Shellfish reefs as Essential Fish Habitats: complementary insights from eDNA and BRUVs in the North Sea.

Worldwide, biogenic reefs – i.e. reefs shaped by organisms such as corals, tubeworms and bivalves – have faced extensive degradation, resulting in a vast reduction of their extent. The remaining reefs are essential habitat for marine vertebrates, providing food, shelter, and nursery grounds. Despite their ecological significance, the role of natural shellfish reefs as essential fish habitats is poorly understood, particularly in temperate regions. In this study, we assessed the effect of reef presence on the marine vertebrate diversity and composition and compared traits for living habit and feeding strategy of two shellfish reefs in the North Sea. In addition, we assessed the complementary value of two non-invasive techniques: eDNA metabarcoding and baited camera analysis (BRUVs). Selected stations inside and outside reef areas in a mixed blue mussel/pacific oyster shellfish reef in the Dutch North Sea and a horse mussel reef in the Scottish North Sea were compared. Vertebrate species diversity was consistently higher within both reefs than in their surroundings, declining with increasing distance from the reefs up to 400-1500 m from the reefs' border. Vertebrate community composition varied significantly, with several species, such as gobies, gunnels, clingfish, and sculpins, occurring more frequently within the reefs. Commercial fisheries species, including cod, pollock, and mullet, also occurred more often with reef presence, and reefs may represent key juvenile habitats for species like ling. Differences in functional diversity showed that reefs attracted mostly invertivores and pelagic species adapting their diet to food availability. eDNA identified more species overall, while BRUVs detected species both in adult and juvenile stages, highlighting the need for including complementary methods in biodiversity assessments. These findings provide valuable insights into the role of temperate reefs as essential fish habitats and inform more effective conservation and fisheries management strategies.