Government says mass crucian carp die-off in Soyang Lake caused by oxygen depletion and bacterial infection

A government investigation has concluded that the mass die-off of crucian carp in the upper reaches of Soyang Lake in Gangwon Province was caused by oxygen depletion in the lake's lower layers, compounded by bacterial infection during a period when the fish's immune systems were weakened.

The Ministry of Climate, Environment and Energy said Tuesday that a probe conducted at five sites where dead fish were mainly found showed that multiple factors combined to stress the fish.

The investigation, carried out from May 21 to May 29 by the National Institute of Environmental Research — an agency under the ministry — found that dissolved oxygen levels in the lake's lower layers had dropped to 2.0 milligrams per liter or below. The condition, known as hypoxia, occurred as organic matter decomposed and consumed available oxygen.

The hypoxic conditions were worsened by thermal stratification, in which warmer, less dense water sits atop colder, denser water, preventing vertical mixing. This spring, high water levels in Soyang Lake, combined with elevated temperatures and low rainfall, intensified stratification.

With oxygen already scarce, the carp were further stressed by infection with Aeromonas bacteria, a microorganism commonly found in freshwater that does not normally kill fish.

The ministry cited the fact that most of the dead fish were adult carp that feed on the lake bottom as a basis for its analysis.

The ministry drew a line against claims that hydrogen sulfide poisoning was the direct cause of death. Hydrogen sulfide was not detected in the water itself, and only trace amounts — between 0.002 and 0.003 milligrams per liter — were found in pore water trapped between sediment particles.

Earlier, a fish research center affiliated with Kangwon National University's environmental research institute reported in a May 14 study that hydrogen sulfide levels as high as 0.519 milligrams per liter had been detected in water samples, attributing the mass die-off to "complex environmental stress including hydrogen sulfide poisoning."

An official at the National Institute of Environmental Research addressed the discrepancy in hydrogen sulfide readings, saying that if concentrations had been that high, juvenile fish would have died first. The official added that sulfate levels — the source from which hydrogen sulfide forms — were also low.

The official said three institutions verified the institute's hydrogen sulfide findings: Hanyang University, Seoul National University and the Korea Institute of Ocean Science and Technology.

In response to the mass die-off, the government said it would work to prevent organic matter from flowing into Soyang Lake by promoting the conversion of highland fields upstream into terraced farmland and encouraging crop changes.

Livestock waste previously handled by individual farms on a small scale will be taken over by public authorities for systematic management and treatment, and public sewage treatment facilities will be expanded.

On calls to dredge accumulated organic sediment, the government said it plans to first conduct a survey focused on the area near Bridge 38, where organic matter concentrations were highest, before selectively removing deposits.

Alongside this, water circulation devices that mix water vertically will run continuously upstream of Bridge 38 to prevent oxygen depletion in the lower layers. The government also plans to monitor indicators that can predict anaerobic conditions, including dissolved oxygen and oxidation-reduction potential.

On compensation sought by fishermen, the government said Inje-gun will provide fishing gear and nets at half price and expand existing support projects for fishing households, including buyback programs for ecosystem-disrupting species. Korea Water Resources Corporation, which manages the Soyang Lake surface, will build infrastructure — including crucian carp spawning grounds — to allow fishing to resume.

A ministry official said the government would "swiftly push forward fundamental measures, including managing pollution sources upstream of Soyang Lake and removing accumulated organic sediment," adding that it would do its best to help affected fishermen return to their livelihoods as soon as possible and prevent a recurrence.

The mass die-off of crucian carp, carp and eels in the upper reaches of Soyang Lake began in early April, prompting 49 inland fishing households to suspend operations. The incident sparked controversy over its cause, including speculation about the inflow of toxic substances.