Mussel aquaculture installations located in inshore waters have been shown to have a wide range of impacts on the surrounding environment and marine organisms, while also presenting a problem to other marine users. However, there is some evidence that moving these installations further offshore into the open ocean could mitigate these impacts. This thesis aimed to develop and pilot a robust monitoring programme that could be used to inform future management of aquaculture installations. A further aim was to assess the effects of mussel headlines at an open ocean long-line mussel farm, currently under development in Lyme Bay, southwest UK, on benthic and pelagic ecosystems, and on local fishers. Filter feeding mussels can affect the pelagic and benthic ecosystems through the addition of ropes and hard structure to the pelagic environment, the drop-off of mussels, mussel debris, and build up of biodeposits (faeces and psuedofaeces) on the seabed. However, there is a lack of knowledge on the effects of open ocean mussel farms compared to extensive research on coastal farms. There is also the need for a holistic monitoring system that accounts for the effects of the whole ecosystem as well as the perceptions of other marine users (Chapter 1). The Lyme Bay mussel farm, which is located in an area of historic heavy fishing activity, has introduced ropes into the pelagic ecosystem, which are acting as fish aggregation devices and a habitat for epibiota. There were large aggregations of pelagic fishes around the mussel farm headlines, with a greater abundance around vii ropes growing 1-year-old and 2-year-old mussels, which supported a greater species richness of epibiota, illustrating that the fishes may not just be attracted to the structure, but to the food source provided. Furthermore, the presence of mussel headlines did not lead to a detectable reduction in zooplankton abundance (Chapter 3), although this may be because of sampling techniques. The mussel headlines have led to a change in the epibenthic community. Taxa recorded in the epibenthos, including some commercial species, showed a trend for greater abundances beneath the headlines, compared to areas that were still being heavily fished. There was no change to the organic matter or mean particle size of the sediment beneath the headlines or associated infaunal communities, although there were indications that the mussel headlines may be causing a reduction in redox potential (Chapter 4). Perceptions from local fishers were mixed. Some had been displaced by the farm, and had negative views on its development, whereas others noticed an increase in their catch around the farm area and recognised the potential for the farm to have a positive effect on fisheries. There was no evidence that the farm had increased landings in the area so far, so any increases in catch would only be on an individual level (Chapter 5). The monitoring programme used has successfully sampled taxa from both the benthic and pelagic ecosystems, however, a few modifications are provided, along with recommendations for future monitoring (Chapter 6). This thesis has highlighted the importance of taking a holistic approach to assessing the effects of open ocean mussel farming, while increasing the evidence base available to policy makers that could be used to help guide the initiative to move aquaculture installations offshore, supporting the Blue Growth agenda.

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