Sharks embody several major aspects of modern marine management: they are traditionally antagonized, exploited or by-caught by humans, typically vulnerable to extirpation, pursued as luxury food, yet valued as wildlife and essential as top-down regulators in marine food webs. Due to their generally large size, elusiveness, high mobility, and potentially dangerous nature, elasmobranchs pose substantial technical challenges to biodiversity monitoring, which prompted recent efforts to harness the power of environmental DNA (eDNA) as a noninvasive survey method for these taxa. Here we deployed an elasmobranch-specific metabarcoding assay to characterize shark and ray diversity around Reunion Island, during the austral summer, detecting at least 14 species and a strong overall correlation between frequency of species detection and read abundance. Over 90% of sequence reads belonged to three large predators: scalloped hammerhead (Sphyrna lewini), tiger shark (Galeocerdo cuvier) and bull shark (Carcharhinus leucas). While the importance of tiger and bull sharks is well established in Reunion Island, and a major focus of the local shark control program, the prevalence and abundance of scalloped hammerhead has so far been grossly neglected.We also confirm the absence of typical "tropical reef sharks" around the island and reveal an important temporal fluctuation in tiger shark during the study period. Collectively, results show how eDNA can help circumvent barriers, bias and drawbacks associated with monitoring shark populations using visual and capture-based techniques, and generate spatial and temporal biodiversity data on these species for rapid consideration by marine environmental managers.

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