Overlap Between Pelagic and Demersal Fishes at the Mid-Atlantic Ridge

The deep-pelagic zone (\u3e 200 m) is essentially boundless in 3 dimensions for most of its extent, structured only by fluid features of the seawater itself. However, near certain topographic features the deep-pelagic zone intersects the seafloor. The mid-ocean ridge system is by far the largest of these features. Unlike the ecosystems of the continental margins, the mid-ocean ridge systems do not receive terrigenous nutrient inputs. Thus, the deep-water fauna associated with mid-ocean ridges ultimately depend on the generally very limited local surface production. Despite this limited surface production, there is evidence that near-ridge demersal fish biomass is increased above the mid-Atlantic Ridge (MAR). To understand the dynamics of the MAR an international study, MAR-ECO, of the animals inhabiting the northern Mid-Atlantic was undertaken. Utilizing multiple technologies the meso-, bathy- and benthopelagic realms were sampled using pelagic and bottom trawls, ROV video and bottom long-lines. Taxonomic analysis to date has revealed at least 294 species (74 families, 22 orders), with ongoing analysis expected to reveal more species, some new to science. Pelagic sampling collected ca. 206 species, with typical orders dominating (e.g., Myctophiformes, Stomiiformes, Osmeriformes). Bottom trawling collected ca. 175 species, with typical demersal families (Alepocephalidae, Macrouridae, Ophidiidae, Moridae), but also pelagic families occurring in numbers higher than would be expected by contamination during deployment and retrieval alone. Discrete, near-bottom pelagic trawls confirmed this observation. In all, 84 species were caught in both pelagic and bottom trawls, with some species showing enhanced abundances in the near-bottom boundary layer, suggesting that overlap of deep-pelagic and demersal faunas is likely a key process regulating mid-ocean ridge community structure