Feeding niche segregation among the Northeast Atlantic community of oceanic top predators

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, the common dolphin Delphinus delphis and the striped dolphin Stenella coeru- laeoalba. The present 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 diel patterns were inferred from prey compositions and diges- tion conditions. In terms of prey taxa, the blue shark, the albacore and the swordfish segregated fairly well from each other and from the 2 dolphins, whereas the 2 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 2 dolphins and the albacore, which, in turn, overlapped consider- ably. Spatio-temporally, the blue shark and the swordfish appeared to be predominantly diurnal mesopelagic predators, while the albacore and the dolphins were mostly nocturnal epipelagic feed- ers. Prey diversity was higher in the 2 dolphins, which also showed a lower interindividual variabil- ity in stomach content composition. The 2 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): all 3 rely on small gregarious epi- to ver- tically 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 for- age on scattered, deep-living, large-size and low-energy prey. Hence, within the whole community, the energetics of predation and constraints relative to the sea surface are the main structuring factors, and not the relationship between predator size and prey size.