Assessing the Health of Coral Reefs

January 2011
By:
Dr. Stuart Sandin is a quantitative marine ecologist with the Center for Marine Biodiversity and Conservation at Scripps Institution of Oceanography in La Jolla, California.
Today's descriptions of coral reefs are often filled with doom and gloom. Reef fisheries are in decline, corals are dying at unprecedented rates, and the reef waters are filled with harmful bacteria.

Studying Coral during Northern Lines ExpeditionWe know that there are a number of causes for these problems-unregulated fishing, unintended climate change, and uncontrolled pollution-but what we don't know is how to reverse this trend and move toward a healthier and more productive future for coral reefs.

For the coming month, a team of scientists from Scripps Institution of Oceanography, San Diego State University, and a number of other institutes will be studying coral reefs in the central Pacific in search of answers. The expedition will venture to the Northern Line Islands, a remote archipelago over 1000 miles south of Hawaii. The islands, a mixture of US and Kiribati protectorates, span over six degrees of latitude and range from marine protected areas to populated islands with very active local fisheries.

This will be the second time many onboard have visited these uninhabited islands. They, along with other scientists on the 2005 expedition, were among the first to document the astounding live coral cover and fish abundance at the uninhabited atolls in that region. Five years later, we are hoping to find the pristine reefs in similar condition and to advance our studies beyond merely documenting what is present to now determine the sustainability of such productive ecosystems.

Pristine Coral Reefs

Are these so-called 'pristine' coral reefs very productive, with fish growing rapidly and corals showing a capacity to recover quickly following natural disasters? Or are they fragile, teetering on the cusp of vitality? As we board our vessel, the Hanse Explorer, a 158 ft. motor yacht outfitted with an air compressor and about 4 tons of research equipment (including microscopes, entire aquarium systems with lighting and water circulation, dissection kits, and drilling gear), we are gearing up and preparing for epic adventure in an attempt to answer these questions.

To kick it off, the Hanse Explorer departs from Honolulu and steams down to the relatively untouched Palmyra Atoll and Kingman Reef, areas protected as a US Marine National Monument. To put the results from these protected areas into context, we will visit neighboring islands that have human populations, where we will ask similar questions. Our next ports of call, Tabuaeran (Fanning) Atoll and Teraina (Washington) Island, are both inhabited by the I-Kiribati people who practice subsistence fishing. The final set of islands visited lie at and just below the equator: heavily populated Kiritimati (Christmas) Island and uninhabited Jarvis Island. In the end, we hope to determine what ecological characteristics a reef needs in order to keep providing critical services (like fishing and shoreline protection) for the people living nearby.

During the expedition, the research group is actively blogging by publishing stories, observations, and photos from each atoll they visit available here. The Line Islands are a world apart, their reefs little explored. Join the team and take a visit far away, both across the globe and below the surface.

From the Author

I am writing these words while onboard the Hanse Explorer in the Northern Line Islands (NLI), one of a group of researchers from Scripps Institution of Oceanography (SIO), San Diego State University (SDSU), and several other institutions. For many of us on this 2010 expedition, our first trip to the Northern Line Islands had come in 2005. Our goal during that cruise was to establish what a 'pristine' coral reef looks like. For this purpose, we had turned to the most remote reefs that we could find, reefs such as those at Palmyra Atoll and Kingman Reef in the NLI. Being so remote and so seldom visited, the stories about these 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 spectacular legendary remote coral reefs real? We had set out to the NLI in 2005 to see for ourselves.

The reality we found was stunning. The legends were true. 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 far fewer 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. While this finding was not novel in itself, the magnitude and the rapidity of the change was astounding (the major influx of population to these islands having begun in 1987). The fish were smaller and, even though more numerous, when combined they added up to less total 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, reefs often considered to be healthy for lack of a pristine reef for comparison.

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 we needed to create a measuring stick that provides 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 measurement of the current health of any reef. To determine a reef's CHI we ask just three questions, make just three measurements:

  • How much fish biomass is present?
  • How much of the benthos is covered with reef builders (corals and coralline algae)?
  • 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 calculating a reef's CHI, we use the structure of pristine Kingman Reef (and a handful of other pristine reefs scattered across the Pacific) as the baseline. Based on our 2005 study in the Line Islands we know that for reef health more fish is better, more corals and more coralline algae are better, and fewer vibrios are better. We score each of these three metrics for the reef against the pristine yardstick, take their average, and this is that reef's CHI. 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 to track changes over time. This has very practical usefulness. When, for example, new local reef management policies are put into place, the impact can be documented by comparing the CHI before and after. The CHI also has larger implications in that it identifies the three ecological dimensions that are critical for reef health. To understand both coral reef health and decline we need to know about the fish, the benthos, and the microbes. These same three dimensions are critical to reef resilience, i.e., a reef's capacity to recover after external insults (such as tsunamis or the coral bleaching events associated with the warm waters of El Niño).

We are back in the NLI now, five years later, to pursue unanswered questions. We will be further evaluating our CHI metrics by including two additional atolls in our surveys. Numerous experiments are planned to determine whether reefs with the highest CHI are also the most productive. Data collected for all three dimensions will be compiled to help us determine what ecological characteristics are essential if a reef is to continue to provide critical services for the people living nearby (e.g., fish production and shoreline protection). Overall our goal is to determine how we can live with coral reefs, how we can enjoy them and use them without destroying them, and how we can help them to survive the major challenges that inevitably lie ahead due to climate change and ocean acidification. We invite you to join this 2010 expedition by visiting our website where you will find our expedition blog, filled with stories and photos from this latest adventure.

Take Action

You can make a difference. Follow these steps to create a positive future for the ocean.

Take Action
  • Help protect the fish of the reef by never purchasing wild caught fish for your aquarium. Tropical fish are often caught by squirting cyanide into the nooks and crannies of the reef to stun the fish. This cyanide causes damage to the coral and other animals that are left behind. Learn more about this problem and solutions from the Marine Aquarium Council.
  • Deep-sea red and pink corals are the most valuable and widely traded of all coral species, and have the least protections. You can help by not purchasing jewelery made from coral. Learn more about action being taken to stop this by The Tiffany & Co. Foundation and Too Precious To Wear.
  • Visit the Coral Reef Systems website. Here you will find detailed information about coral reefs and the central Pacific; the blog from the 2010 Northern Line Islands Expedition; brief sketches of the researchers and expedition members; and a gallery of superb photos and videos of the life on coral reefs.
  • Read Coral Reef Systems book, Coral Reefs in the Microbial Seas. This timely and engaging book relates the scientific findings from the 2005 Line Island Expedition, and playful stories from the cruise. Its 200 pages provide a well-written introduction to the reef-building coral holobiont, coral diseases, the roles of algae on reefs, ecosystem phase shifts, the effects of nutrient addition, coral bleaching, actions needed for reef survival, and much more.