Bosnia and Herzegovina, September 2025 – A devastating ecological disaster has struck the upper Neretva River on September 12. Just downstream of the newly built Ulog hydropower plant, local fishermen discovered a 7-kilometer stretch of river filled with dead fish.

Experts suggest the catastrophe was caused by a release of oxygen-starved deep water from the dam reservoir, suffocating nearly all aquatic life downstream. The impact was catastrophic, wiping out numerous fish species — including the endangered soft-mouth trout (Salmo obtusirostris) — as well as populations of the highly endangered white-clawed crayfish.

Despite the scale of destruction, authorities have denied the hydropower plant’s responsibility. Reports indicate the operator carried out this destructive flushing without the necessary permits, raising urgent questions of legality and oversight.

Scientists Raise the Alarm

In response, a consortium of river ecology experts with long-standing knowledge of the Neretva and the Ulog HPP issued an open letter of concern to the authorities in Bosnia-Herzegovina on September 26th, co-signed by 192 fellow scientists.

The experts point to multiple lines of evidence that strongly suggest the die-off was triggered by the operation of the Ulog hydropower plant, most likely by the release of oxygen-depleted, hydrogen sulfide–rich deep water from the dam’s reservoir. Witnesses reported a strong smell of rotten eggs and observed fish gasping for air, crayfish crawling to shore, and blackened water downstream – all signs of toxic, oxygen-deficient conditions.

The letter warns that this is not an isolated incident. Fish kills were already documented during Ulog’s construction in 2023, and the first test filling of the reservoir in 2024 raised alarms about worsening water quality. Scientists argue that the Environmental Impact Study (EIA) failed to properly assess these risks and urgently call for its review and revision.

Among their key recommendations are:

• Immediate independent investigations into the die-off.
• Comprehensive monitoring of reservoir water chemistry and downstream river conditions.
• Collection of fish specimens for pathological analysis.
• A reassessment of the Ulog HPP’s ecological risks, including the impact on endangered species such as softmouth and marble trout.
• Consideration of technical mitigation measures, such as selective water withdrawal systems to avoid releasing oxygen-depleted water.

The experts stress that without urgent changes to how the dam is managed, such disasters are highly likely to recur.

Our Call for Action

The Neretva is one of Europe’s last wild rivers and a biodiversity hotspot of global importance. Allowing repeated fish die-offs in the name of “green energy” represents nothing less than ecocide.

We urge the competent authorities to:

• Launch a transparent, independent investigation in line with the scientists’ recommendation.
• Hold the operator fully accountable.
• Re-examine the project’s ecological permits and the flawed Environmental Impact Study.