MEPS prepress abstract  -  doi: 10.3354/meps07318

ABSTRACT: In the Northeast Atlantic, off the Bay of Biscay, the pelagic top predator community is mainly composed of the blue shark Prionace glauca, the swordfish Xiphias gladius, the albacore tuna Thunnus alalunga, and the common Delphinus delphis, and striped Stenella coerulaeoalba, dolphins. This study is aimed at determining the patterns of feeding niche segregation among this oceanic top predator assemblage. Overlaps were measured in terms of prey taxa and prey sizes. Preferred foraging depth ranges and nycthemeral patterns were inferred from prey compositions and digestion conditions. In terms of prey taxa, the blue shark, the albacore and the swordfish segregated fairly well from each other and from the two dolphins, whereas the two dolphins showed considerable overlap. In terms of prey sizes, substantial overlap was found between the blue shark and the swordfish but these predators differed from the two dolphins and the albacore, which, in turn, overlapped considerably. Spatio-temporally, the blue shark and the swordfish appeared to be predominantly diurnal mesopelagic predators while the albacore and the dolphins would be mostly nocturnal epipelagic feeders. Prey diversity was higher in the two dolphins which also showed a lower inter-individual variability in stomach content composition. The two dolphins have the highest energy needs and are bound to the surface for breathing, the albacore is also bound to the surface layer for physiological reasons (swim bladder development and body temperature control): they all three rely on small gregarious epi- to vertically migrating mesopelagic prey species of high energy content. The swordfish and the blue shark have much lower energy needs and are not restricted to the surface layer; they are better able to forage on scattered, deep living, large size and low energy prey. Hence, within the whole community, the energetics of predation and constraints relative to sea surface would be the main structuring factors, and not the relationship between predator size and prey size.