South Korea's Soft Corals Are 'Slumping' and Dying as Ocean Warms Off Jeju

Soft coral colonies off South Korea's Jeju Island are once again showing a 'slumping' collapse this year, according to the Paran Ocean Citizen Science Center, a group of volunteer divers who monitor the island's UNESCO-listed soft corals. Roughly half of one coral cluster off the islet of Beomseom has been struggling in recent months.

Unlike hard corals, which are supported by rigid calcium carbonate skeletons, soft corals rely on internal fluid pressure to stay upright. Environmental changes can disrupt that pressure, causing the corals to sag or collapse. Researcher Kim Taihun of the Korea Institute of Ocean Science and Technology (KIOST) said that with repeated slumping, partial recovery and renewed collapse, the corals' capacity to recover keeps declining.

Kim and KIOST colleague Anna Beate Jöst documented Jeju's first recorded soft coral slumping event from 2024 in a paper published in the journal Scientific Reports in May. Kim said this year's event looks similar to 2024's, but could worsen if high water temperatures persist through September and if El Nino-linked typhoons and heavy rains follow. Over the past decade, marine heatwaves around Korea have grown to last an average of 83.3 days, nearly four times longer than before, according to weather authorities.

Jeju's soft coral communities sit at the northernmost limit of coral distribution in the East China Sea, already living at the edge of tolerable conditions. Coral researchers point to a record heatwave on the Korean peninsula alongside heavy rains in the south, as well as freshwater flowing from China's Yangtze River affecting local salinity, as potential triggers for the slumping. South Korea's Oceans Ministry said it plans a detailed investigation into the scale of the damage, while the Korea Heritage Service, which manages the coral colonies as a national monument, said it will continue long-term monitoring to identify the causes.