The concept of ocean multi-use (MU), the co-location or combination of complementary maritime activities, emerged in the early 2000s from research on integrating offshore aquaculture with wind farms in the North Sea. The European Commission subsequently adopted MU as a marine policy concept, promoting it within the Blue Growth Strategy to create economic opportunities, mitigate spatial conflicts, and reduce environmental pressures. However, MU's definition remains fluid, encompassing various forms and interpretations, from repurposing existing infrastructure to fully integrating multiple uses. The limited success in operational "win-win" multi-use combinations highlights a gap between theory and practice. Challenges include stakeholder cooperation, regulatory frameworks, immature technologies, and understanding of cumulative ecological impacts. This paper addresses key questions regarding MU's popularity as a marine policy concept, its comparison with other approaches to maritime management, its applicability beyond European seas, and the need for improved integration across scientific disciplines, research, policy, and society.

Existing literature reviews on ocean multi-use have limitations. They often employ descriptive approaches focusing on milestones, adopt an applied-science perspective lacking broader context, and overlook related work on coexisting marine uses. This paper addresses these shortcomings by analyzing a broader corpus of publications on multi-use, including those focused on MPAs and MSP. The analysis reveals that the concept of multi-use is not new, with early publications dating back to the 1970s, but the specific focus on synergistic combinations of marine uses is more recent, accelerating significantly in the past decade. The rise of multi-use is linked to the European Commission's Blue Growth Strategy and related research programs, which have significantly shaped the field's focus and direction.

This study uses a bibliometric approach to analyze the scientific literature on ocean multi-use. Data was extracted from Scopus, focusing on publications related to 'multi-use', 'multiple use', 'multifunctional use', or 'co-use' within a marine context. After excluding papers on terrestrial topics or those focusing solely on individual marine uses, 311 publications were analyzed in a 'large collection,' 68 of which specifically focused on synergy-based multi-use in a 'short collection.' The Bibliometrix R package and Biblioshiny were employed to analyze various bibliometric indicators, including publication year, authorship, countries of origin, co-authorship networks, and keyword co-occurrence networks. This quantitative analysis was complemented by a qualitative examination of key publications to gain deeper insights into the evolution and structure of the field.

The bibliometric analysis reveals that multi-use research is driven by extensive and dense research groups, particularly within the short collection (synergy-based multi-use). Co-authorship networks illustrate distinct communities focused on MPAs versus MU, with limited collaboration between the groups. Keyword co-occurrence networks highlight the central role of MPAs in the large collection, with keywords reflecting marine conservation, fisheries management, and spatial planning. In contrast, the short collection emphasizes 'offshore wind energy' and 'aquaculture', linked to the concept of 'Blue Growth'. The analysis shows a discrepancy between MU's theoretical transdisciplinary nature and its predominantly natural and engineering science-focused research community, highlighting a lack of social science integration. While MU research originated in multidisciplinary projects, social sciences have often been relegated to providing tools rather than shaping core research questions. There is also a limited focus on environmental issues such as cumulative impacts. In comparison to MPA and MSP research, multi-use studies show a lower level of integration in terms of both topics and disciplinary frameworks.

The findings demonstrate that ocean multi-use research should be considered within the broader context of existing work on MPA and MSP. While MU emphasizes economic synergies and functional integration, MPAs and MSP often focus on regulation and spatial management. However, both approaches aim to manage the increasing diversification and intensity of human activities at sea. The limited practical implementation of MU suggests that it remains largely a theoretical construct, potentially promoting contested activities and new enclosures without sufficient consideration of power dynamics, resource sharing, and environmental impacts. The predominantly positivist, applied-science approach in MU research needs to be complemented with reflexive approaches to fully integrate social and political considerations. A more transdisciplinary approach, embracing sustainability sciences and critically addressing power dynamics and societal impacts, is vital to bridge existing research gaps and promote the successful and sustainable implementation of multi-use systems.

Ocean multi-use has gained traction, but its practical implementation lags behind its theoretical development. While offering promising economic and environmental opportunities, MU needs to address crucial challenges by strengthening transdisciplinary collaborations, integrating social science perspectives, and critically assessing its societal implications. Future research should focus on bridging the gap between theory and practice, advancing understanding of cumulative environmental impacts, and developing robust governance frameworks that ensure equitable and sustainable multi-use systems.

The study's reliance on Scopus as the primary database may have introduced biases, potentially underrepresenting publications from social sciences and non-English language journals. The analysis focuses on research publications and may not fully reflect practical implementations and policy developments. Future research could explore other databases and qualitative methods to broaden the scope and refine the findings.