'Run the moment you see one': Giant jellyfish swarm Korea's beaches

A Nomura's jellyfish found in the waters off Gyeongju, North Gyeongsang Province [Provided by reader]

"Stop swimming and get away the moment you see one." — Ministry of Oceans and Fisheries safety advisory for beachgoers

As summer gets fully underway, beaches across the country are filling with crowds. But an unfamiliar and unwelcome visitor has been showing up in growing numbers, threatening those enjoying the water.

The culprit is the jellyfish — sightings of which have been increasing of late. Among them, venomous giant jellyfish that can grow up to 2 meters are now appearing with alarming regularity in Korean waters.

A fishing boat arriving at Guryongpo Port in Nam-gu, Pohang, North Gyeongsang Province, on Aug. 19, 2025, loaded with jellyfish. The city was purchasing jellyfish at 300 won per kilogram. [Provided by reader. Redistribution and DB prohibited] Yonhap

Giant jellyfish inject toxic substances through their tentacles on contact. Curious children who approach them without thinking are among those most frequently and seriously injured.

The appearance of venomous jellyfish — once thought to be a problem confined to other countries — is being driven by rapidly rising sea surface temperatures caused by climate change.

South Korea in particular has seen an especially steep rise in sea surface temperatures. The frequency of venomous jellyfish sightings has roughly doubled over the past decade.

A Nomura's jellyfish. [Provided by the Korea Institute of Ocean Science and Technology]

An analysis of nationwide jellyfish monitoring data conducted by the National Institute of Fisheries Science found that the appearance rate of Nomura's jellyfish in the Jeju region during the first week of July rose from 36 percent in 2016 to 80 percent in 2026.

The appearance rate reflects the proportion of fisheries monitoring personnel in a given area who observed jellyfish. About 80 percent of this year's Jeju monitoring personnel spotted Nomura's jellyfish. During the same period last year, the rate stood at 55.6 percent.

Nomura's jellyfish is the species considered most likely to cause harm. Adults can grow up to 2 meters in diameter and weigh as much as 200 kilograms. Their venomous tentacles can cause sting injuries on contact.

As full summer conditions have set in, the appearance rate of Nomura's jellyfish has been climbing. The nationwide rate stood at just 2.9 percent on June 18 but rose to 11.4 percent by Thursday. After Jeju, Busan recorded the next highest rate at 35 percent.

Nomura's jellyfish do not originate in Korean waters. They develop along the Chinese coast and in the northern East China Sea before riding ocean currents into the seas surrounding the Korean Peninsula, then spreading from Jeju and the South Sea northward into the central coastal waters.

Rising sea surface temperatures are accelerating this process. Warmer water speeds up jellyfish growth and increases their survival rate, meaning more individuals survive the journey from their spawning grounds off China to Korean waters.

Sightings tend to spike during periods of particularly high water temperatures. South Korea's sea surface temperatures have reached record highs this year, and jellyfish-related damage is expected to increase as a result.

According to the Ministry of Oceans and Fisheries' comprehensive high-water-temperature and red tide countermeasures plan for 2026, released June 4, the National Institute of Fisheries Science's seasonal ocean forecasting system projected that sea temperatures around the Korean Peninsula this summer would run more than 1 degree Celsius above the historical average.

Concerns about a high-density jellyfish bloom had already been raised since last spring. Those concerns materialized when the jellyfish mass-bloom crisis alert was raised from "Attention" to "Caution" on June 8. The "Caution" level is triggered when preliminary jellyfish advisories are issued for two or more sea zones, or when a full advisory is issued for at least one zone.

Jellyfish caught as bycatch aboard a fishing vessel in Gangwon Province waters in 2022 [National Institute of Fisheries Science]

Mass jellyfish blooms already caused significant damage in 2024, recorded as the hottest year on record. Jellyfish sting incidents at beaches were among the most visible consequences.

According to the Ministry of Oceans and Fisheries, jellyfish sting incidents at beaches nationwide totaled 4,224 cases from January through September 2024 — 5.6 times the 753 cases recorded in 2023. Over the same period, the volume of jellyfish removed by the ministry and local governments also rose by about 5.37 times.

A fishing boat arriving at Guryongpo Port in Nam-gu, Pohang, North Gyeongsang Province, loaded with jellyfish. [Yonhap]

The greatest concern, however, is the damage to the fishing industry. When Nomura's jellyfish, moon jellyfish and other species that bloom in large numbers become entangled in fishing nets, they tear the gear and mix in with the catch. This degrades the freshness and marketability of the haul, and in some cases forces fishermen to delay or abandon their operations altogether.

Beyond jellyfish, rising sea surface temperatures are also bringing unfamiliar venomous marine species into Korean waters with increasing frequency. The National Institute of Fisheries Science has found that since the 2000s, venomous creatures from tropical and subtropical waters around Taiwan and Okinawa have been entering Jeju's coastal waters along the Tsushima Warm Current. Blue-ringed octopuses and broad-banded sea snakes are among the species now being spotted along the southern coast and around Jeju.

A blue button jellyfish, a small subtropical species [Provided by the National Institute of Fisheries Science]

The variety of venomous jellyfish species is also growing. According to the National Institute of Fisheries Science, eight venomous jellyfish species were observed in 2025. The blue button jellyfish, a subtropical species, first appeared in Jeju waters on May 9 of that year before emerging in large numbers in mid-to-late July and spreading along the East Sea coast through September.

Beachgoers who spot a jellyfish can report the sighting through the jellyfish reporting web portal. The Ministry of Oceans and Fisheries will give small gifts to 400 people who submit reports during the official beach season in July and August. The National Institute of Fisheries Science also plans to provide a jellyfish sighting alert service for fishermen and others through its website.

Kim In-gyeong, director of fisheries resource policy at the Ministry of Oceans and Fisheries, said the ministry would carry out its jellyfish damage prevention measures without fail to protect public safety and livelihoods. "We also ask the public to actively help prevent harm by reporting jellyfish sightings immediately," she added.

woo@heraldcorp.com