The impact of an offshore mussel farm on the mobile epibenthic and demersal community.

Supervisor: Emma Sheehan (University of Plymouth, UK)
Traditionally, aquaculture is located in sheltered coastal waters, however there is a recent shift in opinion towards the benefits of developing offshore farms. Mussel farming is a major sector of aquaculture which has far fewer negative environmental impacts than finfish farming. Offshore bivalve aquaculture could, in fact, benefit the environment while the stronger hydrodynamics and increased water depth mean biodeposits are less likely to accumulate on the seabed. The impact of this on the epibenthic and demersal mobile species is, however, as yet unknown. This study aims to assess the impact of an offshore mussel farm at Lyme Bay over two years of rapid growth of its harvest to determine whether it affects the community inhabiting the seafloor and water column below it. Baited remote underwater video systems were used in the summers of 2018 and 2019 to collect video footage from three treatments- far control, close control and mussel farm. A total of 31 taxa were observed across the two years. There was a significant difference in community structure between the treatments in both years, with Pagurusspp., Asteriasrubens, Ophiuraophiura, Trisopterusluscusand Scyliorhinuscaniculabeing observed at much higher abundance within the mussel farm. Merlangiusmerlangusand Goneplaxrhomboideswere more abundant at the far control sites. The main driver for increased populations within the mussel farm appeared to be fallen mussel clumps as a food resource- sites with >10% ground coverage of mussel clumps had significantly higher abundances of all schooling fish, as well as Asteriasrubensand Paguruspagurus. This indicates that it is not only the mussel ropes acting as fish aggregation devices which increases biodiversity and population densities within the farm, but also the presence of mussel clumps in the benthic habitat, providing food and hard substrate. Of the commercially important species observed or known to be in the area, only Merlangiusmerlangusappeared to benefit from the farm. The Gilthead seabream, Sparusaurata, was observed for the first time at the farm. If the population of this species grows in the future, it may have serious consequences for the future of the mussel farm as it feeds directly on the mussels. Future monitoring of this species in the farm is highly recommended.