The nocturnal distribution of deep-pelagic fish on the continental slope of the Bay of Biscay

The biomass of deep-sea pelagic fishes could represent more than 90% of the total fish biomass on earth, which represents an important potential for exploitation. However, this community plays multiple key ecological roles in biogeochemical cycles and food webs. Information on their ecology and function is needed to plan effective sustainable conservation measures. In particular, the distribution of deep-sea pelagic fish biomass and the environmental factors that control it remain poorly understood on slope areas at the interface of coastal and oceanic habitats. The combined use of biological data collected by pelagic trawling between 20 and 2000m depth at night and 16 environmental variables allowed us to study the distribution of this community on the continental slope of the Bay of Biscay, NE Atlantic. Multivariate regression tree analysis suggested that immersion depth was the most important variable structuring this community by night, defining four depth assemblages with different indicator species (i.e. organisms whose presence, absence, or biomass reflects specific environmental conditions): the epipelagic (20-175m), the upper mesopelagic (175-700m), the lower mesopelagic (700-1000m) and the bathypelagic assemblage (1000-2000m). The use of generalized additive models indicated a positive relationship between bottom proximity and biomass for three of the four assemblages. This contrasts with the paradigm in open ocean areas where the biomass of meso- and bathypelagic fishes decreases with depth. The echograms also showed low acoustic detection at the surface at night, which differs from the open ocean models where nocturnal migration results in a high density of mesopelagic organisms in the epipelagic layer. Different mechanisms could explain this relationship, such as the concentration of food resources in the benthic boundary layer, an ontogenetic change of some species, active horizontal migration, or a demersal spawning behavior. This specific distribution on the continental slope may influence the transition of carbon and energy flows within this ecosystem located in a Special Area of Conservation (SAC).