November 28, 2025 — Today, a coalition of international and Bosnian scientists, together with leading environmental NGOs, reveal evidence linking the mass mortality of fish and other aquatic life on the affected stretch of the upper Neretva on September 12, 2025, to operational practices at the Ulog Hydropower Plant (HPP). These findings are being formally presented today at a conference hosted by the Zeleni Klub (Green Club) at the Parliamentary Assembly of Bosnia and Herzegovina in Sarajevo.

On September 12, 2025, large numbers of dead fish including the softmouth trout (Critically Endangered) and European bullhead as well as the white-clawed crayfish (Endangered),  were observed along the affected stretch of the upper Neretva River, accompanied by observations of fish gasping for air, crayfish crawling onto the shore, and an intense smell of rotten eggs—clear indicators of hydrogen sulfide abundance and severe oxygen-depleted conditions. The recently completed Ulog HPP cuts through the most valuable and previously intact river stretch of the Neretva, home to endangered species and high biodiversity, which is now directly impacted by the dam’s operations.

The operators of the Ulog HPP do not hold a permit for hydropeaking. Yet, operators discharged oxygen-depleted water from the reservoir in a series of flush releases, directly causing lethal conditions for sensitive and threatened species. Scientists and NGOs stress that the environmental permit must be strictly adhered to and that the Environmental Impact Study (EIA) must be urgently reviewed and updated to prevent further ecological damage.

Prof. Dr. Gabriel Singer, University of Innsbruck, explains: "Measurements we took on September 28, 2025—16 days after the fish kill—confirmed that the cause of the mortality was the release of severely oxygen-depleted deep water from the HPP reservoir. Ulog HPP is really an example of how NOT to do hydropower: these observable implications were predictable based on the reservoir’s expectable stratification over summer and the current operational regime, and they are incompatible with the river’s ecological health."

"Our goal for this case is to serve as an example that energy development must be balanced with environmental preservation, and that decisions must be made based on scientific facts, not short-term commercial interests. The Neretva is one of the most valuable ecosystems in Southeast Europe, and its protection is the collective responsibility of all of us," concluded Prof. Dr. Muriz Spahić, former Dean of the Faculty of Science and Mathematics at the University of Sarajevo.

For more information about the findings, please refer to the factsheet and the summary of the scientific investigative report, as well as an open letter co-signed by 192 fellow scientists.

Call to Action
The Ulog HPP is still operating in violation of the environmental permit. Experts and NGOs emphasise that safe operation requires ecologically responsible reservoir management. Hydropower in the 21st century must not come at the cost of repeated ecological catastrophes. Immediate measures proposed include:

1. Safe operation: The Ulog HPP must cease hydropeaking and operate strictly in accordance with its existing environmental permit in run-of-river mode. There must be no further releases of deep, oxygen-deprived water, and any future permit must explicitly prohibit hydropeaking to prevent repeated ecological damage.
2. Improve monitoring: The plant must monitor and report, in real time, reservoir levels, water discharges, oxygen concentration, and temperature throughout the depth profile and downstream.
3. Technical improvements: Introduce technical solutions that ensure safe water quality downstream. This includes eemploying ecological and environmental models to forecast downstream risks, establishing minimum oxygen requirements and environmental flow criteria, improving deep-water oxygen levels and installing equipment that allows releases from different depths.

Ulrich Eichelmann, CEO of Riverwatch / Save the Blue Heart of Europe Campaign:  “The catastrophe in September not only killed fish, it wiped out all life in that section, including water insects, molluscs etc. If the Ulog hydropower plant is allowed to keep operating illegally in a peaking regime, unleashing near-daily floods and droughts on the Neretva, the devastating damage will not only be limited to the river stretch below the plant, but the fish populations throughout the entire Neretva, all the way to Konjic, will suffer irreparable harm.”