Abstract
In order to identify the obstacles to the self-rehabilitation of the branching Acropora coral communities in the Sekisei Lagoon, one of the largest coral reefs in Japan, we investigated the basic parameters for establishing coral communities, such as larval recruitment, adult survival and growth, and nutrient concentrations in water column and sediment. We compared these parameters between 2 areas: “damaged” area, where branching corals have not been recovered for >20 years, and a “healthy” area, where corals continue to dwell at present or did until the recent past (3 years ago). Larval recruitment, which is important for natural rehabilitation of coral communities, was very low in all study sites (less than 3 settlers/176cm2), particularly in the sites of the “damaged” area (less than 0.5 settlers/176cm2 in average). Although adult survival and growth did not differ significantly between the 2 areas, they did differ among sites and coral transplant methods (touching the sea floor or raised up by a stake). Nutrient (NO2 and PO4) and chlorophyll-a concentrations in the water column were significantly high in the “damaged” area. It remains to be determined whether these slightly higher concentrations in water influence the competition between corals and algae. The results suggest that low larval recruitment rate and slight eutrophication in the water column are one of the main causes for the delayed rehabilitation of branching Acropora corals in the Sekisei Lagoon. Further, the optimal method of transplantation must be investigated at each site, even within a narrow area, when restoration is conducted by transplanting cultured corals.
