Isolation and no-entry marine reserves mitigate anthropogenic impacts on grey reef shark behavior

International audienceReef sharks are vulnerable predators experiencing severe population declines mainly due to overexploitation. However, beyond direct exploitation, human activities can produce indirect or sub-lethal effects such as behavioral alterations. Such alterations are well known for terrestrial fauna but poorly documented for marine species. Using an extensive sampling of 367 stereo baited underwater videos systems, we show modifications in grey reef shark (Carcharhinus amblyrhynchos) occurrence and feeding behavior along a marked gradient of isolation from humans across the New Caledonian archipelago (South-Western Pacific). The probability of occurrence decreased by 68.9% between wilderness areas (more than 25 hours travel time from the capital city) and impacted areas while the few individuals occurring in impacted areas exhibited cautious behavior. We also show that only large no-entry reserves (above 150 km²) can protect the behavior of grey reef sharks found in the wilderness. Influencing the fitness, human linked behavioral alterations should be taken into account for management strategies to ensure the persistence of populations. Beyond overexploitation and the global defaunation of all ecosystems on Earth 1 , humans can induce behavioral alterations on animals with potential consequences on their fitness 2,3. For instance, terrestrial wildlife like mammals 4 and birds 5 are more vigilant near humans, resulting in decreased food intake and reduced reproductive success. Similarly, marine mammals show behavioral alterations (i.e. decreased food intake and reduced reproductive success) due to nautical activities, resulting in additional energetic costs to avoid interactions or nuisance 6,7. Sharks exhibit high learning abilities similar to most terrestrial vertebrates 8. Behavior experiments demonstrate that sharks can learn discriminative tasks and retain information for long time periods 9,10. These abilities make sharks also likely to demonstrate long-lasting adaptive responses to external stimuli including interactions with human activities 11. For instance, the catchability of blacktip reef sharks (Carcharhinus melanopterus) decreases after several catches and releases 12 while bull sharks (Carcharhinus leucas) show a modified behavior related to food provisioning 13. Although human impacts on shark behavior have been documented, we still lack empirical evidence of behavioral alterations along a wide gradient of human activities from densely populated areas to the last wilderness areas. Reef sharks are emblematic species providing socioeconomic benefits through tourism while playing multiple ecological roles in coastal ecosystems 14,15. Their conservative life history traits such as slow growth rate, late sexual maturity and low fecundity, coupled with overfishing, induce severe population depletion worldwide and high extinction risk 16,17. Consequently, reef sharks are of high conservation priority 18. Marine reserves, a restrictive subset of marine protected areas 19 , are recognized as management tools to counteract human pervasive impacts by prohibiting extracting activities (no-take) or even human presence (no-entry) 20. By lessening human pressure including extraction of their prey, marine reserves may attract sharks and restore their behavior under quasi-wilderness or undisturbed conditions 21. However, only a very small portion of marine reserves ar