Abstract
AbstractSharks 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.
Highlights
Elasmobranchs (i.e., “sharks and rays”) remain the least thoroughly assessed vertebrates on Earth (Dulvy et al, 2014), despite their established ecological significance (Ferretti, Worm, Britten, Heithaus, & Lotze, 2010) and their diverse, conflicting value to human societies (Dulvy et al, 2017)
We identified 16 sites as representative of the shelf reef habitats (Pinault et al, 2014) in Reunion Island, with a special emphasis on coral reefs, which are more prevalent on the west side of the island (Figure 1), and are the most vulnerable ecosystems, where increasing anthropogenic pressures have been observed in the last decade (Lagabrielle et al, 2018; Magnan & Duvat, 2018)
When we assessed the relationship between G. cuvier and S. lewini in function of time, we found that tiger shark abundance was inversely correlated to both scalloped hammerhead abundance (F = 5.7, p = .02) and time (F = 31.7, p = .001), but most of the variance was explained by time (R2 = 0.33), rather than the abundance of S. lewini (R2 = 0.06)
Summary
Elasmobranchs (i.e., “sharks and rays”) remain the least thoroughly assessed vertebrates on Earth (Dulvy et al, 2014), despite their established ecological significance (Ferretti, Worm, Britten, Heithaus, & Lotze, 2010) and their diverse, conflicting value to human societies (Dulvy et al, 2017). Concerns over the potential negative impacts of sharks on watersports and other coastal leisure activities have simultaneously prompted the activation of a shark control programme, and an unprecedented level of research effort on the status of the elasmobranch fauna around the island (Blaison et al, 2015; Guyomard et al, 2019), mostly focused on the two species responsible for the unprovoked bites, bull shark (Carcharhinus leucas) and tiger shark (Galeocerdo cuvier) (Ballas, Saetta, Peuchot, Elkienbaum, & Poinsot, 2017; Guyomard et al, 2020) Such a scenario presents a major conservation challenge, because management strategies are being introduced reactively, in an environmental context with large gaps in ecosystem knowledge, and before scientists are in a position to form a comprehensive understanding of the broader ecological consequences of these management actions. Results amass several lines of evidence that can be of immediate relevance for the management of the island's coastal marine environment
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