Index | Overview | Coral standards | Fish standards | Calibration | Data processing | Data sheets Coral Condition
The AGRRA method is focused on assessing the condition of the principal scleractinian and hydrozoan corals that contribute most to the three-dimensional structure and complexity of reefs. The vitality of these corals responsible for the construction and maintenance of reef framework is important for the long-term persistence of a coral reef (Dustan 1987, Done 1997). The AGRRA method assesses overall coral cover, and for each coral, assesses the amount of partial coral mortality (both recent and old), size and height, incidence of bleaching/diseases, causes of mortality, and number of damselfish.
This is a useful distinction because the amount of “recent dead” can approximate the RATE of coral mortality occurring within the previous days-months, while the amount of “long dead” represents an integration of disturbances influencing coral mortality over longer time scales (below).
Causes of partial mortality include grazing (e.g., parrotfish bites), extended bleaching, sedimentation, algal overgrowth, disease (e.g., black band), corallivores, etc. In order to regenerate injured tissue, corals must reallocate energy that would otherwise be used for energy storage, growth or reproduction. It is fairly common for corals to experience partial mortality and to regenerate these areas if the lesions are not too large, relative to their remaining live tissues. Most partial mortality is small in size, but above certain sizes, which vary with species and the kind of injury that has been received, dead skeletal areas tend to become overgrown or eroded by algae or other bioeroding organisms (Meesters et al. 1996). In cases of large lesions, corals often will put energy into marginal growth instead of regenerating new tissue over the dead area. By quantifying the amount of recent mortality, you sum the extent of any damage and obtain information on whether the corals are likely to recover. Several studies have shown that the extent and amount of partial mortality is related to differences in size, morphology, species identity, and differences in the spatial and temporal distribution of predators. Normally, small corals often have either no injuries or total mortality, while larger corals are likely to survive partial mortality (Hughes and Jackson 1980, Babcock 1985, Meesters et al. 1996). The AGRRA data can be used to calculate size frequency distributions as well as mortality patterns related to size and, when sample sizes are large enough, species identity. This allows the added benefit of being able to examine coral size-frequency statistics to assess a population structure (Bak and Meesters 1998) and interpret levels of mortality. Although there are studies showing the importance of size frequency distributions in population dynamics (e.g., Hughes and Jackson 1980, Done 1987, Bak and Meesters 1998), additional research is required to determine “normal” size frequency distributions for major reef building species in various habitats. In general, we expect a “healthy” reef will show relatively little evidence of recent coral death (e.g., <5%), while reefs with high levels of recent mortality (e.g., >10%) would indicate a major disturbance event that has recently occurred (previous days to months) or is presently occurring. Reefs with many “standing dead” coral colonies or that have high old mortality (e.g.,>50%) would indicate significant past disturbance events.
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