Healthy shark populations may aid the recovery of coral reefs whose futures are threatened throughout the globe, according to a new study from the Australian Institute of Marine Science (AIMS).
The link has been found by long-term monitoring of reefs off Australia's northwest coast, and showed that where shark numbers were lower due to fishing, herbivores – important fishes in promoting reef health – were also significantly lower in number.
"At first glance the result might seem strange", says Dr Mark Meekan, Principal Researcher at AIMS and co-author of the publication that appears in the scientific journal, PLOS ONE.
"However, our analysis suggests that where shark numbers are reduced we see a fundamental change in the structure of food chains on reefs. We see increasing numbers of mid-level predators – such as snappers – and a reduction in the numbers of herbivores – such as parrotfishes. The parrotfishes are very important because they eat the algae that would otherwise overwhelm young corals on reefs recovering from natural disturbances," he adds.
The study comes at an opportune time in the life of coral reefs, which are facing a number of pressures both from direct human-activity, such as over-fishing, and from climate change, as explained by lead author, Dr Jonathan Ruppert, of the University of Toronto.
According to Dr Ruppert: "The reefs we studied are about 300 kilometres off the coast of northwest Australia and the only human impacts are Indonesian fishers who primarily target sharks, a practice stretching back several centuries, which continues under an Australian-Indonesian memorandum of understanding. These reefs provided us with a unique opportunity to isolate the impact of over-fishing of sharks on reef resilience, and assess that impact in the broader context of climate change pressures threatening coral reefs."
Dr Meekan adds: "On reefs where sharks are fished we found much lower numbers of herbivorous – algae eating – fishes, providing evidence that over-fishing sharks can have detrimental knock-on effects at least for some species further down the food chain. With many of the changes from a warming climate already locked in, there may be little we can do to prevent increased frequency of disturbances on coral reefs in the near future."
"However, this is not case with the loss of reef sharks," he says.
"Tracking studies show that in many cases individual reef sharks are closely attached to certain coral reefs, so even relatively small marine protected areas could be an effective way to protect the top-level predators, which may ultimately mean that coral reefs are better able to recover from coral bleaching or large cyclones. This makes the declines that are occurring in reef sharks due to overfishing throughout the world of great concern, because our study shows that a healthy reef means healthy populations of sharks," Meekan concludes.
The paper "Caught in the middle: combined impacts of shark removal and coral loss on the fish communities of coral reefs," by Jonathan Ruppert, Michael Travers, Luke Smith, Marie-Josée Fortin and Mark Meekan can be found here.