Research Highlight: Protect Herbivores, Preserve Coral Reefs

Scientists from Scripps Institution of Oceanography at the University of California San Diego predict that protection of seaweed-eating fish and invertebrates will help maintain the health of a Hawaiian coral reef by leveling the competition between fast-growing seaweed and slow-growing coral.

In a paper published in the journal Ecosphere, the Scripps team used data from Hawaii’s Kahekili Herbivore Fisheries Management Area, the world’s first herbivore reserve, to evaluate the benefit of a new management policy that specifically protects parrotfish (Uhu, in Hawaiian), surgeonfish (Api), and other herbivores. Such marine organisms keep in check the spread of seaweed that would otherwise overtake and damage or destroy coral reefs.

According to the study’s lead author Emily Kelly, a marine ecologist at Scripps and former Scripps graduate student, their results are very promising.

Kelly and her team, which included researchers from NOAA’s Pacific Island Fisheries Science Center, University of Hawaii, and Hawaii’s Division of Aquatic Resources, gathered data from Kahekili on herbivore grazing rates and algal growth rates from 2009 (the first year of the herbivore protection) to 2014, to track the amount of seaweed eaten each year. The team’s results showed that throughout the first five years of herbivore protection, seaweed growth consistently exceeded grazing by herbivorous fishes, but by a shrinking margin since 2010. In that year, the amount eaten was 20.8 percent of the amount that grew. By the end of the study in 2014, consumption was 67.0 percent of production.

Then they used the data collected from Kahekili and predicted how the grazing deficit would change if the area, which had been protected for only five years, had the same quantity of grazers as the neighboring Molokini Shoal Marine Life Conservation District, which has been protected for more than forty years.

“We’re trying to provide a window into what Kahekili could look like in the future,” Kelly said of the comparison. “I’m excited to see that the grazing deficit – the difference between the rate of algal growth, and the rate at which herbivores eat the algae – is getting smaller through time.”

“I am very surprised with the speed that this reef ecosystem has responded to the management action,” said Russell Sparks, a coauthor of the paper and an aquatic biologist in Hawaii’s Department of Land and Natural Resources, Aquatic Resources Division. “As a result of the successes we have measured in Kahekili, I suspect herbivore management will become a much more commonly utilized tool throughout Hawaii and the world.”

By making projections for the reserve using a neighboring reef, “we’re not trying to hold this reef to a standard of some reef far away from humans, but rather we’re looking at an example of a reef in the main Hawaiian Islands,” Kelly said. “We’re trying to use a realistic reef future scenario to show that there could be enough herbivore biomass in the future that herbivores will in fact be grazing down the seaweed.”

Kelly says these results can hopefully support herbivore management as a conservation strategy, which is advantageous for policymakers because herbivore protection allows mixed use of a reef area.

“Herbivore management can be a great option because it allows for fishing of other types of fishes. Fishing is really important in Hawaii, it has a lot of cultural importance and of course is important for people getting fish for food, for both day-to-day meals and for celebration,” Kelly said. “Hopefully herbivore management can be a balance of allowing fish to graze down the reef and keep it healthy but also allow people to fish and use the reef in ways that they have for many generations.”

Kelly added that healthy coral reefs with plenty of fish provide many benefits in addition to supporting food supplies. They also protect shorelines from storms and are a big tourist draw, providing a boon for local economies.

– Mallory Pickett

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