Monitoring deep-sea cold-water coral ecosystems: 16-years of protection but no recovery on the Darwin Mounds (Bathyal NE Atlantic)

The Darwin Mounds marine protected area has been afforded protection from bottom trawl fishing since August 2003, following the discovery of cold-water coral (CWC) communities in June 1998. Surveys of the area in 2000 provided evidence that deep-water trawling activities were impacting the corals, prompting the development of a conservation response. Here we report the most recent survey of these CWCs in 2019, contrasting those data with a prior survey in 2011, and the earliest observations in the area (1998–2000). Our assessment is focussed on the colonial scleractinian corals Desmophyllum pertusum and Madrepora oculata. The status of the CWCs was determined using seafloor visual imagery from a remotely operated vehicle (2011), off-bottom towed cameras (1998–2000, 2019) and additionally draws on images from an autonomous underwater vehicle seabed survey (2019). Considering the numerical density, seabed cover, and size distribution of living CWCs, no evidence was detected that the previously impacted corals had recovered. The order of magnitude reduction in live coral abundance in impacted areas remained evident in the 2019 survey after 16-years of protection. Given the likely growth rates of D. pertusum and M. oculata, we suggest that a multi-decadal recovery period should be expected. Our interpretation of long-term change was complicated by the evolving monitoring methodology employed, a common problem and tension in the development of long-term offshore ecosystem monitoring programmes. We further consider a prospectus for effective and efficient future monitoring, noting that autonomous systems and computer vision techniques are likely to play an increasingly important role.