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Forming almost two-thirds of the global ocean, the High Seas and seabed areas beyond national jurisdiction (ABNJ) support some of the highest levels of biodiversity on the planet. However, these vital areas are increasingly under threat from human and climate change-induced pressures. The current governance regime related to the conservation of marine biodiversity beyond national jurisdiction (BBNJ) is fragmented (in terms of institutional mandates, powers and resources) and characterised by significant gaps (in terms of species and geographical coverage). In response, the United Nations has negotiated an internationally legally binding instrument (ILBI) to protect the High Seas. A key aspect of the draft agreement is that that new instrument should "not undermine" existing bodies, instruments and frameworks, which raises key questions relating to interplay between the new ILBI and existing bodies. My research seeks to understand how the agreement can be effectively operationalised by analysing two related strands: (1) The need for the agreement to be effectively implemented by existing institutions, and (2) Due to the migratory nature of BBNJ and governance gaps, the need for existing institutions to work together effectively. I argue under (1) that four candidate conditions are likely to be important for implementation and deploy Qualitative Comparative Analysis (QCA) to identify one condition necessary for successful implementation (multi-party coordination) and three conditions (access to/management of data, multi-party coordination and adaptive management) which are sufficient to lead to a successful outcome. Under (2) a case study of the Northern Atlantic institutional regime is used to characterise and help explain the forces and factors influencing institutional interplay. Taken together, the two parts to the research generate insights into effective institutional arrangements for the future governance of BBNJ.

For millennia, the ocean has been seen as beyond human reach. However, human activities are increasingly advancing into marine spaces. As persistent submerged or semi-submerged structures, human-made reefs (HMRs) represent the embodiment of human influence in the ocean and create new opportunities for social meaning and culture underwater. Though some HMRs such as shipwrecks, fishing traps and oil rigs may be long-established, others are increasingly being used to advance political or artistic aims, and as tools to restore or transform the marine environment. Their creation can have significant implications for marine life. In order to carry out marine conservation in a changing ocean, it is crucial to understand both the ecological and social roles of HMRs. While ecological assessment is making progress, it remains focused on a few HMR types, and social assessment is lagging behind. In this thesis, I explore the historical and current use of diverse HMRs and propose a social and ecological framework to evaluate their conservation potential, regardless of origin. I apply this framework to a case study on the island of Cozumel, Mexico. Here, I locate and characterise over 70 unique HMR sites - over ten times as many sites as previously identified. These included shipwrecks, sculptures, debris, coral restoration modules, infrastructure and rock piles. I observe differences in stakeholder awareness across types and locations of HMRs, typify modes of encounter and describe location sensitivity and barriers to access. Through a socio-cultural analysis, I characterise the provision of a wide range of ecosystem services including cultural services, and identify key factors affecting people's attitudes to HMRs, as well as their costs, benefits and impacts. The results indicate HMRs play complex, unacknowledged and powerful social and cultural roles. I analyse conservation-relevant synergies and conflicts between stakeholders, considering differences in activities and preferences in relation to marine life and suggesting ways forward for collaborative conservation. Finally, I suggest a method for conducting rapid ecological assessments on a variety of HMRs and demonstrate its usefulness in a survey of 70 individual HMR structures, identifying 78 species of fish and five species of mobile invertebrates. The results suggest that structures which are deeper, larger, and contain more holes and internal space are likely to have higher species richness and abundance. Overall, this thesis suggests that HMRs have significant social and ecological conservation value for marine management, and may already be much more prevalent than expected. HMRs offer a wide variety of ecosystem services and are culturally and emotionally important to the people who create and use them. They can host diverse and abundant communities of marine organisms, which may be curated through processes such as fishing, cleaning or coral "planting". Cooperation between stakeholders to fulfil conservation aims offers challenges, but also many potential rewards, enriching the relationship between human endeavour and marine life in future oceans.