Whale shark sighting near Jeju signals changing ecosystem as waters warm

JEJU ISLAND — At around 10 p.m. on July 4 off Jeju Island, two tourists hunting for blue swimmer crabs found themselves stranded on a rock as the tide rushed in. Days earlier, passengers aboard a squid-fishing boat watched an endangered whale shark glide beneath the hull. Once unusual, such encounters are becoming vivid signs of a new reality around the island, where rapidly warming seas are allowing tropical and subtropical species to move into waters long considered temperate. 

The sightings of whale sharks and blue swimmer crabs have drawn particular attention because they were witnessed firsthand by local residents and tourists, which later spread through social media. Unlike earlier photos and videos of subtropical marine life, such as butterflyfish, yellow boxfish, stripey and lionfish, which were mostly captured by professional divers, these encounters occurred in everyday settings. 

The Subtropical Fisheries Research Institute under the National Institute of Fisheries Science said it plans to conduct research into the possible use of subtropical marine species as fisheries resources as the warming of Jeju waters accelerates. Jeju’s waters are already believed to be approaching the threshold for subtropical seas in terms of annual average temperatures. Subtropical waters are generally defined when the annual average sea temperature reaches 20 degrees Celsius (68 degrees Fahrenheit) or higher. Evidence of Jeju’s warming seas has been confirmed through multiple studies. 

According to the National Institute of Fisheries Science, the surface temperature of waters around Korea rose by 1.6 degrees Celsius over the past 58 years, from about 16 degrees Celsius in 1968 to 17.6 degrees Celsius in 2025. That increase is more than twice the global average rise of 0.76 degrees Celsius over the same period. 

A survey by the Jeju Marine and Fisheries Research Institute (translated) also found a sharp annual increase in coastal water temperatures last May. The surface temperature of Jeju’s coastal waters reached 18.7 degrees Celsius, 2.13 degrees Celsius higher than the 16.57 degrees Celsius recorded during the same period last year.  

One of the most noticeable recent changes in Jeju’s sea has been the appearance of blue swimmer crabs. On July 4, two tourists were rescued by the Korea Coast Guard after becoming stranded by the rising tide while foraging at night for blue swimmer crabs on rocks near Hando Bridge in Seongsan-eup, Seogwipo. During the rescue, one Coast Guard officer injured his ankle. Blue swimmer crabs mainly live in tropical and subtropical waters, including the Indian Ocean and the western Pacific Ocean. 

The species was reported to have been caught numerous times in coastal waters around Jeju over the past five years. As social media posts highlighting the crab’s striking blue shell and claiming that it tastes similar to the blue crab commonly eaten in Korea have spread, the species has also drawn attention as a potential new seafood item. 

The National Institute of Fisheries Science has launched a study into the crab’s route of entry, changes in distribution, impact on the marine ecosystem and potential industrial use.  

“Larvae likely traveled along warm currents from Taiwan and Okinawa, Japan, to Jeju before settling in nearby waters,” said Yoon Byeong-il, a researcher at the National Institute of Fisheries Science. “We are conducting a comprehensive study on their route of entry, distribution, population changes, ecological impact and potential industrial uses to determine whether they can be developed as a new ocean resource.” 

There is other evidence that suggests Jeju’s waters are becoming increasingly subtropical. 

On June 30, a whale shark estimated to be 3 to 4 meters (10 to 13 feet) long was spotted off the coast of Aewol-eup, Jeju. The shark was filmed slowly swimming beneath a fishing boat, called the Seungryong, while tourists on board were fishing for squid. Whale sharks, which mainly inhabit subtropical waters and are also an endangered species, are the largest fish in the world and can grow to more than 10 meters in length. While whale sharks have previously been found in nets or dead in Jeju waters, it is rare for a living shark to be seen swimming near the coast.  

The sighting is also seen as connected to recent reports of sharks appearing off Korea’s east coast. According to the Gangneung Coast Guard, reports of shark sightings were made on July 4 in waters east of Gyeongpo Beach and about 4 kilometers (2.5 miles) east of Anmok Beach. On June 9, a 1.8-meter great white shark was accidentally caught in a net near Samcheok Port. Experts believe whale sharks and great white sharks may have moved to Jeju and Gangneung, Gangwon, due to changes in the food chain. As sea temperatures rise, phytoplankton and zooplankton are becoming more abundant, drawing in small fish and larger migratory species that feed on them.  

“Winter water temperatures have risen by about 2 degrees Celsius, increasing cases in which subtropical marine species survive and reproduce,” said Ko Joon-cheol, a researcher at the Subtropical Fisheries Research Institute. “We are continuing to monitor and study changes in the marine ecosystem caused by the appearance of species such as blue swimmer crabs and whale sharks.” 

BY CHOI CHOONG-IL, PARK JIN-HO [[email protected]]