The 854 km River Oder (Odra in Polish), with a catchment area of 118,861 km², flows through Czechia, Poland and Germany. In late July 2022, it became severely polluted and its fish and aquatic molluscs suffered unprecedented mass mortality (Save the Oder Coalition, 2022, saveoder.org).

The disaster began in the Polish part of the river on 27 July. At the beginning of August, toxic golden algae Prymnesium parvum were detected by the German research institute IGB (Leibniz-Institute for Freshwater Ecology and Inland Fisheries). As these algae are associated with saline waters, the primary cause of their appearance in the river must have been contamination by such waters, most likely from coal mines. The toxins produced by golden algae are lethal to gill-breathing organisms and could have been responsible for the mass die-off. This is the first record of these invasive algae in Poland.

The conservation programme for the Critically Endangered Baltic population of the Atlantic sturgeon Acipenser oxyrinchus oxyrinchus turned out to be in vain; some 20,000 young fish, due for release into the river, perished before this could happen as they were being kept in tanks through which contaminated river water was flowing. From the end of July to 12 September, over 249 t of dead fish were taken out of the river. The deaths of millions of fish and molluscs will have a catastrophic effect on the whole ecosystem, including on the hundreds of thousands of waterbirds that winter in the area.

The lower Oder valley is a diverse ecosystem with several dozen protected areas. The Oder influences the ecosystems at its mouth and in the southern Baltic Sea, and these areas are the most important wintering areas of the threatened velvet scoter Melanitta fusca, long-tailed duck Clangula hyemalis and greater scaup Aythya marila.

There has never before been a disaster on such a scale in this region, so its consequences are difficult to predict. Although human activities were undoubtedly responsible, no precise cause has been identified. To prevent a similar catastrophe, scientists have recommended ceasing all regulation of the river, beginning its restoration and improving monitoring. Unfortunately, the Polish authorities are planning the opposite: the implementation of extensive projects for cascading, regulating and deepening the Oder (Ławicki et al., 2017, Oryx, 51, 397), a process that has already begun. In addition, the discharge of industrial sewage continues unabated, as evidenced by the sustained high salinity of the river. In the context of this disaster, the Brandenburg Ministry of the Environment has filed a legal complaint against Poland.

Photographic documentation of the mass mortality is available at doi.org/10.17632/kw5pd8ckwy.1.