Relative influence of predators, competitors and seascape heterogeneity on behaviour and abundance of coral reef mesopredators

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.

Predator removals, trophic cascades and outbreaks of crown-of-thorns starfish on coral reefs

The removal of mesopredatory fishes by fishing may be a key factor driving outbreaks of crown-of-thorns starfish on coral reefs. Evidence for this idea has been derived from correlations between starfish densities and fishing pressure. However, dietary analyses using DNA, studies of the trophic role of mesopredatory fishes and experiments that have invoked threat responses suggest that outbreaks could also result from a trophic cascade driven, in part, by changes in the anti-predator behaviours of these fishes. If corroborated, this hypothesis could inform management decision-making, slowing the frequency of outbreaks and improving the health of coral reefs in the Anthropocene.

Intraguild processes drive space-use patterns in a large-bodied marine predator community

Interspecific interactions, including predator-prey, intraguild predation (IGP) and competition, may drive distribution and habitat use of predator communities. However, elucidating the relative importance of these interactions in shaping predator distributions is challenging, especially in marine communities comprising highly mobile species. We used individual-based models (IBMs) to predict the habitat distributions of apex predators, intraguild (IG) prey and prey. We then used passive acoustic telemetry to test these predictions in a subtropical marine predator community consisting of eight elasmobranch (i.e. shark and ray) species in Bimini, The Bahamas. IBMs predicted that prey and IG prey will preferentially select habitats based on safety over resources (food), with stronger selection for safe habitat by smaller prey. Elasmobranch space-use patterns matched these predictions. Species with predator-prey and asymmetrical IGP (between apex and small mesopredators) interactions showed the clearest spatial separation, followed by asymmetrical IGP among apex and large mesopredators. Competitors showed greater spatial overlap although with finer-scale differences in microhabitat use. Our study suggests space-use patterns in elasmobranchs are at least partially driven by interspecific interactions, with stronger spatial separation occurring where interactions include predator-prey relationships or IGP.

Behaviour of mesopredatory coral reef fishes in response to threats from sharks and humans

Both sharks and humans present a potentially lethal threat to mesopredatory fishes in coral reef systems, with implications for both population dynamics and the role of mesopredatory fishes in reef ecosystems. This study quantifies the antipredator behaviours mesopredatory fishes exhibit towards the presence of large coral reef carnivores and compares these behavioural responses to those elicited by the presence of snorkelers. Here, we used snorkelers and animated life-size models of the blacktip reef shark (Carcharhinus melanopterus) to simulate potential predatory threats to mesopredatory reef fishes (lethrinids, lutjanids, haemulids and serranids). The responses of these reef fishes to the models and the snorkelers were compared to those generated by three non-threatening controls (life-size models of a green turtle [Chelonia mydas], a PVC-pipe [an object control] and a Perspex shape [a second object control]). A Remote Underwater Stereo-Video System (Stereo-RUV) recorded the approach of the different treatments and controls and allowed accurate measurement of Flight Initiation Distance (FID) and categorization of the type of flight response by fishes. We found that mesopredatory reef fishes had greater FIDs in response to the approach of threatening models (1402 ± 402-1533 ± 171 mm; mean ± SE) compared to the controls (706 ± 151-896 ± 8963 mm). There was no significant difference in FID of mesopredatory fishes between the shark model and the snorkeler, suggesting that these treatments provoked similar levels of predator avoidance behaviour. This has implications for researchers monitoring behaviour in situ or using underwater census as a technique to estimate the abundance of reef fishes. Our study suggests that, irrespective of the degree to which sharks actually consume these mesopredatory reef fishes, they still elicit a predictable and consistent antipredator response that has the potential to create risk effects.

Drivers of variation in occurrence, abundance, and behaviour of sharks on coral reefs

Quantifying the drivers of population size in reef sharks is critical for the development of appropriate conservation strategies. In north-west Australia, shark populations inhabit coral reefs that border growing centres of human population, industry, and tourism. However, we lack baseline data on reef sharks at large spatial scales (hundreds of km) that might enable managers to assess the status of shark populations in the face of future development in this region. Here, we examined the occurrence, abundance and behaviour of apex (Galeocerdo cuvier, Carcharhinus plumbeus) and reef (C. amblyrhynchos, C. melanopterus, Triaenodon obesus) sharks using > 1200 deployments of baited remote underwater stereo-video systems (stereo-BRUVs) across > 500 km of coastline. We found evidence for species-specific influences of habitat and fishing activities on the occurrence (probability of observation), abundance (MaxN) and behaviour of sharks (time of arrival to the stereo-BRUVs and likelihood of feeding). Although the presence of management zoning (No-take areas) made little difference to most species, C. amblyrhynchos were more common further from boat ramps (a proxy of recreational fishing pressure). Time of arrival for all species was also influenced by distance to boat ramp, although patterns varied among species. Our results demonstrate the capacity for behavioural metrics to complement existing measures of occurrence and abundance in assessing the potential impact of human activities on shark populations.