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In light of past and present ecosystem and fisheries management failures leading to the continued decline of global fish stocks and ongoing degradation of many marine habitats, ecosystem-based approaches to management have been favoured. Marine Protected Areas (MPAs) have been championed as tools that allow the holistic management of marine resources. Faced with national and international marine conservation targets, the UK has adopted and implemented MPAs based on guidelines from various legislature. Many of these MPAs are multiuse meaning the MPA restricts some anthropogenic activities, typically certain types of commercial fishing, while permitting other activities to continue on account of their compatibility with conservation and fisheries management. The Lyme Bay MPA, introduced in 2008 to protect sensitive benthic reef habitats, is one such multi-use MPA that has restricted bottom towed fishing, while authorising alternative commercial fisheries to continue within the MPA. Commercial potting inside the Lyme Bay MPA has increased in response to the removal of bottom towed fishing, and is now the dominant Lyme Bay fishery. The ecological effects of current, and increasing, levels of commercial potting effort were unknown and thus increases in potting were cause for concern, particularly when allowed to continue unregulated within the MPA. This thesis therefore developed a three year experimental potting study that manipulated potting densities in order to gather evidence on the ecological impacts of increasing potting density on both the ecosystem and fisheries, in order to address existing knowledge gaps. This thesis contains original contributions to knowledge in each of its chapters. Ecological research into the ecosystem impacts (Chapter Two, Three) of commercial potting was carried out using underwater video methods showing that, contrary to previous understanding, commercial potting reduced the number of two key sensitive sessile reef species in Lyme Bay when potting density was high. Research into the fishery impacts (Chapter Four) of increasing potting density showed that under medium to high potting densities, selective fishing pressures alter the population densities and overall condition of key species (brown crab (Cancer pagurus L.) in particular) targeted by commercial potting. The knock-on ecological impacts of these impacts are theorised (Chapter Six) and using this new evidence MPA regulators should now decide what level of commercial potting is compatible in Lyme Bay, and other MPAs, on account of individual MPA ecosystem and fisheries objectives. Overall the thesis has provided evidence on the potential ecological impacts of increasing commercial potting, as well as providing robust evidence for the ecological sustainability of low levels of commercial potting. It has highlighted that when applying an ecosystem-based approach to ecosystem and fisheries management, it is necessary to consider commercial potting, along side other fisheries, when managing multi-use MPAs.