Caught in the Middle: Combined Impacts of Shark Removal and Coral Loss on the Fish Communities of Coral Reefs

Abstract

Due to human activities, marine and terrestrial ecosystems face a future where disturbances are predicted to occur at a frequency and severity unprecedented in the recent past. Of particular concern is the ability of systems to recover where multiple stressors act simultaneously. We examine this issue in the context of a coral reef ecosystem where increases in stressors, such as fisheries, benthic degradation, cyclones and coral bleaching, are occurring at global scales. By utilizing long-term (decadal) monitoring programs, we examined the combined effects of chronic (removal of sharks) and pulse (cyclones, bleaching) disturbances on the trophic structure of coral reef fishes at two isolated atoll systems off the coast of northwest Australia. We provide evidence consistent with the hypothesis that the loss of sharks can have an impact that propagates down the food chain, potentially contributing to mesopredator release and altering the numbers of primary consumers. Simultaneously, we show how the effects of bottom-up processes of bleaching and cyclones appear to propagate up the food chain through herbivores, planktivores and corallivores, but do not affect carnivores. Because their presence may promote the abundance of herbivores, the removal of sharks by fishing has implications for both natural and anthropogenic disturbances involving the loss of corals, as herbivores are critical to the progress and outcome of coral recovery.

Evidence for rapid recovery of shark populations within a coral reef marine protected area

“…Long-term (15 years) monitoring studies on the Scott Reefs and Rowley Shoals found no evidence that abundances of sharks and teleost mesopredators varied in response to very large changes in the composition of benthic communities (loss of 80% of coral cover) and the replacement of live coral by turfing algae, or during the process of recovery of coral habitats (Ruppert et al 2013). Recent studies have shown that the differences in relative abundances of sharks between the unfished Rowley Shoals and the fished Scott Reefs have now remained unchanged for almost 20 years (Barley et al 2017b;Ruppert et al 2013), despite these reefs undergoing long cycles of disturbance and recovery in benthic communities during this time (Gilmour et al 2013).…”

“…Hussey et al 2017). Additionally, targeted fishing for sharks also captures large mesopredatory teleosts, which are likely to act as competitors to reef sharks (Barley et al 2017a, b;Roff et al 2016;Ruppert et al 2013). …”

“…Declining populations of sharks in the tropics are of concern because of increasing evidence of their important trophic role Roff et al 2016;Ruppert et al 2013). The presence of sharks has been shown to affect the diet, condition and morphology of their prey (Barley et al 2017a, b) and food chain structure (Barley et al 2017a).…”

“…There is also evidence that the absence of predators may affect the ability of reefs to recover from disturbances that remove coral cover (e.g. cyclones and bleaching) (Ruppert et al 2013) and promote outbreaks of corallivores (e.g. crown-of-thorns starfish Acanthaster plancii (Dulvy et al 2004;Ruppert et al 2013), which is a major issue in a world facing climate change (Hughes et al 2003).…”

“…cyclones and bleaching) (Ruppert et al 2013) and promote outbreaks of corallivores (e.g. crown-of-thorns starfish Acanthaster plancii (Dulvy et al 2004;Ruppert et al 2013), which is a major issue in a world facing climate change (Hughes et al 2003). However, there is still some uncertainty around the exact functional role reef sharks play in structuring reef communities, which is due to the complex nature of ecosystem dynamics and unknown exploitation histories (Roff et al 2016).…”