IPB University Researchers Warn of Declining Lake Toba Water Levels and Risk of Mass Fish Deaths
The aquaculture fisheries sector, particularly floating net cages (KJA), in the Lake Toba area is facing a serious threat as the lake’s water level recedes. Based on satellite altimetry data, the lake’s water level has continuously declined, reaching approximately 1,6 meters from June 2025 to March 2026.
IPB University’s Satellite Remote Sensing Expert, Prof Jonson Lumban Gaol, stated that the lake’s water level could potentially drop by up to 2 meters if the dry season persists. This phenomenon threatens various activities around Lake Toba, including the aquaculture fisheries sector.
Prof Jonson warned that this hydrological phenomenon could be exacerbated by the Meteorology, Climatology, and Geophysics Agency (BMKG) forecast of a possible simultaneous occurrence of El Niño and positive phase Indian Ocean Dipole (IOD) in 2026.
“This combination could trigger a prolonged dry season in Indonesia, including the Lake Toba area, ultimately accelerating the shrinkage of the lake’s water volume and potentially causing mass fish deaths in KJA,” explained Prof Jonson.
Prof Jonson added that declining water levels are often linked to mass fish deaths in KJA, such as the 2016 incident where thousands of tons of fish in KJA died when the water level dropped by 2 meters. Similar events occurred in 2018, 2020, and 2023 when water levels were relatively low, though on a smaller scale.
He revealed that the water level drop is not the direct cause of fish deaths but rather a triggering factor for water mass mixing during extreme weather and strong winds. In relatively shallow waters, strong winds stir up organic waste sediments at the lake bottom. Fine sediments rise to the surface and can clog fish gills.
“At the same time, oxygen-poor water from the lower layers rises to the top, causing a drastic drop in dissolved oxygen levels, resulting in fish deaths in KJA,” explained this Professor from the Faculty of Fisheries and Marine Sciences at IPB University.
Another triggering factor is the accumulation of organic and household waste at the lake bottom. Prof Jonson stated that under normal conditions, this waste is broken down by bacteria with the help of oxygen. However, when oxygen is depleted, the decomposition process becomes anaerobic, producing toxic gases such as hydrogen sulfide and methane.
“Hydrogen sulfide can damage the fish respiratory system, while methane further degrades water quality. The combination of low oxygen, high toxic gas content, and increased water turbidity causes mass fish deaths in KJA,” he clarified.
Prof Jonson urged fishermen in Lake Toba to heighten their vigilance. If signs of extreme weather appear, such as high wind speeds and turbid water color, he recommends moving KJAs to deeper waters or harvesting the fish immediately.
“On the other hand, local governments and stakeholders need to act more proactively, not just with warnings, but with systems capable of providing accurate and rapid early warnings,” he emphasized. (MHT) (IAAS/PRO)
