Developing a comprehensive picture of the geographic location and monitoring the health status of the earth's coral reefs is an enormous task, but one necessary to the development of sustainable resource managment strategies, practices and policies. Below is a list of mapping and monitoring techniques currently employed:
Satellite-based sensors: Satellite imagery can be used for low-cost, albeit coarse-scale mapping of coral reefs, and as such is probably the most effective way to build a comprehensive picture of where the world's reefs are located. Satellite data can also provide information on sea-surface temperatures, wave height and direction, and primary production in upper waters. They may also be useful for distinguishing living from dead coral in very shallow waters. Military agencies have the most comprehensive satellite data, often at a much finer resolution; however, these data are rarely available for public use.
Aerial photography and sensors: Photos and data from overflights of reefs can provide a more detailed picture of reef location, and can yield bathymetric data to depths of several tens of meters. However, aerial surveys and the analysis of their products are far more costly than those derived from satellite information and are difficult or impossible to conduct legally in many countries because of security concerns. These data can determine living from dead coral, but only within very shallow water. Costs have been reduced by using ultralight aircraft, balloons, kites, and other devices. With improvements in computer technology, it will be possible to survey reefs with remote-controlled aircraft, further cutting costs.
Ship and boat-based sensors: Research vessels carry a range of sensors useful for detailed mapping of coral reefs. Various types of sonar can be used to produce three-dimensional images of coral and distinguish between different types of bottom substrate. Passive acoustic analysis, along with sonar in some instances, can distinguish between live and dead reefs. Research vessels play a vital role in surveying and mapping coral reef habitats. However, they are costly to operate (generally ranging around US$10,000 per day). One way to reduce costs, and better utilize existing research vessel fleets, is to conduct reef surveys during the course of other oceanographic and fishery investigations.
Submersibles: Manned and unmanned submarines play an essential role in assessing coral reefs in waters below a 30-meter depth-beyond the practical working limits for scuba diving. Although the technological capacity available for exploring the world's oceans is highly developed, there are very few submersibles in the world that are available for undersea research. Promising new technologies are coming on line for conducting transect surveys, distinguishing live from dead coral cover using laserline sensing devices, and conducting rapid, large area assessments at various depths including shallower waters accessible by scuba divers.
Diving surveys: Scuba-diving scientists are the main source of information on reefs in shallower waters today (down to 30 meters in depth). However, the specific objectives, taxa of focus, and sampling approaches severely limit the comparability of the data among regions and over time. In addition, scuba-based assessments and monitoring are limited by the number of scientists available for this work and the small area that can be covered by one individual. Survey protocols are being developed so that recreational divers and others can help gather data, often on a volunteer basis. This offers tremendous potential for gathering new information on reefs, since there are several million scuba divers in the world and several times as many people proficient at skin diving with mask or goggles. Similarly, residents of coastal communities can be recruited to evaluate their reefs through participatory resource mapping. This low-tech approach is particularly relevant in developing countries, where few can afford expensive scuba equipment. Here, villagers are trained to gather general information on the coverage of various ecosystems, supplemented with descriptions of simple factors such as hard coral cover, and then transfer the data to a map using a simple compass. Work on this type of approach is underway through various programs, such as the Coastal Resource Management Program in the Philippines.
Source: D. Bryant et al. Reefs at Risk: A map-based indicator of threats to the world's coral reefs. (Washington DC: World Resources Institute, 1998) Introduction by IOC.