An extremely low-cost and accessible tool for conservation is on the horizon after decades of research into the natural phenomenon of coral reef halos, scientists say.

Jack Randall, the former senior ichthyologist at Bishop Museum in Honolulu, first wrote about coral reef halos over 50 years ago. Since then, Elizabeth Madin, a marine ecologist and assistant professor at the Hawaii Institute of Marine Biology, has become a leading expert in the field.

Her latest research would be a game-changer for reef managers in places where there are a plethora of reefs to monitor and fewer resources. Instead of spending time and money to send out dive teams down to count fish and examine the reef in person, they could glean much of the same information just by looking at satellite imagery — using the halos that appear in the photos as indicators for reef health.

Madin and her nine-member team at HIMB are working to understand more about how the halos are being formed, what the key components or drivers are that allow the halos to persist, and how the halos change over time.

Coral Halos Satellite Image
A satellite image of coral halos in the Northern Red Sea. Google Earth

An estimated 1 billion people benefit from coral reefs for food, coastal protection and income, according to the Environmental Protection Agency.

But in the last 30 years, scientists say more than half of the world’s reefs have succumbed to bleaching as the ocean’s temperature continues to warm and stress the coral. That’s part of the reason scientists are more keen than ever to monitor reef health worldwide.

Reef halos are believed to be formed by the interaction between predators and prey, says Zack Rago, a PhD student working at Madin’s lab who was featured in the Netflix documentary Chasing Coral. Herbivores like to graze close to the protection of the reef so they tend to stay around the same area. As a result, they eat all of the algae and leave behind the barren sand, forming a ring or halo around the reef that can be seen even from space.

Rago is currently researching whether the distance around the reef that is barren can change based on the number of predators in the water. Others in Madin’s lab are interested in topics such as the environmental factors that play a role in the reef halo formation, and how to measure and monitor the halos using satellites and how the dynamics change over a year or longer.

“There are halos as far back as you can go with satellite and aerial imagery,” Madin said. “We see halos that were there in the 1960s that are still there today in the exact same place.”

Although halos are resistant over the long term, she said they can change in size over the short term — which is where Rago’s work comes into play.

HIMB Coral Halo Experiment Enclosure
Zack Rago’s 12 enclosures built out of PVC pipes where he tests the correlation between predators, herbivores, and their effects on forming coral reef halos. Courtesy: Aviv Suan/2022

Rago is exploring if and when halos get bigger or smaller, whether it tells us more about the predators, the herbivores, or both. He designed a system that allowed him to observe and study how the predators influence the herbivores and where they may decide to graze, and the correlation to coral reef halos.

In his experiments, Rago has been using omliu, or bluefin trevally, as the predators, while the herbivores he uses are manini, or convict tang. The algae Rago uses is an invasive species called Acanthophora spicifera, which is highly palatable and has been commonly used in research.

HIMB Coral Halo Experiment Enclosure
A bluefin trevally or omilu, in front of central refuge inside one of the 12 enclosures at HIMB. Courtesy: Zack Rago/2022

The system is built inside an old fish pen at the HIMB lab on Moku o Loe, also known as Coconut Island. It’s essentially is a floating cage made of PVC pipes with 12 enclosures, all with a different number of predators — ranging from zero to three — controlled by Rago.

Each enclosure has a central point — an upside-down flowerpot with holes drilled into the bottom, which are big enough for the herbivores to get inside but not for the predators.

Rago also planted 20 pieces of algae at fixed distances away from the pot, and twice a day, he goes in and measures the 240 data points from all 12 enclosures to see how much algae the herbivores have removed.

“We’re really trying to think about this concept of a landscape of fear,” Rago said.

He explained that when the prey — in this case manini — are perceiving a fear, that fear changes everything about their behavior, which is a component of a trophic cascade. Vegetation patterns are shaped by where and how the fish spend their time.

Madin first heard of coral reef halos in the midst of her Ph.D. program at the University of California Santa Barbara. She was studying the difference between Palmyra Atoll, where there were essentially no people, and the Republic of Kiribati, where there is a larger population, and how it affects fish behavior when predators have been fished out, leading to changes in the food source of the prey.

This is when Madin made the connection that if reef halos are the result of predators and prey interacting, and if there is a healthy predator population, it’s enough to scare the prey and can be used to indicate aspects of reef health.

Madin later saw reef halos for herself the following year in 2010, while stuck on an island in Australia due to bad weather. Her 12 years of work and research started as a side project that she thought she could complete with one short paper.

“For me, why I got into the game of science in the first place was to do work that is relevant for conservation, that helps promote wise and sustainable use of our resources.” Madin said, adding that she hopes to soon develop her work on reef halos into a freely accessible resource in the next few years.

Civil Beat’s coverage of climate change is supported by the Environmental Funders Group of the Hawaii Community Foundation, Marisla Fund of the Hawaii Community Foundation and the Frost Family Foundation. 

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