Cold-water coral (CWC) habitats dwell on continental shelves, slopes, seamounts, and ridge systems around the world’s oceans from 50 to 4000 m depth, providing heterogeneous habitats which support a myriad of associated fauna. These highly diverse ecosystems are threatened by human stressors such as fishing activities, gas and oil exploitation, and climate change. Since their life-history traits such as long lifespan and slow growth rates make CWCs very vulnerable to potential threats, it is a foremost challenge to explore the viability of restoration actions to enhance and speed up their recovery. In contrast to terrestrial and shallow-water marine ecosystems, ecological restoration in deep marine environments has received minimal attention. This review, by means of a systematic literature search, aims to identify CWC restoration challenges, assess the most suitable techniques to restore them, and discuss future perspectives. Outcomes from the few restoration actions performed to date on CWCs, which have lasted between 1 to 4 years, provide evidence of the feasibility of coral transplantation and artificial reef deployments. Scientific efforts should focus on testing novel and creative restoration techniques, especially to scale up to the spatial and temporal scales of impacts. There is still a general lack of knowledge about the biological, ecological and habitat characteristics of CWC species exploration of which would aid the development of effective restoration measures. To ensure the long-term viability and success of any restoration action it is essential to include holistic and long-term monitoring programs, and to ideally combine active restoration with natural spontaneous regeneration (i.e., passive restoration) strategies such as the implementation of deep-sea marine protected areas (MPAs). We conclude that a combination of passive and active restoration approaches with involvement of local society would be the best optimal option to achieve and ensure CWC restoration success.

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Cold-water gorgonians dwelling on the continental shelf are a common by-catch of bottom-contact fishing practices. Given the slow growth and limited recruitment of coldwater gorgonians, impacts derived from fishing activities may seriously compromise the conservation of the highly complex coral gardens which they generate, as well as the abundant and highly diverse associated fauna. For this reason, the development of effective active and passive restoration methods is nowadays a priority to enhance the natural recovery of impacted cold-water coral gardens. However, ecological restoration of mesophotic and deep-sea communities remains extremely limited, due to their technological requirements and associated costs bringing their wide-scale and longterm application into question. This study reports the results of the first large-scale active restoration of more than 400 cold-water gorgonians on the Mediterranean continental shelf. By actively involving local fishers during two consecutive fishing seasons, by-catch gorgonians were recovered and returned to the continental shelf (at 80-90 m depth). Two-years monitoring performed through Autonomous Underwater Vehicle (AUV) surveys revealed that 460 gorgonian transplants survived over an area of 0.23 ha. This reintroduced cold-water gorgonian population is compared to a reference natural population in terms of size and spatial structure. The cost of the restoration amounted to 140 000 €/ha, which is significantly less than for any deep-sea restoration actions performed to date. The success of this cost-effective activePowered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation restoration highlights the viability of large-scale restoration of impacted cold-water coral communities, with promising results for the conservation and recovery of mesophotic and deep-sea ecosystems.