A mass mortality event has hit fish farming in Lake Batur, Bangli Regency, Bali. Approximately 37 tons of fish belonging to local farmers were reported to have died suddenly on Monday (20/7/2026) early in the morning at around 03:30 WITA. This shocking event was accompanied by a strong sulfur smell around the lake area.
The Geological Agency explained that this event was purely caused by a natural process related to limnological dynamics and the hydrothermal system within the Mount Batur caldera. The phenomenon, known as an overturn or upwelling, was triggered by cold weather changes and strong winds that suddenly stirred the lake's water column.
Scientifically, the oxygen-rich water on the lake's surface swapped places with the oxygen-depleted water at the bottom of the lake. Turbid bottom water containing hydrogen sulfide (H₂S) gas and organic waste rose to the surface. This water mass rotation caused oxygen levels in the farming zone to drop drastically, making it difficult for the fish to breathe, resulting in instant death.
Head of the Fisheries Division of the Bangli Livestock and Food Security Department, I Wayan Agus Wirawan, mentioned that this annual cycle also frequently occurs in other lakes in Indonesia. For instance, a similar disaster struck Lake Toba in 2018 with losses reaching 180 tons of fish, and Lake Maninjau which has repeatedly experienced similar phenomena, locally known as "Tubo Belerang," including one that killed up to 1,428 tons of fish in late 2025.
Given that the timing of this natural phenomenon is difficult to predict accurately, the department urges fish farmers to better understand weather patterns and upwelling cycles. Farmers are advised to delay stocking new fingerlings or harvest fish early to minimize economic losses if signs of water rotation begin to appear.
On the other hand, the Geological Agency ensured that this phenomenon is not at all an indication of increased volcanic activity or eruption of Mount Batur. Based on seismic monitoring results, the volcanic activity remains at Level I (Normal). Although the sulfur gas emission has reportedly subsided, the public and tourists are urged to remain vigilant and stay away from the affected area.
