CHI – The Three Dimensions of Reef Ecology

by Stuart Sandin, head researcher on the Fish Team

Grouper at Palmyra Atoll. Photograph by Stuart Sandin.

For many of us on this expedition, our first trip to the Northern Line Islands had come in 2005. Our goal during that cruise was to establish what is a ‘pristine’ coral reef. Seeking reefs that could tell us this, we turned to the most remote islands that we could find, islands such as Palmyra Atoll and Kingman Reef. These islands lie so far from most people on the planet and had been visited so seldom that the stories about their reefs seemed to have grown to mythical proportions. They were fabled to hold multitudes of sharks and other big fish, lavish coral gardens, and water so clear you could see forever. These descriptions sounded like fairy tales compared to our real-life diving experiences on reefs near populated islands in the Pacific, the Caribbean, and the Indian Ocean. There it was a fabulous dive if we saw even one shark or a fish over two feet long. Our research surveys of those reefs suggested that corals were vying for space with the seaweeds, and frequently losing. Our photos reminded us that clear water days were rare. Were those legendary remote coral reefs real? We set out to the Northern Line Islands in 2005 to see for ourselves.

The reality we found was stunning. We surveyed, counted, and collected to give a quantitative scientific description of the reefs at these remote and uninhabited locations. The coral reefs at Kingman and Palmyra really did support massive fish assemblages including an abundance of large animals; their surfaces really were covered by lots of corals and reef-building coralline algae; the amazingly clear reef waters really did contain shockingly few bacteria and viruses. When we compared these reefs to those at nearby inhabited islands, the contrast was striking. With just moderate human populations harvesting fish from the reef, the structure of the reef ecosystem was dramatically altered. The fish were smaller and even when combined added up to less fish biomass, the corals and other reef builders were less common, and the microbes were more abundant. In other words, we were back to the reefs that were familiar to us all.

Black tip reef shark at Palmyra Atoll. Photograph by Jen Smith.

Our 2005 data demonstrated that the effect of human activities on coral reefs was straightforward, yet profound. Through poorly managed fishing and waste systems, humans can change the entire appearance of a coral reef, and quickly. This finding was not novel in itself, but the magnitude of the change was shocking. We wanted to communicate this message to citizens and resource managers alike: the reefs you know and consider healthy are but a pale shadow of what a coral reef should be. For this purpose, we needed to create a measuring stick against which other reefs can be compared—a quick-and-dirty assessment of reef health. So was born the CHI.

The Coral Health Index–endearingly called CHI by Les Kaufman of Boston University to signify the holistic and balanced spirit of a healthy reef–provides a straightforward and reliable metric of coral reef condition. Using this method we can provide a snapshot measurement of the health of any reef.

To determine a reef’s CHI we ask just three questions, we measure just three properties:

  1. How much fish biomass is present?
  2. How much of the benthos is covered with reef builders (corals and coralline algae)?
  3. How abundant are the potentially pathogenic bacteria? (We count the number of vibrios in this case, as that group of bacteria is easy to sample and is known to cause many diseases. Heard of cholera? You can blame a vibrio for that!)

In scoring the values obtained from each reef, we use the structure of Kingman Reef (and a handful of other pristine reefs scattered across the Pacific) as the baseline. Based on our previous study in the Line Islands we know that for reef health more fish is better, more coral and corallines are better, and fewer vibrios are better. We score each of these three metrics, take the average, and we have the CHI for that reef.

CHI measured at reefs with varying human impacts throughout the Line Islands. (Tabuaeran and Kiritimati are the Gilbertese names for Fanning and Christmas.) Credit: Coral Health Index. *

Remote and uninhabited islands have a high CHI. Remote islands with small populations have lower CHI. Heavily populated islands have even lower CHI. But, importantly, heavily populated islands with strong local management (e.g., fisheries protection and sewage control) have higher CHI than those without protection.

By creating CHI, we have provided a way to readily assess and compare the health of reefs around the globe and over time. This has very practical usefulness. When, for example, new local reef management policies are put into place, the impact can be measured by comparing the CHI before and after. It also has larger implications in that it identifies the three ecological dimensions that are critical for reef health and that are critical to assure that a reef maintains the capacity to recover after external insults (like tsunamis or hot-water events associated with El Niño). It also tells us that to understand coral reef health and decline we need to know about the fish, we need to know about the benthos, and we need to know about the microbes.

From this awareness arose the mighty triumvirate of our research team and this current expedition.

* Kaufman, L., Sandin, S., Sala, E., Obura, D., and Tschirky, J. 2010. Coral Health Index (CHI): Measuring Coral Community Health. Science and Knowledge Division, Conservation International, Arlington, VA, USA. (In preparation. Available late November, 2010, at: