Trophic ecology and coastal habitat use of two sympatric shark species in the Azores using CNS stable isotope analysis

• Main Author: Priester, Carl Robert
• Other Authors: Afonso, Pedro, Abecasis, David
• Format: Master Thesis
• Language: English
• Published: 2020
• Subjects: Stable isotope analysis; Trophic ecology; Essential fish habitats; Shark nursery; Sphyrna zygaena; Galeorhinus galeus; Domínio/Área Científica::Ciências Naturais::Outras Ciências Naturais; North Atlantic
• Online Access: http://hdl.handle.net/10400.1/15288

The anthropogenic exploitation of marine resources has severely altered ecosystems and caused drastic declines of large predatory fishes. Amongst these, sharks are the species of major conservation concern due to their critical role as top predators and high vulnerability to overfishing. The Azores are an oceanic archipelago in the mid-North Atlantic thought to serve as essential fish habitat (EFH) for some oceanic or semi-oceanic sharks such as coastal pupping and nursery grounds for tope (Galeorhinus galeus, Linneaus 1758) and smooth hammerhead (Sphyrna zygaena, Linneaus 1758) sharks. Yet, to date, the dependency of those juvenile sharks on coastal resources has not been investigated and crucial information on their trophic ecology is missing. This knowledge gap is relevant as it would allow to ascertain the importance of these areas for conservation and help developing management plans. Here, simultaneous δ13C, δ15N and δ34S (CNS) stable isotope analysis is used to investigate the trophic ecology, ontogenetic shifts and habitat use of the coastal life stages of G. galeus and S. zygeana around the Azores. A Bayesian ellipse approach (nicheROVER) and generalized additive mixed models (GAMM) are applied and interpreted in reference to isotope values of coastal and pelagic food web samples. The results show high diet overlap between tope and juvenile smooth hammerhead sharks with coastal-associated values. Tope shark showed a significant ontogenetic shift to higher trophic level coastal-benthic prey with growing size. Smooth hammerhead sharks exhibited significant decreases in δ34S, also suggesting a shift towards more coastal-benthic prey with increasing size. The diet of both species support their co-occurrence in shared nurseries with no evidence of sexual segregation or interspecific niche partitioning, but instead highly trophic competition, emphasizing the importance of healthy coastal habitats for conservation of these highly mobile sharks in the wider Atlantic. A exploração antropogénica dos recursos ...