Global Coral Reef Monitoring Network (GCRMN) method

The Global Coral Reef Monitoring Network (GCRMN) methods describe several elements of the coral reef ecosystem to be surveyed:

  1. Abundance and biomass of reef fish taxa
  2. Relative cover of reef-building organisms (corals) and their dominant competitors
  3. Assessment of health of reef-building corals
  4. Recruitment of reef-building corals
  5. Abundance of key macro-invertebrate species (sea urchins and sea cucumbers)
  6. Water quality (i.e. water transparency)
  7. Macroalgae height

See details below.

For the Dutch Caribbean this method has been implemented on:

  • St.Eustatius: The survey location is the St. Eustatius National Marine Park (SNMP), which surrounds the island from the high water mark to a depth of 30 meters.  To maximize comparability across the region, GCRMN data will be collected solely from forereef habitats at depths ranging from 8 – 15 meters. An effort was made to include sites within the industrialized harbor area along with sites with perceived lower anthropogenic influence on the north and south ends of the island. Within the SNMP 20 sites were selected: Crooks Castle, Humps, Twin Sisters, Ledges, Valley of the Sponges, Five Fingers South, Double Wreck, Northman, The Blocks, The Cave, Aquarium, Anchor Point, Dump, White Wall, Gibraltar, Outer Jenkins Bay, Barracuda Point, Triple Wreck, Mushroom Gardens and Hangover.
  • Saba bank: in progress
  • Saba: Grouper sightings during tourist dives. All year round tourists dive together with a guide trained in species recognition. The guides registers the species and number of individuals sighted per dive


1. Abundance and biomass of reef fish taxa.
To measure fish density, all fish present (of all species) within a belt transect (30m length x 2m width) were counted, with the survey time limited to approximately 6 minutes per transect. To measure size structure and calculate biomass, the length of each fish was estimated and assigned to the following size categories: <5cm, 6-10cm, 11-20cm, 21-30cm, 31-40cm and >41cm.  At each site, 5 transects were surveyed and the data pooled to provide an average assessment of the density and size structure of all fishes at the site.  Such high resolution estimations of the fish assemblage provide the core information (snappers, groupers, parrotfish and surgeonfish), while also providing fundamental information about other members of the fish assemblage that may serve important roles in fisheries or ecosystem management.

2. Relative cover of reef-building organisms (corals) and their dominant competitors
Percent cover of key benthic taxa was estimated using the photoquadrat method. This approach depends upon taking digital photographs of the reef surface in standardized quadrat areas (0.9m x 0.6m). Photographs were taken along the 5 transect lines set for counting fish, capturing 15 images per transect line (i.e., one image taken at every other meter marker on the transect tape).   Data was captured from the images through post – processing using Coral Point Count (CPCe) software.  25 random points on each image were identified and classified into a standardized benthic category, such as Seagrass, Sponge, Zooanthid and Cyanobacteria.  Reef building corals were identified to species level; soft corals and macroalgae to genus level.

3. Assessment of health of reef-building corals.
Disease prevalence in corals was estimated using the photoquadrats from the benthic cover assessment. Data was recorded as the proportion of images collected that contained a coral with any disease pathology. For example, if there were four colonies in a particular photoquadrat and any of these colonies showed signs of disease, this image would be tagged as “with disease”. The number of images that were “with disease” was divided by the total number of images (15 per transect) to generate a proportional estimate of disease prevalence.

4. Recruitment of reef-building corals.
Coral recruits are defined operationally for this assessment as any stony coral that is greater than 1.0 cm² and less than 4.0 cm². Estimates of coral recruit density were recorded from replicate 25cm x 25xm (625cm2) quadrats. 5 quadrats were surveyed along each of the first three transects used for fish and benthic surveys. Coral recruit quadrats were placed at 2-m intervals along the transect line, i.e. with the lower corner of the quadrat placed at the following meter marks: 2, 4, 6, 8 and 10m. Each coral within the target size range was recorded to the finest taxonomic level.

5. Abundance of key macro-invertebrate species (sea urchins and sea cucumbers)
The 15 benthic photoquadrats from each of the 5 transect lines (75 photographs per site) were used to estimate the density of sea urchins and sea cucumbers. The number and species identity of each sea urchin and sea cucumber were recorded for each image. The density was calculated by dividing the total number of sea urchins and sea cucumbers recorded by the product of the number of images (75) and the size of each photoquadrat (0.54 m2 [i.e., 0.6 m x 0.9m]).

6. Water quality (i.e. water transparency)
The Secchi disk (a black-and-white disk 20 cm in diameter) was used to estimate the concentration of particulates in the water column.  The Secchi disc was placed on the seafloor and the diver swam away along the transect line until the Secchi disc could no longer be seen. The diver then swam back until the Secchi disc could be seen. Recording this distance gave a measurement of water clarity.

7. Macroalgae height
The macroalgae height was estimated to calculate macroalgae biomass. Algal impacts scale with biomass and canopy heights are the best (nondestructive) way to get at that.  The canopy height of algae in the recruit quadrat was measured at a sufficient number of points relative to algal biomass.