Environmental niche, spatial distribution and size structure of shelf-dwelling populations of the gorgonian Eunicella cavolini (Koch, 1887) in NW Mediterranean

Extensive knowledge exists about the ecology of gorgonian species dwelling in coastal areas of the Mediterranean, but less information is available regarding populations found in deep circalittoral and bathyal environments. In recent years, the use of marine imaging technology has led to the discovery of dense coral gardens below SCUBA depths. This is the case of Eunicella cavolini (Koch, 1887) populations dwelling on the continental shelf off Cap de Creus (northwestern Mediterranean), an area included in a Site of Community Importance (SCI) of the Natura 2000 Network. We characterized the environmental niche of this species using a combination of density data extracted from 60 underwater video transects (80–400 m depth) and a set of environmental factors of a geomorphologic, oceanographic and human nature. Populations of E. cavolini develop within a restricted combination of environmental conditions, predominantly in areas of the mid continental shelf (95–110 m depth) characterized by a mixture of substrates (gravels, pebbles and rocks) with intermediate yearly average bottom current velocities (0.11–0.13 m s⁻¹) and where no records of bottom trawling activity were registered in the years prior to the surveys. The species forms dense aggregations that average 6–8 col·m⁻², with local high-density peaks above 15 col·m⁻². Areas suitable for the development of E. cavolini were modelled by means of a Random Forest algorithm, which predicted that an area extending over 14 ha could potentially host populations with densities over 5 col·m⁻². Bottom current velocity emerged as the most important factor determining the spatial distribution of the species, followed by depth and presence of rocks. Colony height measurements indicated that populations are dominated by small-to-medium sized colonies, with average colony height positively correlated with gorgonian density. The reduced average colony height observed might respond to the strong bottom current regime characteristic of Cap de Creus, but also to the increased likelihood of larger colonies to be captured as by-catch by trammel nets, a common type of fishing gear used in the area. This study provides relevant information about the ecology of E. cavolini populations dwelling on deep circalittoral habitats, useful for policymakers when formulating specific management measures to foster the conservation of this species inside the SCI. Additionally, the data provided could assist on the restoration actions already taking place in Cap de Creus, and be used as baseline data to monitor the evolution of E. cavolini populations through time once management plans are implemented.