Recovery of reef shark populations invokes anti-predator behaviours in mesopredatory reef fishes on a coral reef.

Natural experiments where populations of large predators have recovered following management initiatives offer insights into the potential for these animals to structure communities via consumptive and nonconsumptive effects on their prey. Ashmore Reef, a coral reef off the coast of Western Australia, provides such an opportunity. Here, reef shark populations have increased significantly since the enforcement of a no-take MPA in 2008. This change has been accompanied by an increase in the abundance of medium and large mesopredatory teleosts, but a decline in small mesopredatory teleosts. We explored whether these changes in abundance were accompanied by changes in anti-predator (nonconsumptive) behaviours of mesopredators due to an increase in both acute and chronic risks of predation. We analysed videos from Baited Remote Underwater Video Systems (BRUVS) collected prior to the enforcement of no-take status in 2004 and after enforcement and shark recovery in 2016 to quantify any changes in anti-predator behaviours of small (<50 cm TL), medium (50-100 cm TL) and large (>100 cm TL) size classes of teleost mesopredators. Comparisons of the effect of chronic and acute risk on the total time teleosts spent in the BRUVS videos, proportional time spent on activities associated with various risks, and foraging intensity were made both within years (acute risk: in videos where sharks were absent compared to present) and between years (chronic risk: 2004 and 2016). Our results indicate that both small- and medium-sized mesopredatory fishes exhibit behaviours indicative of risk-induced trait responses (anti-predator behaviours) in the presence of reef sharks and that these seem to occur as a joint response to the interaction of acute and chronic risks. Our study suggests that the decline of small mesopredatory fishes following the recovery of reef sharks could be due to both the consumptive and nonconsumptive impacts of sharks as predators in this system. These results show that both chronic and acute risks seem to play significant roles in shaping behaviours of mesopredators.