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CRITTER: Invasive marine fish damage our coral reefs

Fish that evolved on our coral reefs here in Hawai‘i all have predators that may eat them, but they recognize the predators and normally only the weak or old fish get eaten.

This keeps our coral reefs healthy. Our butterfly fish feed on sick corals, which makes room for new corals to grow. When the butterfly fish gets old, sick or wounded it will get eaten by a large ulua or shark, and this creates space on the reef for new young butterfly fish to grow. This is all just the balance of nature and our coral reefs have figured out how to achieve this balance for a very long time, until humans came along.

Back in the 1950s, the state of Hawai‘i introduced three fish species from French Polynesia into Hawaiian waters to try and have more fish for people to eat. Hawai‘i is classified as a “marine desert” because we are so isolated that our coral reefs have very few fish species compared to other island chains that are closer to large land masses. Adding in more good eating fish may have sounded like a good idea at the time, but it turned out to be a disaster.

One of the fish species that was introduced is the the roi or peacock grouper. This fish grows to be about 20 inches long and it feeds close to the reef or in caves on smaller fish. When I dove with the roi in Tahiti in its native habitat, the other reef fish like the aweoweo and u’u know it is a predator. When the roi came into their cave, the smaller red fish spit to keep from being eaten. Just a normal predator prey relationship on a healthy reef.

In Hawai‘i, it is different between the roi and the smaller red cave fish because the roi did not evolve on Hawaiian reefs. When the roi goes into a cave, the aweoweo swims right up to the roi’s face to see what it is and they instantly get eaten. The butterfly fish do the same thing with the roi, so this new predator fish eats healthy fish, which it could not normally catch on their native reef. By the time the red cave fish and butterfly fish figure out that the roi is a large predator, they have been all eaten. This upsets the balance on the reef, and can even cause corals to die because there are no healthy fish left to clean them.

Many of my dive sites in Hawai‘i have hundreds of roi along with the two other invasive fish that were introduced, the ta‘ape and to‘au. One small isolated rock pile in the marine reserve at Sharks Cove on O‘ahu had more than 40 aweoweo living there 10 years ago, and now there is nothing left except a huge pair of fat roi. This is not good for the future of our native marine fish.

In the past, spear fishermen and women have put on “roi roundups” where they go out spearfishing for a day and only remove invasive fish. And, at an event at ‘Anini Beach about 10 years ago, we all caught more than 400 roi in one day and the fish were used to make organic fertilizer. Maybe we should do more of these events and try to get a balance back on our coral reefs between predators and prey.


Terry Lilley is a marine biologist living in Hanalei Kaua‘i and co-founder of Reef Guardians Hawai‘i, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawaii go to
Source: The Garden Island

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