Brown seaweed returns to stink up Jeju coastline

A thick mass of dark brown seaweed known as Sargassum horneri is once again blanketing the coastlines of Jeju Island, bringing foul odors, disrupting tourism and threatening local marine industries.

A thick mass of dark brown seaweed known as Sargassum horneri is seen coating the black volcanic rocks along the coast of Sinheung-ri in Jocheon-eup, Jeju City, on May 1. [JOONGANG PHOTO]

JEJU — A thick mass of dark brown seaweed known as Sargassum horneriis once again blanketing the coastlines of Jeju Island, prompting concern from residents and authorities as the seasonal marine invader brings foul odors, disrupts tourism and threatens local marine industries.

In Sinheung-ri in Jocheon-eup, Jeju City, a thick layer of dark brown seaweed coated the black volcanic rocks along the coast on Thursday afternoon. The pungent, fishy odor of the seaweed filled the air as it clung to every crevice of the rocky shoreline.

Entangled among the matted seaweed were plastic bottles and discarded fishing gear. Many of the plastic bottles bore simplified Chinese characters. Beneath the surface of the water, dense clusters of seaweed drifted, while ducks struggled to forage through the tangled mess.

The area is a popular stretch of coastal road, frequented by tourists visiting Hamdeok Beach and its nearby cafes and restaurants.

“I had just stepped out of a restaurant nearby when I was hit by the stench, and I looked out to the sea and was even more shocked,” said Kim Ji-yeon, a 38-year-old tourist from Seoul. “It’s unpleasant to look at and worse to smell. I hope they clean it up soon.”

A thick mass of dark brown seaweed known as Sargassum horneri is seen coating the black volcanic rocks along the coast of Sinheung-ri in Jocheon-eup, Jeju City, on May 1. [JOONGANG PHOTO]

Jeju authorities are on high alert as Sargassum horneri, often dubbed an unwelcome guest of the island's waters, invades the coast once again.

Between 10,000 to 20,000 tons of the seaweed drift into Korean waters — including those around Jeju and South Jeolla Province — every year. The seaweed, which can grow up to five meters long (16 feet), travels in massive floating mats.

It clings to aquaculture nets and structures, disrupting fishing operations, and often wraps around ship propellers, posing hazards not only to fishers but also to tourists using boats. Unlike Codium fragile, a native seaweed used in Jeju’s traditional momguk soup, Sargassum horneri remains tough even when boiled and is inedible.

Scientists believe the seaweed originates from the southern coast of China. It begins growing along rocky shores of the East China Sea during spring, when water temperatures rise, and breaks off due to wind and waves. It then drifts northward along the Kuroshio Current and reaches Korean waters via the Tsushima Current.

Plastic bottles with Chinese characters are seen tangled among a thick mass of dark brown seaweed known as Sargassum horneri coating the black volcanic rocks along the coast of Sinheung-ri in Jocheon-eup, Jeju City, on May 1. [JOONGANG PHOTO]

In fact, around 2015, China began mass-planting Sargassum horneri as part of marine forest projects and ecological restoration efforts to boost the ocean economy. That timing aligns with when large-scale invasions of the seaweed began appearing in Jeju.

Genetic analysis by Korea's National Institute of Fisheries Science confirmed the 2015 influx matched specimens from the East China Sea.

The seaweed tends to reach Jeju's coasts in large quantities between March and June. Jeju’s collection volumes spiked from 860 tons in 2019 to 5,181 tons in 2020 and 9,755 tons in 2021.

Since 2022, however, annual volumes have dropped to around 500 tons. This year, approximately 50 tons had been removed by February. On April 17, 20 tons of seaweed — believed to be a type of kelp — washed ashore near Iho Beach, prompting a cleanup operation.

Jeju City officials are seen clearing away seaweed at Iho Beach in Jeju on May 7, 2024. [JOONGANG PHOTO]

Jeju has experimented with using the collected seaweed as fertilizer. Its salt content and minerals can neutralize acidic soil, benefiting farmland.

In 2021, most of the 9,755 tons collected were distributed to local farmers. However, since 2022, with lower collection volumes, the seaweed has been mostly incinerated.

“We believe recent strong winds pushed large amounts of seaweed ashore,” a Jeju official said. “Since January, we’ve seen unusually high volumes washing in and are continuing close monitoring.”

Translated from the JoongAng Ilbo using generative AI and edited by Korea JoongAng Daily staff. BY CHOI CHOONG-IL [lim.jeongwon@joongang.co.kr]