The 21st century faces a global challenge; understanding the ecosystem changes and regulating fish resources is essential to provide food for humankind and to preserve the oceans. Marine ecosystems are being increasingly altered because of global, regional, and anthropogenic stressors such as climate change, Non-Indigenous Species (NIS), pollution, fisheries, eutrophication, and habitat loss. In an era of escalating transformations of marine ecosystems, the Mediterranean Sea is recognized as a hotspot of global biotic and abiotic changes. In recent decades, human-mediated introductions of NIS have notably reshaped the marine ecological structures and processes of the region. The Suez Canal (constructed in 1869 and subsequently expanded) has established a permanent sea-level waterway connecting Red Sea with the Mediterranean Sea. Lack of preventive measures and climate change allow more thermophilic NIS of Indo-Pacific origin to find their niche in the Mediterranean Sea. Over 750 multicellular NIS have established viable populations in the region and a complete halt of their spread is impossible. Non-indigenous species have become members of a complex system of interactions between economy, society, and environment, affecting the livelihoods of local communities. Managers are faced with a complex task of navigating the changing conditions. They must find ways to utilize the potential benefits that some NIS may provide, while simultaneously protecting ecosystems from the harmful impacts that some invasive species can induce. This PhD started in 2018 at a moment when the Mediterranean Sea was experiencing the lionfish (Pterois miles) expansion in one of the fastest population expansions ever recorded for marine invasive species. With a documented invasion history in the Western Atlantic, and proven possible management measures, the lionfish invasion in the Mediterranean Sea offered a critical opportunity for research and acquisition of fundamental knowledge about management possibilities during escalating invasion stages. With an emphasis on lionfish, this PhD employed social (surveys and questionnaires) and ecological research methods (visual census surveys) to provide novel knowledge on four management action priorities for NIS and guide a holistic management in the region. The priority management actions were (i) Education and public awareness (Chapters 1, 3, and 4), (ii) Rehabilitation of the environment (Chapter 2), (iii) Commercial and recreational utilization (Chapters 2 and 4), and (iv) Targeted removal of the species (Chapters 2 and 3). Considering the identified gaps in public awareness and knowledge about lionfish and NIS in the Mediterranean Sea (Action 1), questionnaire surveys conducted in Chapter 1 revealed high awareness of stakeholders about lionfish and its impacts but limited public awareness and support for new fisheries from lionfish. The Chapter analysed stakeholder motivations, the guided the implementation of communication initiatives aiming at involving the local communities in lionfish management. Chapter 4 revealed lack of knowledge and awareness of fishers regarding the origin of many common NIS, and divergent perceptions on the ecological impacts that were correlated with the price of the species in the market. There is potential here for collaborative and communicative management processes to harmonize divergent views amongst fishers. The goal would be to mutually agree management measures that can support livelihoods and restore the ecosystem. Rehabilitation of the environment (Action 2) and development of a healthy functioning ecosystem has the potential to control the distribution and abundances of introduced species. Two experiments that conducted visual census surveys in Chapter 2 demonstrated that recent fishery restrictions in a Marine Protected Area (MPA) of Cyprus likely benefited the spatial and temporal expansion of lionfish populations. Additional management measures, such as selective fishing and targeted removals, to offset invasive species spread would help MPAs recovery processes especially in the early years of MPAs’ designation and contribute to their conservation objective. The decreased lionfish densities in fished areas compared to strictly MPA zones (no fishing) found in Chapter 2 demonstrated the potential contribution of fisheries (commercial and recreational utilization – Action 3) to regulate NIS populations. Chapter 4 used fishery-dependent data and questionnaire surveys to demonstrate the emerging and important contribution of NIS catches in Cyprus fisheries, both in terms of volume and value. The Chapter has shown that recreational fishers avoided common NIS and even easy targets, such as lionfish. Recreational fishers are potentially driven by motivations beyond economic gain, such as traditional norms for larger and ‘trophy’ fish which are often keystone top-predators within marine ecosystems. Conversely, targeting of NIS by commercial fishers was largely driven by the price and their familiarity with the species. By reforming both recreational and commercial fisheries to adapt to NIS and launching market campaigns to address motivational factors, it could be possible to mitigate the negative impacts of NIS and generate greater profits and yields for local communities. Chapters 2 and 3 demonstrated the efficiency of targeted removals (Action 4) with volunteers in controlling lionfish populations from priority sites. Social and environmental benefits from the execution of such events outweigh the economic costs, as demonstrated by their potential to incentivize lionfish fisheries, collect biological and ecological data, enhance public knowledge, awareness, secure support, and promote cooperative efforts. Chapters 5-6 used the knowledge from Chapters 1-4 to guide reforms in the management and policy frameworks of NIS in the Mediterranean Sea and ameliorate the impacts of established NIS. Chapter 5 documented challenges encountered with a proposal to include lionfish in the European Union List of Invasive Alien Species (IAS) (EC/1143/2014) and provided recommendations on the basic IAS Regulation and the Delegated Regulation on risk assessments (EC/2018/968) that could be applied to improve relevance, coverage, effectiveness, and management of marine IAS at a European and regional level. To move beyond current problematic management approaches that do not adapt to the presence of non-indigenous species, Chapter 6 proposed major fishery reforms and an Ecosystem Based Fisheries Management framework that could direct a concerted and harmonized holistic approach against NIS to limit the socioeconomic and ecological impacts. Finally, results were summarized and a comprehensive tailored-made guide for the management of lionfish in the Mediterranean region was also developed and supplemented the major outputs of the PhD as Appendix. The guide provides information about key topics for the management of lionfish such as targeted lionfish removals, development of markets, outreach and communication activities, research and monitoring, and regional cooperation.

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