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 non-consumptive
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 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 (non-consumptive) behaviours of mesopredators
due to an increase in both acute and chronic risk of predation. We
analysed videos from Baited Remote Underwater Video Systems (BRUVS)
collected prior to 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
non-consumptive impacts of sharks as predators in this system. These
results show that both chronic and acute risk seem to play significant
roles in shaping behaviours of mesopredators.