Biogenic reef creation and biodiversity enhancement by an offshore longline mussel farm

• Offshore mussel farming allows seabed recovery after halt in fishing activities. • Creation of biogenic reef after seven years of mussel farming. • Offshore mussel farming increased benthic species abundance and diversity. • This study reports how low-trophic aquaculture support nature restoration. • This study report the potential of offshore mussel farms to act as de facto MPAs. As the mussel farming industry moves offshore, there is a need to fill evidence gaps on its ecological impacts. Over eight years, this research presents the results of a long-term monitoring study of the UK’s first large scale offshore mussel farm following a BACI (Before-After Control-Impact) approach to evaluate the temporal and spatial effects of the farm on benthic habitat and epibenthic species assemblages. Situated in historical fishing ground, the farm is located on a degraded seabed ecosystem dominated by soft sediment. After seven years of farming, damaged seabed increased its complexity, with biogenic reef being created (30 % increase). The structure is attracting a significantly increasing diversity and abundance of sessile, sedentary and mobile species to the area compared to control sites. The abundance and diversity of commercial species increased compared to far control sites and certain commercial species were exclusively recorded beneath the mussel headlines: Cancer pagurus, Maja squinado and Necora puber . Results are discussed through a three mechanisms framework: Species recruitment, settlement, and subsidy. Results from this study are crucial to support policymakers develope a sustainable offshore industry as part of the blue economy that can both produce sustainable protein and contribute to marine conservation objectives as ‘other effective area-based conservation measures’ (OECM). This study reports how low-trophic farming can support nature restoration and recovery.