"Despite that, we're not getting the negative impact on the grazers, which are so important to keep the [seaweed] in check," Brumbaugh added.
Mumby and his colleagues found that larger parrotfish, which are vulnerable to fishing outside the reserve, have thrived in the protected area along with the grouper.
The researchers also found that large parrotfish escape predation by grouper, because they are too big for the predators to eat.
Once parrotfish reach a length of about 11 inches (28 centimeters), they become too big for even the largest grouper to swallow, the scientists explained.
So while the grouper preyed more on smaller parrotfish species, populations of larger parrotfish have grown. And these big parrotfish eat the most seaweed.
As a result, the researchers found that grazing within the reserve has doubled, which corresponds to a fourfold reduction in seaweed cover compared to outside the reserve.
"Those are the conditions necessary to help corals return to the reef and grow free of competition from [seaweed]," Mumby said.
In an accompanying article Ove Hoegh-Guldberg, a marine scientist at the University of Queensland in Australia, writes that the findings "illustrate some of the complexity of how marine protected areas might influence coral reef ecosystems."
Mumby and his colleagues were initially concerned about a potential decline in parrotfish numbers in the reserve, because a mysterious disease wiped out a species of sea urchin there in 1983.
The sea urchins were the only other species known to eat the seaweed that grows on the coral reefs. Without either the sea urchins or the parrotfish, the coral would almost certainly suffer.
For coral reefs to grow, larval coral must settle on dead coral or rock, Mumby explained.
"The first thing a lot of seaweed does is reduce the amount of space corals have to settle on, and even if they do settle, they can be overgrown by [seaweed]," he said.
Worried that further stress on the reefs could prove devastating, Mumby and his colleagues were skeptical at first that marine reserves could protect reefs.
Now, armed with this new research, Mumby and his colleagues are ready to further investigate how protected areas can help save the world's fast-disappearing reefs.
"The use of reserves to protect fish stock is totally compatible with using them to protect corals," he said.
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