Coral reefs, the endangered rainforests of tropical seas, can be saved by local fishes — but only if we stop overfishing them.
A lawn that is not mowed is soon smothered by weeds. A similar thing happens on reefs. The lawnmowers of the sea — also known as “herbivores” — are animals that eat seaweeds or limu. Herbivores are extremely important to the health of the reef because they keep dead coral surfaces clean so new corals can grow. Once seaweeds smother a reef, corals can never return unless something removes the limu.
Coral reefs are extremely valuable to Hawaii, providing seafood, coastal protection, surfable waves, tourism, medicines, and spiritual connection. The U.S. Geological Service has reported that reduced coastal flooding provided by reefs saves over $395 million per year on Oahu alone. And tourism centered on Hawaii’s coral reefs generated more than $1.2 billion per year before the pandemic.
There are five groups of herbivores on our reefs, all of which are important for saving corals because they eat different limu in different ways. It’s like the various tools we need to keep our lawns well-manicured.
The heavy lifters are the “scrapers,” the parrotfishes or uhu that scratch dead coral surfaces with their beak-like teeth. This scraping causes uhu to poop sand. A single large parrotfish can produce up to a half a ton of sand per year! Think of that the next time you wish there was more sand at your beach.
Leafy seaweeds are eaten by “browsers,” which include the chubs or nenue and a couple other species. The largest group of herbivores eat smaller turfy limu. These are the “grazers,” dominated by surgeonfishes, such as zebra-striped manini and bright yellow lau‘ipala. Other surgeon fishes include the kole, “detritivores” that delicately brush sediment and tiny limu off larger seaweeds. Finally, spiny sea urchins also clean dead coral surfaces.
Unfortunately, many of our lawnmowers of the sea are severely overfished, especially on Oahu. The National Marine Fisheries Service found that parrotfishes and surgeonfishes are particularly overfished. And it’s not enough that herbivores are technically not overfished — they must also be abundant to control seaweeds. Comparative studies with the unfished northwest Hawaiian Islands — the Papahanaumokuakea Marine National Monument — have shown that Oahu has only 5% of its potential herbivore population.
Importantly, in addition to the local stresses of poor water quality, our reefs are increasingly threatened by ocean warming and acidification. Coral bleaching, where corals stressed by ocean heat waves lose the single-celled plants that live within them, feed them, and give them color, is steadily becoming more severe, often killing corals. Scientists predict that Hawaii’s coral will bleach every year by about 2040. Only if there are many herbivores of many different kinds will affected corals have a chance to endure.
How can we save the lawnmowers of the sea? Obey fishing regulations and take only what you need, no more. Call out those who pillage our reefs. Avoid eating herbivores — especially uhu — there are plenty of other tasty fish. And support marine protected areas.
Gov. David Ige’s “Holomua: Marine 30×30 Initiative” has the goal of effectively managing Hawaii’s ocean by establishing at least 30% of nearshore waters as marine managed areas by 2030. Local communities working with ocean managers and scientists now have the opportunity to replenish populations of herbivores on our reefs so that corals can recover following whatever hazards they face in the future.
The Hawaiian creation chant, the Kumulipo, begins with the coral polyp. Let us work together to ensure that corals, the foundation of our reefs, our islands, and our way of life, thrive for present and future generations.
Mark Hixon is the Hsiao Endowed Professor of Marine Biology at the University of Hawaii-Manoa School of Life Sciences.
