Mesophotic Animal Forests of the Ligurian Sea (NW Mediterranean Sea): biodiversity, distribution and vulnerability

Knowledge about deep-sea megabenthic communities has greatly increased during the last two decades thanks to the improvement of technical diving equipment, submersibles, remotely operated and autonomous vehicles. In the last 10 years, several studies, targeting the Mediterranean deep continental shelf or mesophotic zone (40-200 m), have reported the occurrence of rich hotspots of biodiversity, namely marine animal forests, dominated by habitat-forming species such as corals and sponges thriving in dim light conditions. This bathymetric range hosts the deepest extension of shallow-water animal forests as well as proper mesophotic assemblages thriving on the so-called roche du large ecosystems. These complex aggregations play a fundamental ecological role in the marine ecosystems (refuge effect, nursery area, benthic-pelagic coupling, biogeochemical cycles) and the characterization of their diversity and distribution is considered of primary interest worldwide. Explorations have depicted not only the diversity of these forests but also their vulnerability towards mechanical impacts inflicted by demersal fishing activities, especially in the overexploited Mediterranean basin. This leads to an urgent need to quantify the damages burdening on these sensitive ecosystems and to develop easy-to-apply tools to evaluate and monitor their environmental status, in order to provide effective conservation measures. This thesis aims to address these topics focusing on the marine animal forests of the Ligurian deep continental shelf. The Ligurian Sea represents one of the most studied Mediterranean basins due to several extensive researches, which have characterized the benthic and pelagic fauna mainly through SCUBA diving (shallow waters) and trawl surveys (bathyal waters) since the beginning of the last century. On the other hand, a large knowledge gap still exists for this region regarding the deep circalittoral megabenthic communities and, overall, on the anthropic threats insisting on deep-sea communities. Indeed, considering the significant amount of professional and recreational fishermen operating at these depths in the Ligurian region, it was crucial to carry out an extensive investigation in this area. In the present study, about 80 sites have been investigated during a four-years ROV survey conducted along the Ligurian deep continental shelf representing the most comprehensive study so far for the Italian waters. ROV footage was used to characterize the benthic biocoenoses thriving on soft and hard bottoms, determine the most relevant environmental factors driving their spatial distribution, and quantify the anthropogenic disturbance. Twelve major assemblages have been identified by means of community analysis, including gorgonian and hydrozoan forests, dense keratose sponge grounds, a Dendrophyllia cornigera facies, bryozoan beds and soft-bottom meadows of sabellids and alcyonaceans, some of them reported for the first time. Forests of black corals and Paramuricea macrospina aggregations have also been reported from the video footage, although with a more scattered distribution. The distribution of these assemblages is determined by different environmental parameters, namely depth, substrate type and inclination. The abundance of marine litter herein observed is among the highest ever recorded in Europe, with maximum values up to 7900 items ha-1. Abandoned, lost or discarded fishing gears represent the 81% of the total sightings, with the highest abundance recorded on the deep rocky shoals facing the largest fishing harbors and fleets. The majority of the fishing gears observed is attributable to artisanal and recreational fishing activities, suggesting that these represent the main source of litter in the Ligurian Sea. Urban litter, mainly represented by plastic, has also been observed, generally with higher abundances nearby large cities and river mouths. A multidisciplinary approach based on ROV explorations, background fishermen interviews and on-board bycatch monitoring has also been applied to obtain quantitative data on the impact of different fishing methods adopted within two Ligurian case studies. The study areas have been selected following a bathymetric point of view to study two different deep-sea ecosystems (a deep circalittoral coralligenous shoal exploited by a small-scale fishery and a trawling ground located nearby a cold-water coral thanatocoenoses), and the investigated parameters were designed to fulfil the criteria proposed by FAO for the identification of Vulnerable Marine Ecosystems (VMEs). A fine characterisation of the megabenthic assemblages and the fishing effort was obtained, as well as the quantification of no-target catches rates (with particular attention to structuring megabenthic invertebrates and rare species), and the frequency of gear entanglement and loss. This information allowed to initiate the process to delineate Fisheries Restricted Areas (FRAs), over these sensitive deep areas. Finally, to evaluate the environmental status of megabenthic assemblages thriving on mesophotic coralligenous or rocky reefs at temperate latitudes, a multiparametric index (MACS) integrating all possible sources of ROV-based information regarding community structure and impacts has been developed and validated. This practical tool was thought to have a large-scale application within the on-going Marine Strategy monitoring programs of these ecosystems. In conclusion, a georeferenced database of the location of diversity hotspots and sites of litter accumulation has been created, in order to set the baseline for the monitoring of Ligurian mesophotic sensitive animal forests. This dataset will provide managers and stakeholders the scientific information and the tools for the creation of an extensive network of deep-sea marine protected areas in the Ligurian basin.