Ecological Assessment of Coral
Corals are the defining species in one of the highest diversity marine ecosystems, coral reefs. CRED studies of corals address basic questions concerning the distribution, abundance, and condition of corals and coral reefs throughout selected areas of the tropical and subtropical Pacific under U.S. jurisdiction.
The suite of questions addressed by CRED studies includes:
- What is the diversity and relative abundance of corals?
- How can the habitats in which corals occur be qualitatively and quantitatively analyzed?
- What is the spatial distribution of habitats in which corals occur?
- What is the capacity of corals to replenish populations over space and time?
- What are appropriate indicators of reef health?
- Are the reefs healthy?
- How are the reefs changing over time?
Several methods are used to address these questions. Any one method can address only a certain subset of questions; no one method can address them all. Methods used by CRED and its partners include towed divers surveys, Rapid Ecological Assessments (REAs), permanent transects, recruitment plates, and sampling for assessment of reproductive status .
Coral (Rapid Ecological Assessment) REA Methods
After descending to the bottom, the coral biologist videotapes along at least two of the three transect lines laid out by the fish team using a Sony PC digital PC100 camcorder in a Gates underwater housing, while slowly swimming ~1 meter above the length of the line. These video sequences enable later, computer-assisted quantitative analyses of percent coverage of corals, algae, and substrate types. Additionally, at the beginning of each of the transect lines, a 360° pan of the surrounding reef area is slowly videotaped to document the topography and general nature of the surrounding area.
The coral biologist then swims back along as many of the transect lines as bottom time permits and lists coral species occurring within 0.5 m² of each side of the transect lines and the size class to which the maximum diameter of the colony belongs ( 5 cm; 5–10 cm; 10-20 cm; 20-40cm; 40-80 cm; 80-160 cm; or 160 cm). In geographic regions such as the Northwestern Hawaiian Islands where coral richness and abundance are reduced relative to lower latitude regions with higher species richness, the protocol is amended to include all corals occurring within 1m of each side of the transect lines.
A random swim is then conducted in the vicinity of the transect lines within an area of about 5,000m2 in which all coral species are listed and assigned a DACOR abundance code based on visual estimation (dominant, abundant, common, occasional, and rare). If bottom time permits, corals showing signs of disease, bleaching, or abnormal growth are tallied, described, and photographed.
|Acropora cytherea, French Frigate Shoals NWHI|
Analysis of digital video taken along the transect lines is conducted using duplicates of the videotapes rather than the originals, which are archived as a permanent record of the state of the reefs at a known point in time.
The quantitative analysis of the benthic habitat, as documented by the digital videos, involves three major steps:
- The selection of single, sequential, non-overlapping still frames
- The import of individual still frames into the computer program SigmaScan for identification and tracing of key benthic components and calculation of their percent cover within each still frame, and
- The compilation and summarization of SigmaScan's quantitative data using Microsoft Excel.
This sophisticated, computer-assisted method of transforming qualitative photographic images into quantitative data that describe the relative contribution of key ecological components to the benthos is also being successfully used in the quantitative analysis of digital video collected during towed-diver surveys over larger spatial scales.