Ligurian seamounts: known vulnerability of unknown ecosystems

Seamounts are prominent features of the world’s underwater topography, with estimates of about 250 structures for the Mediterranean Sea (WÜRTZ and ROVERE, 2015). Very few data are available on the benthic assemblages of Mediterranean seamounts (BO et al., 2011); explorations carried out so far, however, do highlight lush filter-feeding communities strongly coupled to local dynamics of food, plankton, and pelagic species, including top predators.

The Ligurian Sea embraces five major underwater structures: three of these seamounts (St. Lucia, Ulisse and Janua) have been investigated by means of ROV exploration for the first time. The aim of this study was to depict the benthic biodiversity of the top of these three mounts differing in depth (145, 400 and 810m, respectively) as well as in supposed fishing effort, based on their distance from the nearest continental coast (30, 25 and 30NM, respectively).

All seamounts host ecosystems dominated by structuring species. St. Lucia hosts a well-diversified coral community of black corals and small sea fans. The most striking biological feature of the Ulisse Seamount is a conspicuous forest of the fragile gorgonian Callogorgia verticillata on the top, surrounded by a rich, basal coverage of small sea fans and sponges along the most exploited flanks. These two seamounts share a conspicuous amount of species that are also common in deep continental margin and slope ecosystems. Outstanding, instead, is the deep Janua Seamount hosting black corals, gorgonians and hexactinellids never reported before in the Ligurian or Mediterranean Sea.  

Significant amounts of lost fishing gears (mainly long lines) were found on St. Lucia and Ulisse. Both sites are highly frequented by professional and recreational fishermen, moreover Ulisse has also been well-known, back in the 70s, as an exceptional fishing site for deep groupers (CANESE and BAVA, 2015). Not related to distance from the coast, but rather to bathymetric inaccessibility, Janua Seamount shows the lowest abundance of lost gears.

The occurrence of long-living, slow-growing, rare and fragile species contributes to increase the vulnerability of these ecosystems that we just started to explore. Among the many paradigms concerning seamounts’ ecology, the one relating low resilience of communities to fishing disturbance is the most supported (ROWDEN et al., 2010); our data agree with this consideration, suggesting that these habitats may no longer be biological refugia.