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Toxic red tide blooms devastate fish farms along South Gyeongsang coast

Published: 31 Aug. 2025, 14:22 Updated: 31 Aug. 2025, 19:04

A mass die-off of fish is seen at an aquaculture farm in Namhae, South Gyeongsang, on Aug. 29. [YONHAP]

Toxic red tide blooms have erupted along the southern coast of South Gyeongsang, a region home to a high concentration of fish farms, resulting in mass die-offs of flatfish and other species. After suffering record losses last year due to extreme heat and high sea temperatures, fish farmers in the region are now bracing for another wave of damage — this time from harmful algal blooms.

According to the South Gyeongsang provincial government on Sunday, about 268,000 fish, including flatfish, mullet, blackhead seabream, sea bass and red seabream, were killed over four days starting Aug. 27 at 20 aquaculture farms in Namhae County, Hadong County and other coastal areas. The damage is currently estimated at 662 million won ($476,000), though the final toll is expected to rise as more farms report losses.

The National Institute of Fisheries Science identified Cochlodinium as the main microorganism responsible for the red tide off the coast of South Gyeongsang. When this algae proliferates in large numbers, it depletes oxygen in the water. The cells also cling to fish gills, causing tissue damage that leads to death. The province last experienced severe damage from red tide in 2019, when 2 million farmed fish died, resulting in 3.6 billion won in losses. No such damage had been recorded in the five years since.

On Aug. 26, a red tide watch — issued when more than 10 harmful micro-organisms are found per milliliter of seawater — was announced for the western coast of Namhae. By Sunday, the advisory had been expanded from the western coast of South Gyeongsang to the western shores of Geoje Island, with concentrations exceeding 4,000 organisms per milliliter in some areas.

Cochlodinium thrives in temperatures between 25 and 28 degrees Celsius (77 to 82.4 degrees Fahrenheit). Throughout August, surface temperatures across the entire South Gyeongsang coastline remained above 28 degrees, resulting in continuous heat alerts and likely contributing to the spread of the bloom.

Government officials test out an anti-red-tide sprinkler off the coast of Tongyeong, South Gyeongsang, on Aug. 27. [GYEONGSANGNAM-DO]

"The heat wave last year caused massive damage to farms," said a flatfish farmer in his 60s in South Gyeongsang, who has been in the aquaculture industry for more than 30 years. "This year’s heat wasn’t as intense, but with this red tide outbreak, most fish farmers are on edge again.”

According to the provincial government, last year’s marine heat wave killed more than 29 million farmed fish and shellfish across 952 aquaculture sites in the region, causing losses of 65.9 billion won.

“Over the past four to five years, warming waters in the South Sea have introduced subtropical toxic plankton and a greater diversity of red tide species," said a representative from the National Institute of Fisheries Science. "Torrential downpours — reaching several hundred millimeters per hour — are also rapidly changing the ocean’s nutrient balance, contributing to these shifts.”

The official added that the institute has been intensively monitoring red tide conditions since June and is sharing forecasts with local governments and fish farmers in an effort to minimize damage. Authorities in South Gyeongsang continue to track reports from affected farms to assess whether the losses are escalating.

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.
BY KIM MIN-JU [[email protected]]