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Study: Sharks boost reef health

<p>Apex predators such as tiger sharks are at the top of the food chain in Papahanaumokuakea Marine National Monument. They are dependent on bottom-dwelling algae as the base of the chain.</p><p>James Watt/
Special to Stephens Media Hawaii</p>


Stephens Media Hawaii

A recently published study by University of Toronto researchers links the number of sharks on a reef to the reef’s health, with more sharks indicating a healthier reef.

The proposed West Hawaii fisheries rules package, still awaiting Gov. Neil Abercrombie’s signature, included provisions to protect nine species of inshore sharks and rays, as well as two invertebrate crown-of-thorns predators.

Those species included the tiger shark, whale shark, whitetip reef shark, blacktip reef shark, gray reef shark, spotted eagleray, Hawaiian stingray, broad stingray and pelagic stingray.

A spokeswoman for the governor said the package was still under review this week. The rules package was controversial, particularly for measures that would ban scuba spearfishing in West Hawaii waters. The Board of Land and Natural Resources approved the plan, including the scuba spearfishing measures, over the objections of Department of Land and Natural Resources Chairman William Aila. Abercrombie cannot veto the package, but he can let it sit unsigned, effectively preventing the package from being implemented.

Dr. William Walsh, a Department of Land and Natural Resources Division of Aquatic Resources Management marine biologist, said West Hawaii’s waters don’t have an abundance of inshore sharks, but the University of Toronto study illustrates the import role sharks play in keeping the reefs balanced.

“If the sharks aren’t there, what happens?” Walsh said.

Losing the “top of the line” predators such as sharks allows for more mid-level predators, Walsh said, citing the study. Those predators in turn eat more of the herbivorous fish, which are responsible for keeping algae levels on the reef in check. That, researchers said, can then affect the reef’s health.

“You wouldn’t even think it has anything to do with sharks,” Walsh said. “It’s very clear the big difference is the top-level predators were missing.”

The study shows, as other research has, just how interconnected and complex reef systems are, he added.

“If you start mucking with one part of it, things happen you wouldn’t think of happening,” Walsh said. “Who would think coral survivorship would be connected to big predators?”

The study, available at PLOS One, compared two remote atolls north of Australia, looking at the impact of shark removal from the reefs. One was a marine protected area, with fishing restrictions, while the other was open for fishing, including shark fishing. Researchers found the fished reefs had more algae than the non-fished reefs.

Walsh said the study bolsters the idea that protecting the reef shark species, some of which show up only occasionally in West Hawaii waters, is “really critical.”

Sharks do not reproduce as quickly as smaller predators, which means any shark that is taken from the reef isn’t replaced as quickly as a species that lays multiple eggs at a time.

“It’s the kind of rule that isn’t going to have a dramatic effect,” Walsh said, adding the shark rules will play an important role in protecting the reef.

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