Pelagic ecosystem interactions of an offshore longline mussel farm

Long-term interactions of the UK’s first large scale offshore mussel farm with the surrounding pelagic environment were studied to assess the farm’s ecological impact. Pelagic fishes, zooplankton community and oceanographic parameters such as temperature, salinity, density, chlorophyll- α, dissolved oxygen and CDOM were quantified over seven years. Effects to oceanographic parameters were observed locally comparing samples collected on the east and west parts of the farm, evidencing turbulent mixing of the water column enhanced by the farm. There was no difference between zooplankton assemblage composition inside and outside the farm, only naturally occurring temporal changes were detected, supporting the hypothesis that mussel farms in high hydrodynamic conditions do not exert a ‘top-down’ effect on planktonic communities. The mussel farm increased the number of fish species (from one in the first year, to six species) observed and the abundance over time by > 90 % with sightings of important commercially valuable species such as European bass ( Dicentrarchus labrus ) and grey thick-lipped mullet ( Chelon labrosus ) only present within the farm, while control sites were dominated by jellyfish species, demonstrating the potential fish recruitment effect of the farm. This study demonstrated the potential of such offshore developments to avoid the environmental and ecological impacts of inshore, sheltered farms. Providing a three-dimensional habitat, food source to the ecosystem and attracting larger pelagic species and other invertebrates, our findings are key to inform knowledge gaps on the ecosystem interactions of offshore shellfish developments crucial for the sustainable development of the Blue Economy. • Offshore mussel farming provides a three-dimensional habitat and food source to the ecosystem. • This study reports oceanographic interaction producing a degree of turbulent mixing. • It attracts larger pelagic species and other invertebrates demonstrating its capacity as fish recruitment structure. • It avoids the environmental and ecological impacts of inshore, sheltered farms. • This study reports how low-trophic aquaculture can boost marine biodiversity.