Who Will Save the Coral Reef Saviors of Hawai’i?

Mark Hixon | August 3, 2023 | Leave a Comment

Parrotfishes and other herbivores clean reefs so new corals can grow after human activities kill corals, but these living lawnmowers of the sea are terribly overfished near human population centers

Imagine an idyllic Hawaiian beach and there’s a good chance a beautiful coral reef will appear in your mind: vibrant corals being the trees of a colorful undersea rainforest, teaming with an astonishing variety of species, ranging from minuscule invertebrates of every kind to enormous fishes, sea turtles, and monk seals.

Healthy coral reefs provide extremely valuable ecosystem goods and services for humanity. These natural freebies include productive fisheries, coastal protection from increasing erosion due to sea level rise and worsening storms, exceptional recreational opportunities, breathtaking beauty, spiritual inspiration, cultural practices, and even new medicines. A quarter of all known ocean species inhabit coral reefs, which occupy less than 1% of the ocean surface, and a billion humans depend on reefs for their livelihoods.

Unfortunately for humanity, thoughtless abuses have already resulted in the loss of about half our coral reefs worldwide. Historically, reefs were killed by intentional dredging and burying (still happening full-blast in the South China Sea), by pollution of various kinds that can stimulate outbreaks of coral predators (e.g., the crown-of-thorns seastar on the Great Barrier Reef and elsewhere), by mud flowing off poorly managed coastal zones and smothering reefs, and by excess fertilizers and leaching wastewaters that stimulate seaweeds to overgrow and outcompete corals (major problems in Hawai’i, which has more cesspools than any other state). 

Now, as climate disruption rapidly worsens coral mortality events, many of our reefs are on the brink of collapse.

Ironically, the one ecological service of healthy reefs that makes them resilient to such assaults–herbivory in scientific jargon–is also the victim of human shortsightedness. By consuming what scientists call benthic algae (i.e., seaweeds), parrotfishes (uhu in Hawaiian), surgeonfishes (kala, kole, manini, and others), chubs (nenue), and others keep dead coral surfaces clean so new corals can settle, survive, and grow. The different species remove different seaweeds in different ways, so it takes a diversity of herbivores to keep a reef clean. Yet many of these fishes, especially uhu and major surgeonfishes, are terribly overfished near human population centers. For example, around the island of O’ahu in Hawai’i, total reef herbivore populations are at less than 5% of their estimated unfished abundance.

The same Bullethead Parrotfish (Chlorurus spilurus), or uhu, at work. Credit: Jeff Kuwabara.


While large coral colonies can hold their own against encroaching seaweeds, baby corals cannot survive such overgrowth. When a coral dies for any reason, the dead coral surface will most often become covered by either seaweed or new coral, depending upon the local abundance and species diversity of herbivorous fishes and some sea urchins.

Think of a healthy reef as a living, self-reinforcing, positive feedback loop: herbivores control seaweeds, so corals thrive and provide shelter and living space for not only herbivores but all reef life. Kill the corals and remove the herbivores, and the reef ecosystem collapses.

Amazingly, in my home state of Hawai’i and elsewhere in the world, few of those in power–from both the top down (e.g., politicians) and the bottom up (e.g., fishermen)–understand (or want to acknowledge) the critical link between healthy herbivore populations and healthy coral reefs. Overfishing of herbivores is seen as only a fisheries issue, with managers merely trying to bring populations up to about a quarter of their unfished biomass, typically considered to be the overfishing threshold. But even if not overfished technically by this narrow-minded definition, reducing herbivore populations by only half may destroy the ability of these living lawnmowers to keep our reefs clean and resilient to coral mortality events.

Here in Hawai’i, the new 800-pound gorilla in the room is coral bleaching caused by ocean warming. 

Bleaching occurs when an ocean heat wave breaks down the mutual relationship between the coral animal–the polyp–and symbiotic plant-like microbes living within them. Corals cannot survive long without their symbionts, and coral bleaching has already killed vast tracts of the Great Barrier Reef and other reefs globally. Hawai’i is sufficiently far north that there have been only four substantial bleaching events so far: 1996, 2014, 2015, and 2019. Only the 2014 and 2015 events caused considerable coral mortality, and only in some regions. 

Now at the end of an unprecedented 2020-2022 cool-water streak in the Pacific–3 years of La Niña conditions–we are entering what is predicted to be an intense El Niño ocean phase that will likely cause severe heat waves and coral bleaching throughout the tropical Pacific. And the situation will only worsen. Scientists predict that Hawaiian coral reefs will bleach every single year by 2040. Reefs with more herbivores and coral are known to resist and recover from bleaching events much more rapidly than degraded reefs.

Unfortunately, Hawai’i’s reefs, like others, are sitting ducks. With our reefs already stressed by poor coastal water quality, and now increasingly by coral bleaching, the herbivores that could save the day are few to be found. Is all hope lost? 

If Hawai’i’s luck persists, we avoid major coral mortality for another decade, and we act to replenish herbivore populations, then there is hope.

A new public education campaign has been launched to spread the word about the critical importance of healthy herbivore populations: 

Fish Pono—Save Our Reefs (fishpono.org).

Only time will tell whether this and other conservation initiatives will finally inspire the silent majority of Hawai’i to demand an end to the status quo of ignoring the loss of our reef saviors: the uhu and friends.

Mark Hixon is the Hsiao Endowed Professor of Marine Biology in the School of Life Sciences at the University of Hawai’i at Mānoa. He has studied coral reef herbivores in Hawai’i on and off since the 1970s.

The MAHB Blog is a venture of the Millennium Alliance for Humanity and the Biosphere. Questions should be directed to joan@mahbonline.org 

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