Mass coral spawning in the Houtman Abrolhos Islands, Western Australia

In March 1987, corals from the Beacon Islands in the Wallabi Group of the Houtman Abrolhos Islands were monitored in order to observe the timing of spawning. Corals were tagged and sampled starting on the 11th March and all were resampled on the 27th March. A total of 422 colonies from 107 species were sampled two or more times over the period of the study.Corals were examined for reproductive status by in situ observations of broken sections, microscopic examination of live material in the laboratory or dissections of preserved material. Sperm development was monitored daily in a range of corals to assess when spawning was imminent. Night dives were conducted from the 22nd to 26th March and tagged colonies and other colonies in the vicinity of the study site were monitored for spawning activity. Selected species were also kept in aquaria to provide additional observations. The time and date of spawning was recorded for each species.Water temperature and depth were recorded every 15 minutes during the study period using a data logger deployed at a depth of about 2m at the study site. The subtropical reefs of the Houtman Abrolhos Islands are the southernmost reefs in the Indian Ocean. Field and laboratory studies were undertaken to monitor and observe coral spawning patterns in the Houtman Abrolhos Islands in relation to temperature and tidal cycles. Observations were compared with spawning at lower latitude reefs in Western Australia and reefs in the Great Barrier Reef, which exhibit significant differences in tidal phase and amplitude.