Coral Bleaching and Disease

Example of healthy corals. NOAA photo by Bernardo Vargas-Ágel. Example of bleached corals. NOAA photo by Bernardo Vargas-Ágel.
Examples of (left) healthy corals and (right) bleached corals. NOAA photos by Bernardo Vargas-Ángel.

Coral bleaching and disease have become central topics of discussion among coral reef managers, scientists, and the general public, largely because of the alarming rate at which these threats have increased in the last two decades. For example, coral bleaching, once thought to be a localized phenomenon affecting selected reefs in the Caribbean, eastern Pacific, and Indonesia, is now documented and considered as a major threat to all major coral reef ecosystems worldwide.

Diseased coral exhibiting skeletal growth anomalies. NOAA photo by Bernardo Vargas-Ángel.
Diseased coral exhibiting skeletal growth anomalies. NOAA photo by Bernardo Vargas-Ángel.

Since 1979, nine major coral bleaching episodes have affected important coral reef provinces around the world, with an alarming reported increase in scale and intensity in recent years. Mass coral bleaching has resulted in significant losses of live coral cover globally, with an estimated 16% mortality of the world's reef corals during the 1997–98 bleaching episode. This episode is the most geographically extensive and severe on record. Because reef-building corals provide much of the primary productivity and topographic complexity of the ecosystem, alterations in the abundance and diversity of corals severely affect the majority of other reef dwelling organisms, including fishes, algae, and non-coral invertebrates. As such, the socioeconomic ramifications of mass coral bleaching to coastal communities can be long-lasting and severe, with potential detrimental effects to fish stocks, coastal erosion, and tourism. NOAA's Coral Reef Watch has developed metrics that link satellite-derived sea-surface temperatures and coral bleaching, as well as tools that provide near real-time monitoring of environmental conditions and identify areas at risk of mass coral bleaching. Timely monitoring of sea-surface temperature provides researchers and managers with tools to better understand and better address the complex interactions and potential effects of mass coral bleaching.

Likewise, back in the 1970s and 1980s, coral diseases were considered to be a problem limited to Caribbean reefs, with some localized outbreaks in the Red Sea, attributable mainly to land-based sources of pollution. Since 1990, however, significant disease outbreaks and associated dramatic coral losses have been reported within all reef systems worldwide, with significant events occurring in the Great Barrier Reef between 2001 and 2003 and Caribbean-wide in 2005.

Diseased coral exhibiting rapid tissue loss off the coast of Kauaʻi. NOAA photo by Bernardo Vargas-Ángel.
Diseased coral exhibiting rapid tissue loss off the coast of Kauaʻi. NOAA photo by Bernardo Vargas-Ángel.

Coral bleaching and disease are health impairments that interfere and modify a coral's ability to perform normal physiological functions (maintenance, growth, and reproduction). They typically are caused by a suite of agents: environmental factors, such as nutrients, toxicants, and climate; pathogens, including bacteria and viruses (infectious agents); and inherent or congenital defects (Wobeser 2006). CRED's Pacific RAMP conducts long-term, broad-scale assessment and monitoring of coral bleaching and disease to document the prevalence of these health conditions on U.S. Pacific reefs and investigate the factors that may be contributing to their occurrence. Since 2006, and with direct support from NOAA's Coral Reef Conservation Program CRED conducts routine coral bleaching disease surveys at the Remote Pacific Island Areas, American Samoa, the main Hawaiian Islands, the Northwestern Hawaiian Islands, Guam, and the Commonwealth of the Northern Mariana Islands (CNMI). Bleaching and disease prevalence estimates, for which no data existed in many of the U.S. Pacific jurisdictions prior to this project (e.g., Howland, Baker, and Jarvis Islands, Kingman Reef, Wake Atoll, and the CNMI), is now better characterized, and a better understanding of disease occurrence and potential effects on U.S. Pacific coral reefs is emerging. These activities support the goals and objectives of the Coral Reef Conservation Program's National Coral Reef Monitoring Plan, which is aimed at assessing the status and trends of the corals and coral reef ecosystems of the United States.

For a detailed description of the various types of coral lesions and diseases commonly recognized on U.S. Pacific reefs and information about their distribution and abundance, please click here.

For more information about coral and algal diseases, contact Bernardo Vargas-Ángel.

References Cited

Wobeser GA
2006. Essentials of disease in wild animals. Blackwell Publishing Professional, Ames, Iowa 243 p.