Pelagic productivity and abundance of competitors modulate trophic niche segregation between large predators

In the open ocean, large pelagic predators often share similar food resources and feeding grounds, probably avoiding competition and coexisting through niche partitioning. Building on this hypothesis, we combined spatial distribution data with isotopic niche metrics and diet reconstructions based on C and N stable isotopes to describe intra- and interspecific competition between three sympatric predators: swordfish (Xiphias gladius), blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus). We then evaluated the role of biological (competitor abundance), environmental (pelagic productivity) and anthropogenic (fishing pressure) drivers in shaping competition metrics within and between species. Shortfin makos had a high isotopic niche overlap with blue sharks (>80%), feeding on similar prey. In the Mediterranean, the isotopic niche of swordfish was narrower than, and highly overlapped with, that of blue sharks, although diet estimates suggest that swordfish rely more on fish while blue sharks rely more on squid. On average, the potential for intraspecific competition was highest for swordfish and lowest for shortfin makos. Our results suggest that pelagic productivity and competitor abundance are key drivers of intra- and interspecific trophic niche segregation between large pelagic predators, respectively. They support the hypothesis that niche partitioning is reduced under a scenario of high resource availability.