Skip to main content
U.S. flag

An official website of the United States government

Coral Concern: The World's Reefs Face Big Issues

Right-click and save to download

Detailed Description

Coral reefs are one of Earth's most beautiful and vital ecosystems—and they are declining at a rapid pace.

Mike Field, Chief of the USGS Pacific Coral Reef Project, talks about the importance of coral reefs and how pollution, climate change, and other factors are affecting them.


Public Domain.


Dave Hebert: Hello everyone! This is Dave Hebert with the USGS, and today I’m joined on the phone by Mike Field, a marine geologist and Chief of the Pacific Coral Reef Project as part of the USGS’s Coastal and Marine Geology Program. Mike, thank you so much for joining us.

Mike Field: You’re welcome. It’s a pleasure for me to do so.

Dave Hebert: So today we are going to be talking about coral reefs, and these are a beautiful and vital part of the ocean’s ecosystems, but they’re also facing some serious challenges around the world. So could you start off by giving us an update on the health of coral reefs around the world? What sort of issues are you and other coral reef researchers seeing?

Mike Field: Certainly Dave. I’m sorry to report that the present status, the health of the coral reefs, is not what we all would like it to be. They are, on a worldwide basis, in a serious stage of degradation. They’re seriously at risk. A lot of this has to do with human impacts to reefs near and far so that they are suffering, as one researcher put it a couple of years ago, “not from one single thing, but from a thousand cuts.”

Everything from people stepping on them to blast fishing and cyanide poisoning in an attempt to get fish, overfishing of fish that normally would keep algae from growing too fast and covering up corals. Climate change is certainly a major factor, and then land-based sources of pollution—runoff from water sheds from construction and housing and agriculture, and so on. So every reef everywhere is facing some sort of threat and those near population centers seen to be faring worse.

Dave Hebert: Right. In relation to that, I read quite a bit about sedimentation being a problem with reefs. Is that the case?

Mike Field: It is. It’s commonly referred to as land-based sources of pollution.

Dave Hebert: OK.

Mike Field: So it includes nutrients and pollutants that run off but by and large, it’s sediment that is coming down the hill from areas that have been developed, areas that have been heavily grazed, or where they have been deforested.

Dave Hebert: What does the sediment do or how does it negatively impact the coral?

Mike Field: Well, that’s a good question. Corals need clear, warm water. They grow in a symbiotic relationship with microscopic algae that live inside the tissue of corals, and so they generate food and energy through photosynthesis. They need clear water to do so.

If sediment comes into the water and clouds the water, the ability for the animals and the plants to grow is seriously threatened. And if sediment sets on top of them, then it affects recruitment. It affects their ability to produce larvae and for the larvae to find a place to settle. And as the sediment moves around, it abrades small corals, knocks them off, disrupts them. Larger corals can push the sediment off—remove it with mucous sloughing, but that costs some energy. They have to do that then it’s energy they can't put in to growing or to reproducing.

Dave Hebert: OK, thank you. And then what about bleaching? I’ve heard about coral bleaching. What’s actually happening there?

Mike Field: Coral bleaching is on the rise, and that is one of the dramatic and incontrovertible impacts of climate change. As the water heats up in many areas as the sea surface temperatures rise, the coral expels microscopic algae that symbiotically produce oxygen for the coral to live—they’re expelled.

They also produce the color of corals, so when they leave, the coral then is white. It's not actually dead at that point, but without the symbiotic algae, within a few days they do die. And so massive coral bleaching in 1998 and then again in 2005 in the Caribbean … there were the highest temperatures in northern hemisphere for a long, long time so there was very heavy bleaching in the Caribbean then.

Dave Hebert: OK. So why are reefs so important ecologically? What’s their significance to their environment?

Mike Field: Ecologically they’re just a major ecosystem for the world’s ocean. Coral reefs occupy somewhere in the order of 0.2 percent of the oceans and yet they account for almost all the biodiversity. Twenty five percent of the fish in the ocean that are caught commercially are found on coral reefs.

There’s a tremendous amount of variation in organisms, and corals themselves are considered a foundation species. They build these three-dimensional structures that extend for miles and miles in tropical areas, and those structures provide a framework or a habitat for many invertebrates, for many fish, and actually for thousands of species that depend on the structure that the corals produce.

Dave Hebert: Wow, so you’re—comparing those numbers, the 0.2 percent versus 25 percent of fish, you’re talking about an incredible concentration of marine life.

Mike Field: It is. It’s an incredible concentration and it’s very, very diverse. The common analogy made is that they are the rainforest of the sea, and that’s true. Although they are actually disappearing faster than rainforests are, but they hold the same very high marks of biodiversity.

Dave Hebert: So what about to human beings, why are coral reefs important to humans?

Mike Field: Well, we can start with the biodiversity. When we lose species, they go extinct. We lose a lot and we lose things that we don’t even know about, and it can translate as potential pharmaceuticals, or resistant species that might survive different kinds of climate shifts and so on.

So the biodiversity is an important aspect for us, but there’s also an economic factor. As I said, 25 percent of the fish that are caught and used locally come from coral reef communities. Coral reefs provide a buffer against waves to protect shorelines. They provide a strong buffer against very large storms and tsunami waves.

In the Indian Ocean tsunami in places that it seemed to have major effect were at places where the reef was dead and had been eroded and so it was lower. So the reefs protect the coastline. They generate sand. They produce sand for the beaches. They produce food for sustainable fishing for local cultures, and of course tourism depends an awful lot.

And increasingly nations are interested in trying to recruit ecotourism to their countries, and the value of coral reefs has been valued at numbers ranging from $25,000 to over $270,000 per square kilometer. So they have a tremendous economic importance to the local communities.

Dave Hebert: What are some of the things that can be done to reverse or slow down some of the negative trends we’re seeing?

Mike Field: That is a tough question. I have to be straightforward and say that I think all the scientists that are involved in coral reefs have been working very hard at this. It’s a major concern that everybody has is as we identify these problems, we have to pause and say, “Now what can be done? What are the solutions to help bring about change?” And the problems are both local and global, so the solutions have to be that way as well.

So for example, in an island that is suffering from sediment runoff onto the reefs, actions can be taken both on the land and in the water. On the land, implementing water shed plans. Removing invasive species, and planting native plants. Removing feral animals like goats and pigs that pull out plants by the roots and make the soil more susceptible to erosion.

Many things can be done in the water shed, and there are many groups starting to work on this. In the water, most people feel that appropriate action would be to establish more protected areas, more marine managed areas. That is wherever we can lessen the other stresses. Limit overfishing. Limit runoff from sediment so that the reefs have a better chance to survive. And the global issue is of course we have to confront climate change, climate warming, and this is a global problem. It requires political will but at the local level again.

We need to identify coral reefs that have the greatest potential for resilience, and we have to protect them locally from some of these other factors like sediment runoff, overfishing, so they have the best chance to withstand the problems that are coming into focus from ocean warming and from ocean acidification.

Dave Hebert: Now, I know you were involved in a USGS publication … well, I know you have been involved in several USGS publications on coral reefs, but in particular … really about as close as the USGS would probably get to a coffee table book on some of the issues we’ve been talking about. Can you tell us a little bit about that publication and then maybe where people can go to learn more?

Mike Field: Sure. That publication is called the “The Coral Reef of South Molokai, Hawaii: Portrait of a Sediment-Threatened Fringing Reef.” We … and “we” includes a very large group of my colleagues and researchers not just in the USGS, but at the University of Hawaii, University of Washington, and other labs for the USGS in Florida and Wetzel, Massachusetts, had been trying to understand patterns of sediment shed from the land and the behavior of the sediment on the reef and the ultimate fate. How long does it stay there? How does it affect the corals?

What makes it move, and what changes do we need to make in order to improve the overall health and condition of the reef? So we embarked on a research program for that 10 years ago in which we examine these different aspects and try to bring it all together in this publication. And the book tries to look at the history of the island. We go back and examine what the natural changes are.

If you want to understand what human-induced changes have happened to the reef, you have to understand what natural changes … how do storm waves, how do currents, and how do tides—how do these things affect the normal growth of the reef so you can separate what is human induced change versus natural change.

So we made an effort to bring all of this together and our goal with the book was to produce a volume that could be used by three different sets of people. And that is the informed lay person—on the island of Molokai, people are interested in protecting the environment—and those that teach children about protecting the environment.

We also aim the book at resource managers in Hawaii and other island states and nations. People who have to make decisions about where to establish marine-protected areas for an example.

Our third target audience are colleagues in different disciplines. So the articles about geology, we hope, will be useful for marine biologists. And the articles about algae will be useful for marine geologists and so on.

The publication on the reef of South Molokai as well as some of our journal publications and a lot more information are all available at, or you can do a search using “coral reefs USGS” and that should take you to our site.

Dave Hebert: Great. Well Mike, we really appreciate your time and just giving us this excellent information on the importance and a lot of the challenging issues facing reefs.

Mike Field: Well, you’re certainly welcome, Dave. It’s been my pleasure.

Dave Hebert: Great, thank you, and thanks to all of you out there for listening. To get more USGS podcasts, just go to, and for other multimedia including a lot of great video and images and other audio, go to This is a product of the U.S. Geological Survey, Department of the Interior. I’m Dave Hebert, and until next time, thank you so much for listening and have an excellent day.

Show Transcript