Coral Bleaching

Coral Bleaching

Coral animals are sensitive to changes in sea water temperatures and other disturbances. Coral bleaching results when the symbiotic zooxanthellae (single-celled algae) are released from the original host coral due to stress (e.g., unusually high or low temperatures, high or low salinities, or excessive sedimentation). The coral loses its pigment (color) when the zooxanthellae are expelled. Bleaching can inhibit the ability of coral to recover from small-scale tissue damage; increase partial or total mortality; reduce reproduction success; or increase susceptibility to disease or other stresses. However, corals can die if the stress lasts for an extended period of time or is very severe.

Coral Health Watch – Report Coral Bleaching

Help us track coral health in the Caribbean!

Please fill out one survey form per site. You have two choices of surveys: Basic and Detailed.

This simple survey form allows you to enter presence/absence data on corals affected by SCTLD and bleaching by coral species and estimates of corals affected, photos and site information. You can identify survey method used such as rover diver, bar drop, general observation etc.

NEW combined SCTLD / coral bleaching survey data sheet can be used for basic or detailed surveys. This corresponds to the online data entry tool and tracking map. Click here or image below to access.

This detailed survey form allows you to input more detailed quantitative data on the number of coral colonies affected by stony coral tissue loss disease and/or coral bleaching (fully bleached, partly bleached, pale), recent coral mortality, photos and site information. You can identify the method used such as rover diver, bar drop, etc.

Watch this Video tutorial on AGRRA’s new Detailed Survey form. Click on image below for video.

Video narrated by Nicole Craig, Belize Coordinator, Healthy Reefs Initiative.

Tracking Coral Bleaching in the Caribbean

Map of coral bleaching recorded throughout the Caribbean – The Coral Bleaching Tracking Map is an interactive map that shows where Coral bleaching has been observed in the Caribbean. Sightings are submitted through the Online Coral Health Watch Survey Form which then appears on the map.

Data from both surveys are presented in two maps: The SCTLD map and the Coral Bleach maps with the option of showing all data layers.


This Caribbean Coral Bleach Map shows site information, presence/absence of coral bleaching and available photos. We have further developed the map to include additional data in the pop up tables (e.g., pale, partly bleached, fully bleached corals, Bleaching severity Index). We will be soon adding a function to access the raw data. Available data from previous years will also be added. Click the “layers” button to explore the different layers. Click on any site marker to open a pop-up table of data. Some sites have photos included and are accessible through the pop-up tables.

Check back soon for further updates!

To cite or use this map in a report or publication: Kramer, P.R., Roth, L., and Lang, J. 2020. Map of coral bleaching in the Caribbean. ArcGIS Online. [access date]. Check back frequently for updates.

NOAA – Coral Reef Watch Products

NOAA’s Coral Reef Watch program produces a variety of satellite data to identify areas at risk for coral bleaching. For more information click here.

What Reefs are at risk of Bleaching?

ESRI’s ArcGIS Living Atlas program has developed an interactive dashboard based on NOAA’s satellite products to explore areas at highest risk. Click on image below or the link above to see more.

What does Coral Bleaching look like?

Bleached tissue may appear white (translucent) or pale, but you can still see the polyp tissue above the skeleton. Bleaching is not always uniform, but may be mottled in appearance. Polyps are not always killed by temporary bleaching, but often regain their pigmentation after several weeks or a few months.

We measure coral bleaching in terms of degree of bleaching (severity of bleached tissue within each colony) and prevalence of bleaching (% of colonies affected by bleaching in a given area).  The degree of bleaching is categorized as:

  • Healthy – No bleaching, normal, healthy tissue color
  • Pale – discoloration of coral tissue
  • Partly Bleached -patches of fully bleached or white tissue
  • Bleached -all tissues are fully bleached, no zooxanthellae visible

What is Mass Coral Bleaching?

Mass, widespread coral bleaching events, due to elevated sea surface temperatures, have resulted in high coral mortality. In the Caribbean, past mass bleaching events occurred in 1995 and 1998 affecting the western Caribbean and Bahamas; 2005 in the central and eastern Caribbean and again in 2010 and 2015. The intensity and frequency of bleaching events have increased over the last decades and continue to pose a threat.

Coral Bleach Watch in the Mesoamerican Reef Region

AGRRA has teamed up with The Healthy Reefs Initiative (HRI) to develop the new detailed Coral Health Watch survey form combing disease and bleaching. HRI has been leading a MAR-wide Coral Bleach Watch monitoring effort since 2015, mobilizing a coordinated network of trained surveyor teams to evaluate the extent of bleaching throughout the Mesoamerican Reef by using a systematic rapid colony-based protocol called the Bar Drop [1] methodology. The Bleach Watch monitoring network activates once there is an alert issued by NOAA through the Coral Reef Watch [2], mainly due to the increase in water temperature due to heat stress[3] (usually happens in September-November). HRI and its partners have conducted BleachWatch monitoring in the years 2015, 2016, 2017, 2019 and 2020. Graduate students from CINVESTAV in Merida, Mexico are analyzing the data for a number of scientific papers[4]  helping to better understand bleaching response.