The trophic blockage hypothesis is not supported by the diets of fishes on Seine Seamount


Stefanie Hirch, Institute of Hydrobiology and Fishery Science, University of Hamburg, Große Elbstraße 133, D-22767 Hamburg, Germany.


Several hypotheses exist about the trophic mechanisms that support fish stocks at seamounts. This study investigated the diets of benthopelagic fish species on the summit plateau of Seine Seamount (NE Atlantic), testing the sound-scattering layer interception hypothesis. A combined approach of gut content, stable isotopes and fatty-acid biomarker analyses was employed. Fish species included zooplanktivores, benthivores, piscivores and species with mixed crustacean/cephalopod/fish diets. Trophic coupling between pelagic food sources and benthopelagic fish consumers was apparent based on three main lines of evidence: (i) dominance of pelagic prey in the guts of zooplanktivores, (ii) stable isotope enrichment of consumers indicative of a pelagic prey source, and (iii) high proportions of fatty acids that are typical of phytoplankton and zooplankton in the storage lipids of fishes and their similarity with fatty acid signatures of pelagic prey. Elevated levels of arachidonic acid in a benthivorous species suggested a minor dietary contribution of rhodophytes. The lack of larger taxa that undergo diel vertical migrations in the fish guts suggests that it is horizontal fluxes of non-, or weakly migrating zooplankton that are the main food supply to the resident fish consumers. Overall, there was no unambiguous support for the trophic blockage hypothesis. Differences in gut contents, trophic position and storage lipid fatty acid signatures of zooplanktivorous fishes indicate some degree of resource partitioning with respect to feeding habitats, prey selection and ontogenetic diet shifts. Irrespective of body size and feeding mode, the benthopelagic fishes occupied intermediate trophic positions between the 3rd and 4th trophic level in a food web composed mainly of omnivorous species.