21 Apr 2017 - As the sky turned to ink on Monday, seven men and women waited with bated breath at a secluded island off Singapore's southern coast to witness a once-a-year spectacle - an orgy, they hoped, of spectacular proportions.
But this year's mass coral spawning ended more with a whimper than a bang, as local reefs are still shaking off the effects of last year's bleaching event, caused by an extended period of elevated sea surface temperatures.
Dr Karenne Tun, director of the coastal and marine division at the National Parks Board's (NParks) National Biodiversity Centre, said: "The impact of the longer coral bleaching last year on this year's coral spawning is a major concern."
A "greatly reduced spawning intensity" was recorded this year compared to past years, Dr Tun said. "However, the species that did not bleach last year were not affected and displayed healthy spawning."
Healthy coral reefs are important as they draw in marine life and function as a nursery for baby fish. Corals depend on symbiotic algae, called zooxanthellae, for energy. Bleaching occurs when abnormally high sea temperatures cause corals to expel the algae, turning the corals white and depriving them of a key source of nutrition.
Last year, Singapore's corals endured the longest bleaching incident on record. It started in June and corals started recovering only at the end of the year. It was not just corals in Singapore that were affected. Last month, a study by Australian scientists found that two-thirds of the 2,300km stretch of the Great Barrier Reef had also suffered serious bleaching.
Scientists here think that the limited spawning this year could be because corals had more critical resource concerns.
Said Dr Toh Tai Chong, a marine biologist from National University of Singapore's Tropical Marine Science Institute (TMSI): "After bleaching, corals devote a lot of energy to recovering. This depletes the amount of energy they store in their tissues and reduces the amount of energy available to initiate or complete the sexual reproduction."
Dr Tun told The Straits Times on Monday, after a dive off Raffles Lighthouse to monitor this year's spawning event: "When bleaching occurs, corals lose their algae. When that happens, it might affect the reproductive cycle."
The reproductive cycle for broadcast coral spawners - or corals that reproduce through the mass release of eggs and sperm into the water column - usually lasts between six and eight months, noted Dr Tun.
For spawning to occur, corals must have enough time and energy to complete the cycle, similar to how human mothers need to carry their babies in the womb for nine months before giving birth. When energy resources are insufficient, corals may skip a spawning event, or produce less sperm and eggs.
In Singapore, the affair takes place once a year, usually beginning on the third night after the full moon in late March or April.
NParks scientists are planning another dive after the full moon next month, just in case there is any delayed spawning.
The last bleaching incident in Singapore occurred in 2010 between June and September. But corals affected then started recovering around the time the reproductive cycle in corals was expected to start. The spawning event in 2011 was observed to be less intense compared to the pre-bleaching 2010 spawning event. However, this year's spawning intensity was observed to be even weaker than that in 2011, said Dr Tun.
Mr Stephen Beng, head of the marine conservation group at Nature Society (Singapore), noted that intense bleaching incidents may become more frequent with climate change. He said: "We should be equally concerned with longer term threats, such as climate change, as we are with immediate ones such as coastal development, overfishing and pollution."