Overfishing of sharks, done mostly for their fins, is causing a chain reaction that could ultimately cause significant damage to coral reefs, researchers say.
“Where shark numbers are reduced due to commercial fishing, there is also a decrease in the herbivorous fishes which play a key role in promoting reef health,” says Jonathan Ruppert, a recent University of Toronto PhD graduate.
Ruppert was part of a team engaged in long-term monitoring of reefs off Australia’s northwest coast.
The results at first glance may seem strange, says team leader Mark Meekan, of the Australian Institute of Marine Science (AIMS).
“However our analysis suggests that where shark numbers are reduced, we see a fundamental change in the structure of food chains on reefs.
“We saw increasing numbers of mid-level predators—such as snappers—and a reduction in the number of herbivores such as parrotfishes,” he says.
“The parrotfishes are very important to coral reef health because they eat the algae that would otherwise overwhelm young corals on reefs recovering from natural disturbances.”
The study comes at an opportune time—coral reefs are facing a number of pressures both from direct human activity, such as overfishing, and from climate change.
For the study, published in the journal PLOS ONE, researchers studied reefs located about 300 kilometers off the coast of northwest Australia where Indonesian fishers target sharks—a practice stretching back several centuries and which continues under an Australian-Indonesian memorandum of understanding.
“The reefs provided us with a unique opportunity to isolate the impact of overfishing of sharks on reef resilience, and assess that impact in the broader context of climate change pressures threatening coral reefs,” Ruppert says.
“Shark fishing appears to have quite dramatic effects on coral reef ecosystems. Given that sharks are in decline on reefs worldwide, largely due to the shark fin trade, this information may prove integral to restoration and conservation efforts.”
Tracking studies show that, in many cases, individual reef sharks are closely attached to certain coral reefs.
This means that even relatively small marine-protected areas could be effective in protecting the top-level predators and allowing coral reefs to more fully recover from coral bleaching or large cyclones which are increasing in frequency due to the warming of the oceans as a result of climate change.
The Natural Sciences and Engineering Research Council of Canada and Woodside Energy of Perth, Australia, funded the research.
Source: University of Toronto