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Korean fish farmers on edge as sweltering heat, 'death water' imperil stocks

Published: 01 Aug. 2025, 15:53 Updated: 03 Aug. 2025, 17:41

Dead fish fill a rubber tub after a mass die-off caused by high sea temperatures in Sanyang-eup, Tongyeong, South Gyeongsang, in August 2024. [TONGYEONG CITY GOVERNMENT]

At around 2 p.m. Wednesday, in a coastal village in Sanyang-eup, Tongyeong, South Gyeongsang, temperatures reached 32 degrees Celsius (89.6 degrees Fahrenheit) as fish cages for red seabream and rockfish bobbed in the waters off the village, covered by shade nets to block the harsh sun.

“It barely rained during the monsoon this year, so the sea warmed rapidly, and we’ve entered the high-temperature zone earlier than usual,” said Lee, 61, a fish farmer with more than 30 years of experience in the area. “Since late June, I’ve been feeding the fish a special diet with antibiotics and nutrients to help them endure the heat, but I still worry about how long this extreme heat will last — and whether we’ll see mass deaths again. I’m constantly checking the thermometer.”

Fishermen fear repeat of fish death

Around 1,800 fish farmers in Tongyeong operate roughly 570 marine farms cultivating species such as rockfish, red seabream, sea squirts and oysters. Last summer, when sea surface temperatures in the area rose above 30 degrees Celsius, mass fish die-offs struck 604 farms, causing damage worth 49.9 billion won ($35.6 million). Across South Gyeongsang, more than 29.2 million farmed fish and shellfish perished, resulting in a total loss of 65.9 billion won for 952 farms.

Tongyeong, the hardest-hit region, has been on high alert since a high-temperature advisory was issued Monday for Jinhae Bay, where seawater temperatures exceeded 28 degrees Celsius.

“From last year’s experience, we learned that larger, market-ready fish are more vulnerable to heat,” said Lee. “We leave early by 5 a.m. to feed the fish and finish up work before the heat peaks. But even people are exhausted on the water in this heat.”

No fish deaths have been reported yet this summer in Tongyeong.

“Before the advisory was issued, we distributed nutritional supplements and shade covers to local farms,” said a Tongyeong city official. Temperature-monitoring devices were installed in each aquaculture zone after last year’s losses, allowing farmers to check sea temperatures on their phones in real time.

A shade cover is draped over fish cages for red seabream and rockfish in the waters off a coastal village in Sanyang-eup, Tongyeong, South Gyeongsang, on July 30. [KIM MIN-JU]

‘Dead water’ threat rises

This year, farmers also face another threat: low-oxygen water masses. The National Institute of Fisheries Science has warned of increasing zones of hypoxia, known as “dead water,” across Jinhae Bay, Bukshin Bay in Tongyeong, and other areas along the southern coast.

These water masses, with dissolved oxygen levels below 3 milligrams per liter, typically form in summer when surface and bottom water temperatures diverge sharply. As organic matter washes into the sea from land, microbial decomposition consumes oxygen, forming hypoxic layers that often start 15 meters (49.2 feet) below the surface and spread upward.

Once dead water enters fish farms, it can cause catastrophic die-offs, earning nicknames among farmers such as “death water” or “clear water.” The dual threat of extreme heat and hypoxia leaves many struggling to cope.

Rockfish float lifelessly at a fish farm in Geoje, South Gyeongsang, in August 2024, after succumbing to high sea temperatures. [JOONGANG ILBO]

“During heat waves, we lower sea squirt farming ropes deeper into the water where it’s cooler,” said Jin, a sea squirt farmer in his 60s. “But dead water rises from the bottom, so we risk running into that instead. We don’t know what to do.”

Low-oxygen water masses caused 1.4 billion won in damage to aquaculture farms in 2023 and 7.3 billion won last year, according to South Gyeongsang officials.

“Dead water is hard to predict and invisible to the eye,” said Park Sung-eun, a researcher at the institute’s marine environment division. “For sea squirt farms, curving ropes into a U-shape or harvesting early are preventive options, but they’re not fundamental solutions.”

“This year, the onset of dead water in the South Sea is about 20 days later than last year and not as intense near the surface,” Park added. “But if a typhoon brings heavy rain and more organic matter flows into the sea, the severity could increase. Caution is needed.”

A sea squirt farmer removes dead sea squirts that appear cloudy and ruptured due to high temperatures at a farm in Tongyeong, South Gyeongsang, on Aug. 26, 2024. [YONHAP]