Trophic ecology and coastal niche partitioning of two sympatric shark species in the Azores (mid-Atlantic)

Abstract

The exploitation of marine resources has caused drastic declines of many large predatory fishes. Amongst these, sharks are of major conservation concern due to their high vulnerability to overfishing and their ecological role as top predators. The 2 protected and endangered shark species tope Galeorhinus galeus and smooth hammerhead Sphyrna zygaena use overlapping coastal areas around the globe as essential fish habitats, but data to assess their trophic ecology and niche partitioning are scarce. We provide the first comparative assessment of the trophic ecology, ontogenetic shifts, and niche partitioning of the co-occurring tope and juvenile smooth hammerhead around the Azores Islands, mid-north Atlantic, based on δ13C, δ15N, and δ34S (CNS) stable isotope analysis of muscle tissue of the sharks and their putative prey species. Overall, isotopic niches of both species indicated a reliance on similar resources throughout the sampled sizes (tope: 35-190; smooth hammerhead 54-159 cm total length), with significant ontogenetic shifts. Topes displayed a gradual shift to higher trophic levels and a more generalist diet with increasing size (increasing δ15N values and isotopic niche volumes, respectively), whereas smooth hammerhead diet shifted towards prey with lower δ34S at a constant trophic level and a more specialized diet than tope of comparable body size (decreasing δ34S and constant δ15N and δ13C values, respectively). Our results indicate contrasting ontogenetic shifts in δ13C and δ34S along with pronounced differences between niche overlap of life stages pointing to intra- and interspecific niche partitioning of habitat and prey.