Sharks Play Critical Role In Coral Reef Health

Brett Smith for – Your Universe Online

Sharks play an important role in the food web of a marine ecosystem and a study recently published in the journal PLOS ONE has found that overfishing of these predators can have a disruptive effect on coral reefs.

“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,” said study author Jonathan Ruppert, a marine ecologist with the University of Toronto.

Co-author Mark Meekan of the Australian Institute of Marine Science (AIMS) said that the team’s finding might seem strange at first given that sharks and coral reefs seem to be completely unconnected.

“However our analysis suggests that where shark numbers are reduced, we see a fundamental change in the structure of food chains on reefs,” he said. “We saw increasing numbers of mid-level predators – such as snappers – and a reduction in the number of herbivores such as parrotfishes.”

“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.”

To reach their conclusion, the team of Canadian and Australian scientists conducted a long-term monitoring program of coral reefs off Australia’s northwest coast. The reefs sit about 190 miles off the coast of Australia where sharks are targeted by Indonesian fishers. For several centuries, sharks have been hunted in these waters under an Australian-Indonesian memorandum of understanding.

“Going by our surveys, around four sharks a day were being taken from these reefs,” Meekan told the Guardian. “This doesn’t sound like a lot, but it has been going on for a long time. The fishermen come in their sailing prows, which can dry an awful lot of shark fin on the decks.”

“The 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,” Ruppert noted.

“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.”

In their report, the researchers said the complex web of prey-predator interactions surrounding coral reefs makes uncovering a specific apparatus linking sharks and corals very difficult.

“Despite the correlation between shark abundance and herbivores, we could not show the mechanism that linked these trophic levels,” the scientists wrote.

Previous studies have shown that reef sharks are closely linked to certain coral reefs, meaning that small, well-targeted protected areas could effectively protect both apex predators and the reefs themselves.

This new study comes as coral reefs are under pressure from direct human activities, as well as the impact of climate change. The use of small protected areas would allow coral reefs to be able to recover from large storms, which some scientists say are increasing in frequency and strength.