Ministry of Environment: Fish Deaths in Soyang Lake Caused by Low Oxygen and Disease

In April, a mass die-off of carp in the upper Soyang Lake in Inje County, Gangwon Province, was found to be caused by a combination of environmental factors, including low dissolved oxygen levels. However, the investigation has sparked controversy regarding the potential impact of upstream pollution sources, such as highland farming and livestock waste.

On June 9, the Ministry of Climate, Energy and Environment announced that a detailed investigation by the National Institute of Environmental Research concluded that the fish deaths were not due to specific pollutants but rather the result of multiple environmental factors.

The die-off in April led to the suspension of operations at 49 nearby fish farms. To determine the exact cause, the government formed a committee involving relevant agencies, experts, and local fishermen to conduct a thorough investigation.

The investigation revealed significant low oxygen levels in the bottom layers of the lake. This was exacerbated by high water levels, elevated temperatures, and low rainfall this spring, which intensified the stratification of the water, preventing adequate mixing and worsening the oxygen deficiency.

Additionally, the ministry explained that adult carp, which were already stressed due to spawning, became infected with Aeromonas bacteria present in natural waters, further compromising their health. Most of the dead fish were confirmed to be adult individuals that feed near the lake bottom.

In contrast, no external toxic substances, such as heavy metals or pesticides, were detected above acceptable levels, and hydrogen sulfide was found only in trace amounts in the pore water between sediments, not in the water column.

However, concerns have been raised that the accumulation of organic matter in the bottom layers contributed to the low oxygen levels, suggesting that fertilizers from highland farms and livestock waste may be fundamental causes.

A ministry official stated, "It is true that land use is a major contributor to nutrient runoff in the upper Soyang Lake basin, but there are various sources of discharge, including domestic wastewater and livestock waste. It is difficult to pinpoint a specific cause."

The government plans to strengthen management of the upper basin to reduce the influx of organic matter identified as a factor in the low oxygen levels. This includes promoting crop rotation and terraced farming in highland fields, expanding public treatment of livestock waste, and enhancing community-based water quality management initiatives. The removal of sediments in areas with high organic matter concentrations will also be considered.

Support for affected fish farms will be expanded. Inje County plans to increase funding for purchasing fishing gear and nets, as well as for the purchase of invasive species. The Korea Water Resources Corporation will assist in establishing infrastructure for resuming fishing activities, including creating spawning grounds for carp.

Additionally, the ministry intends to enhance monitoring of dissolved oxygen and redox potential and establish a system to detect low oxygen levels in advance through the operation of water circulation devices.

Jo Hee-song, head of the Ministry's Water Management Policy Office, stated, "This die-off was analyzed as a result of low oxygen levels in the bottom layers and various environmental factors, rather than pollution from specific substances. We will implement fundamental measures to manage upstream discharge sources and reduce sedimentary organic matter to prevent similar incidents in the future."