy from plankton to higher trophic levels. However, SPF dynamics in shallow coastal areas are often under-studied, despite their importance as crucial prey near breeding sites of fish-eating birds such as common terns ( Sterna hirundo ). This study had two connected objectives: to map the pelagic fish food landscape in the inlets of the Dutch Wadden Sea and to describe its use by common terns. Two hydro-acoustic fish surveys were conducted in May and October 2022, while tern distribution was assessed using aerial counts and GPS tracking. Prey selection was analyzed through camera traps and DNA analysis of faeces. Herring and sprat were the predominant species in the SPF community, with sandeel also abundant in May. Smelt was most common in freshwater-influenced areas. Most fish were below 12 cm, fitting the preferred size range for common terns. SPF energy density ranged from 20.1 to 22.1 kJ/g dry weight, with sprat having the highest values. SPF biomass showed significant spatiotemporal variability across transects, inlets, and seasons. Common terns foraged frequently around the gullies, feeding mostly on herring. A significant correlation was found between fish biomass and common tern density, indicating food landscape may be a good predictor of tern distribution and vice versa. This study presents the first integrated biomass estimates of SPF in the subtidal areas of Dutch Wadden Sea for both spring and autumn, providing essential data for future energy budget models and management strategies.

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