Resumen del producto

Trophic interactions are one of the main indicators used to describe the role of species within marine ecosystems. This is particularly relevant for shark species, which occupy top positions in marine food webs and exhibit a wide spectrum of prey and foraging habits. Stable isotopes of carbon and nitrogen have allowed a tremendous progress in the understanding of shark trophic ecology, by respectively unravelling primary production sources and trophic positions. This methodology has therefore been applied to investigate competition in sympatric shark species but can appear limited for apex predators that can perform large movements over different habitats, such as deep dives and long-distance migrations. The present study analyses the trophic ecology of three shark species co-occurring in the Pacific coast of Baja California Sur (Mexico): the blue shark (Prionace glauca), the scalloped hammerhead shark (Sphyrna lewini) and the smooth hammerhead shark (Sphyrna zygaena). Focusing on muscle tissues, we completed classical stable isotopes analyses (d13C and d15N) with mercury (Hg) isotopes (d202Hg and ?199Hg), which represent promising new ecological tracers. Affected by light radiation and therefore displaying contrasted patterns with depth, these newly developed Hg isotopes reflect preferential foraging layers in the water column. We found that P. glauca showed lower values for all isotopic tracers (i.e. lower d13C and d15N as well as lower d202Hg and ?199Hg) than the two hammerhead shark species. These results suggest that P. glauca had lower trophic position and foraged on mesopelagic prey, while hammerhead sharks occupied higher trophic positions and fed on more epipelagic prey. Regarding S. lewini, an increase in both d202Hg and ?199Hg with increasing size was observed, likely explained by the migration of this species from coastal shallow waters to the open ocean throughout ontogeny. This study shows that mercury isotopes could therefore be a useful complement to traditional biomarker approaches to describe the trophic ecology of large predators which exploit a diversity of feeding niches over a three-dimensional scale.

Palabras clave: Trophic ecology; carbon and nitrogen isotopes; Mercury isotopes; Competition; ontogeny; Sharks

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