Deep-Sea Fishes of the Mid-Atlantic Ridge: Trophic Structure and Interactions

Because deep-sea fisheries are increasing as coastal fisheries decline, fisheries scientists need baseline data on deep-sea ecosystems prior to further development of deep-water fisheries. We present preliminary results and ongoing efforts to characterize the trophic structure and energy flow of the pelagic ecosystems of the northern Mid-Atlantic Ridge, from Iceland to the Azores. This study is one component of the international CoML field project MAR-ECO (www.mar-eco.no). We found a diverse deep-pelagic fish fauna (205 spp.), with unexpectedly high bathypelagic fish biomass and spatial complexity. Based on literature reports of species present, crustacean planktivory is the dominant trophic guild (79% of individuals 47% of species), primarily within the mesopelagial. "Gelativory" was second (12% ind., 4% spp.), primarily within the bathypelagial. Omnivory (3%, 13%), "shrimpivory" (2%, 4%), and piscivory (1%, 21%) were the remaining major feeding guilds. The diets of 22 spp., primarily bathypelagic, are unknown. Based on stable isotope analysis of fish tissue, two distinct trophic modes were identified: a mode at trophic level 4 (18%; crustacean and gelatinous planktivory) and a mode at trophic level 4.5-5.0 (58%; crustacean planktivory and piscivory). The top piscivores were bathypelagic fishes. In terms of fish biomass, the gelativorous taxa dominated, followed by crustacean zooplanktivores and piscivores. Quantitative comparisons of the different trophic pathways are not possible at present, given our limited knowledge of feeding rates of most species. However, microscopical and molecular analyses are currently ongoing to fill this void, including development of techniques to identify/quantify gelatinous prey as an alternate trophic pathway.