Ecological Assessment of Coral
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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
Field Components
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.
Laboratory Components
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| 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.
Preliminary Results
Size-Class Distributions
The size-class distribution of corals at a given site, assessed in combination with knowledge of species occurrence, can be informative in inferring ecological processes that are presently operative at the site. For example, a distribution in which the preponderance of corals occurs within large size classes suggests a region in which disturbance events have not interfered with the growth of corals over long periods of time. Conversely, a distribution in which the preponderance of corals occurs within small size classes suggests either a recent, severe disturbance or frequent recurrences of moderate disturbance, after which recruitment and growth have been relatively recent processes. Size-class distributions, however, must always be interpreted within the context of species-specific life history strategies as well as the salient environmental regimes to which the corals are exposed. ; For example, members of many genera, such as Pocillopora, attain only a moderate maximum size before senescence occurs even in the absence of disturbance events, whereas many large Porites heads escape the effects of severe disturbance such as typhoons due to their massive size. The size-class distribution of a species' population exposed to frequent high-energy waves will likely differ from the size-class distribution of that same species found in a more protected location. Size-class distributions of Acropora on the forereef, backreef, and lagoon at French Frigate Shoals in the Northwestern Hawaiian Islands demonstrate such differences:
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Species Richness
Prior to surveys undertaken in the Northwestern Hawaiian Islands by CRED and its partners in 2000, only 20 species of scleractinian (reef-building) corals had been documented in the NWHI, in contrast to 48 species known from the main Hawaiian Islands (MHI). Following surveys conducted in 2000 and 2001, 52 species were documented from the NWHI, of which 3 to 5 were new species, yet to be described taxonomically and by definition endemic to Hawaii. Of these 52 species, 12 are not known to occur in the MHI. During surveys conducted in 2002, several possible new records and/or new species were documented, the taxonomic analyses for which have yet to be conducted.
Similarly, surveys undertaken in the U.S. Line & Phoenix Islands by CRED and its partners in 2000, 2001, and 2002 have led to significant increases in coral species richness previously known from these regions (Table 1).
Table 1. Number of coral species recorded in the U.S. Line and Phoenix Islands following CRED-initiated surveys in three successive years.
| AREA/YEAR | 2000 | 2001 | 2002 |
|---|---|---|---|
| Baker | 59 | 77 | 80 |
| Howland | 53 | 85 | 91 |
| Jarvis | 32 | 44 | 49 |
| Palmyra | 109 | 156 | 170 |
| Kingman | 102 | 142 | 155 |
Surveys undertaken by CRED and its partners in American Samoa in 2002 added to the coral species lists previously tallied for Tutuila/Manu’a Group and Rose Atoll (255 and 103, respectively). Surveys that included the initiation of a coral species’ list at Swain’s Island were conducted for the first time in 2002, establishing a base list of 40 species.