The purpose of this study was to assess the composition of juvenile corals at different depth intervals and to assess the survivorship and recruitment rates of juvenile corals in La Parguera, Puerto Rico. Two inshore reefs, two mid-shelf reefs, and two shelf-edge reefs were split into multiple 0.25m2 quadrats at four depth intervals. The locations of each quadrat were chosen at random. All juveniles in each quadrat were counted, photographed, and mapped in 2003 and then resurveyed and photographed in 2005.
The juvenile corals had a survivorship of 54.9% for the six reefs surveyed and overall juvenile densities decreased significantly from 2003 to 2005. There was a higher mortality in shallower habitats due to a greater variability in environmental conditions (water temperature, UV radiation, etc.). There was a low recruitment rate at only 78 new recruits in 2005 with the mid-shelf reefs showing the highest number of new recruits. The low number of new recruits was most likely due to a lower reproductive output caused by disease and bleaching given that species less affected by bleaching and disease had a higher number of new recruits. It was also found that a coral’s mode of reproduction (spawning or brooding) did not affect its recruitment rate.
This experiment was conducted in a way that allowed the results to be as fair as possible. The juvenile corals were surveyed in six different locations that varied in distance from the shore and the quadrats selected in each location varied in depth from 0 to >15m. 31 species of brooding and spawning coral were included in the surveys. The article referenced other studies and some data showed some contrasting results.
The results of this experiment could have been different if the experiment was conducted in previous or later years, even if the time between each survey was the same, because coral recruitment rates differ among seasons and between years. Studies conducted on coral reproduction and recruitment should be conducted over years or decades because reproductive attempts often fail due to unfavorable conditions and environmental factors. Coral are long-lived and one or two successful reproduction attempts could be enough to maintain populations.