SINGAPORE – The annual mass-mating of corals in Singapore’s waters in April and May 2026 was a muted affair. The reduced release of eggs and sperm at one of Singapore’s richest coral reefs off Raffles Lighthouse could reflect an overall reduced resilience across the reef, said Karenne Tun, group director of the National Parks Board’s (NParks) National Biodiversity Centre. This needs further investigation, she added. Scientists said the lacklustre reproduction could be an effect of the 2024 marine heatwave – which caused over 40 per
cent of corals to bleach and turn white at one point – and a small-scale bleaching event at Pulau Satumu – where Raffles Lighthouse is located – in September 2025. Mass spawning events occur once a year, and are usually a spectacular underwater show in which the coral reefs appear to “snow”, as they release bundles of coral eggs and sperm. The phenomenon usually happens a few nights after the full moon following the spring equinox, which generally happens here around late March. The larger
the “snowy blizzard”, the more likely fertilisation will occur. The eggs and sperm then join to form larvae, which are carried by the water until some find a hard surface on which to latch on and grow. NParks conducted its annual coral spawning surveys at the southern island of Pulau Satumu from April 4 to 6 with researchers from the NUS Tropical Marine Science Institute and the St John’s Island National Marine Laboratory. “While some coral colonies were observed spawning, the overall activity appeared relatively
muted compared to the spawning activity previously observed in some years, where a larger number of species had spawned,” said Tun. NParks and the researchers returned to Satumu in May to see if more spawning happened. After April’s turnout, they initially suspected that the spawning was split across two months. But no significant activity was seen in May. The muted reproductive event was similar to what happened in 2025, a few months after reefs were hit with mass bleaching in 2024 due to the warming
El Nino climate phenomenon. Back then, NParks and NUS said corals that had recovered from the 2024 episode may not have had enough energy to reproduce. On the latest spawning episode, Tun said: “The reduced spawning capacity may however reflect an overall reef-wide reduced resilience, which requires further investigation.” Some species though, continued to reproduce despite the recent environmental stressors, including the Diploastrea moon coral and the Montipora velvet corals. Higher sea temperatures stress coral reefs, forcing them to expel the algae that give them
their striking colours. This causes corals to bleach, or turn ashen white. The ordeal in 2024 – part of the severe fourth global mass bleaching episode – eventually killed 5 per cent of corals in Singapore. Between early 2023 and mid-2025, 84 per cent of the world’s coral reefs were affected by the marine heatwave. Research fellow Lionel Ng from TMSI – who was among the NUS scientists involved in the surveys in April and May – noted that the combined effects of the 2024
bleaching event and a smaller-scale bleaching episode at Satumu in September 2025 may have reduced reef health or modified the physiological responses of the corals. “The reef’s reproductive ability may not have been able to fully recover in time for the mass spawning event,” he said. While more studies need to be done, scientists suspect that the 2025 bleaching at Satumu is due to a strong south-west monsoon bringing in more water rich in river runoff from the Strait of Malacca. The organic-laden, cloudy water
may have reduced the amount of light entering the corals in deeper zones, causing them to bleach. Ng said: “It may also be useful to understand how successive (environmental) disturbances also impact reproductive capacity and the early life stages of our corals. These have long term consequences for reef replenishment and key ecological functions.” Coral scientists worldwide are dreading another global marine heatwave that could emerge with the upcoming El Nino, which is predicted to be stronger than the previous one that lasted between 2023
and 2024. Back-to-back mass bleaching can further aggravate the health of coral reefs, but is an emerging pattern with climate change. The Meteorological Service Singapore forecasts El Nino conditions to develop from June to July. Nearly every El Nino event since 1998 has coincided with mass coral bleaching that also affected Singapore’s waters. The last super El Nino in 2015 and 2016 caused 10 per cent of local corals to die. To help boost the reproduction of local reefs, scientists here are attempting to fertilise
corals in the lab. In April, the researchers based at St John’s Island National Marine Laboratory and NParks staff collected some of the spawn to fertilise them. “Corals grow very slowly and juvenile corals are more vulnerable, requiring more care during the early stages,” said Ng. Sexual reproduction increases genetic diversity among coral babies. He explained: “This is important for improving our chances of producing corals that possess certain traits which afford resilience during environmental disturbance.”
Singapore, coral reefs, mass spawning, Pulau Satumu, Raffles Lighthouse, NParks, National Biodiversity Centre, marine heatwave, El Nino, coral bleaching, Lionel Ng, Karenne Tun, TMSI, NUS Tropical Marine Science Institute, St John’s Island National Marine Laboratory