Lake Toba Shrinks, IPB Expert Warns of Risk of Mass Fish Deaths

Jakarta, CNBC Indonesia - The water level of the lake in the Lake Toba area is receding, posing a serious threat to aquaculture, particularly floating net cages (KJA). Based on satellite altimetry data, the lake’s water level has continued to decline, reaching around 1.6 metres during the period from June 2025 to March 2026. IPB Institute of Satellite Remote Sensing expert Jonson Lumban Gaol stated that the lake’s water level could potentially drop by up to 2 metres if the dry season persists. This phenomenon threatens various activities around Lake Toba, including the aquaculture sector. Jonson warned that this hydrological phenomenon could be exacerbated by the BMKG’s prediction of the possible simultaneous occurrence of El Niño and a 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,” Jonson explained in a statement obtained from IPB’s official website on Friday (24/4/2026). He added that a drop in the lake’s water level is often associated with mass fish deaths in KJA, as occurred in 2016 when thousands of tonnes of fish in KJA died when the water level receded by 2 metres. Similar incidents also happened in 2018, 2020, and 2023 when the water level was relatively low, though on a smaller scale. Jonson revealed that the drop in water level is not the direct cause of fish deaths, but rather a triggering factor for the mixing of water masses during extreme weather and strong winds. In relatively shallow waters, strong winds stir up fine organic waste sediments from the lake bottom. These fine sediments rise to the surface and can clog fish gills. “At the same time, oxygen-poor water from the lower layer rises to the top, causing a drastic drop in dissolved oxygen levels, resulting in the death of fish in KJA,” explained this professor from IPB’s Faculty of Fisheries and Marine Sciences. Another triggering factor is the accumulation of organic and household waste at the lake bottom. Jonson said that under normal conditions, this waste is broken down by bacteria with the aid of oxygen. However, when oxygen is depleted, the decomposition process occurs anaerobically, producing toxic gases such as hydrogen sulphide and methane. This hydrogen sulphide, he said, can damage the fish’s respiratory system, while methane also degrades water quality. The combination of low oxygen, high levels of toxic gases, and increased water turbidity causes mass fish deaths in KJA. He urged fishermen in Lake Toba to heighten their vigilance. If signs of extreme weather such as high wind speeds and cloudy water colour begin to appear, he suggested relocating KJA to deeper waters or harvesting the fish as quickly as possible. “On the other hand, local governments and stakeholders need to act more proactively, not just with appeals, but also with systems capable of providing accurate and rapid early warnings,” he emphasised.