Mineo Okamoto

A basic experiment of coral culture using sexual reproduction in the open sea

Coral larvae, produced from a mass spawning event, were successfully settled on special stone settlement sticks and raised in situ for eventual transport to other reefs. The test area, Sekisei lagoon, Okinawa Prefecture, Japan, is located close to the warm Kuroshio current and is surmised to be the source from which major Japanese corals are derived. A total of 131 settlement sticks, with small holes in their sides to increase protection from grazing (4 mm in diameter and approximately 5 mm deep), were deployed in the lagoon the day before the coral’s mass spawning. After 3 months, 61 sticks were recovered containing 71 corals, mostly in the holes. After 1 year, three corals were confirmed to be growing well and extending outside the holes of the three sticks out of 70 sticks left in the water at the lagoon site. They survived two potentially lethal conditions, that is, high water temperatures with associated extensive coral bleaching and continuous grazing pressure from predators. This procedure is applicable for large-scale coral transplantation, not only in Japan but also in other tropical countries.

Development of ceramic settlement devices for coral reef restoration using in situ sexual reproduction of corals

A ceramic coral settlement device (CSD) was developed to raise juvenile corals in situ in preparation for transplantation. The CSD is designed to be deployed in the water before mass spawning of corals, allowing larvae to settle and grow on the CSD before it is fixed to the reef structure. CSDs were aggregated and fixed on 0.2-m2 frames (either 648 or 720 CSDs per frame) and deployed in the Sekisei Lagoon around the full moons of April and May in 2002. Seven frames were collected in August 2002 and settled corals were counted in the laboratory. The maximum number of coral colonies settled on CSDs in one frame was 617, with 94% belonging to the genus Acropora. The aggregated CSDs successfully functioned as a coral settlement substratum, with more appropriate arrangement of CSDs on the frame, enabling greater exposure to sunlight and currents, which is expected to increase the amount of settlement.

Evaluation of coral bleaching condition in situ using an underwater pulse amplitude modulated fluorometer

Photosynthetic activities of hermatypic corals (28 genera, c. 68 species and c. 92 specimen) were measured in November 2000 at Sekisei lagoon, Okinawa, Japan, using an underwater pulse amplitude modulated fluorometer. Maximum quantum yield (Fv/Fm) ranged from 0.61 to 0.70 and had an average value of 0.664 (standard deviation [SD], 0.0289). Following the measurement of Fv/Fm, quantum yield (Fm′-F)/Fm′ at eight-leveled actinic-light intensities were obtained from all specimens. Values of electronic transport rates (ETR) were nearly the same under low photosynthetic active radiation (PAR; μmol quanta/m2 per s) condition and varied with increasing PAR. The value of ETR at 100 μmol quanta/m2 per s ranged from 21.3 to 30.0 and had an average of 27.8 (SD, 2.09). In August 2001, a coral bleaching event was observed in Sekisei Lagoon. During the event, photosynthetic activities of 21 specimens of eight species (seven genera) were measured. By applying two indicators, Fv/Fm and ETR at 100 μmol quanta/m2 per s obtained in August 2000, 18 of 21 specimens were suggested under the bleaching process. In case of Porites lutea, seven colonies were visually evaluated for their condition of bleaching and were found to have two ‘normal’, three ‘slightly bleached’ and two ‘heavily bleached’. From the density of zooxanthellae, colonies evaluated ‘normal’ or ‘slightly bleached’ did not show an apparent difference. However, they were clearly identified by Fm/Fv and ETR at 100 μmol quanta/m2 per s.