Oder fish die-off: Salt discharges caused mass proliferation of toxic alga

According to the German expert group report published today, the most probable cause of the fish die-off in the River Oder is the rapid rise in salinity.

Expert report assumes environmental disaster caused by humans

According to the German expert group report published today, the most probable cause of the fish die-off in the River Oder is the rapid rise in salinity, which together with other factors led to the mass proliferation of a brackish water alga that is poisonous to fish. The brackish water alga Prymnesium parvum produces a toxic substance that is fatal to fish and other aquatic organisms. At the same time, the experts had to leave the cause of the unnaturally high salinity open due to a lack of available information. It is also unclear how the brackish water alga, normally found in coastal waters, found its way inland. The results of the Polish report were presented yesterday in Warsaw.

Federal Environment Minister Steffi Lemke commented: "The fish die-off in the Oder is a serious environmental disaster. The key finding of the investigations is that it was caused by human activities. Experts believe the fish die-off was caused by salt discharges. The high degree of salinity in the Oder and other factors led to the mass proliferation of a brackish water alga which produces a toxin that is fatal to fish. This devastating chain of effects is what the experts feel is the most likely scenario. Nevertheless, a number of questions remain unanswered. In addition to determining the causes, the main focus is on the regeneration of the Oder. The Federal Environment Ministry (BMUV) will support the affected regions, for example, in examining all environmental damage and advancing restoration. Engineering activities on the Oder are counterproductive to its successful regeneration. That is why I want to engage in dialogue with my Polish colleague to promote understanding of this and agree the next steps together. The warning and contingency plan for the Oder is already being revised. It is also clear that fish die-off is not just a problem in the Oder. In the face of the climate crisis, we need to seriously consider what we can continue to put our rivers through in future. We need to examine and reduce substance discharges into rivers, for instance from wastewater treatment plants. I plan to discuss this in November with the German federal states (Länder)."

Lilian Busse, Vice-President of the German Environment Agency (UBA) and head of the German delegation: "We can only conclusively confirm our hypothesis once the Polish report has also been evaluated. It is important that we continue to strengthen the resilience of the river floodplain ecosystem. In the midst of climate change with long periods without rain and with high temperatures, diverse uses put too great a strain on our rivers. The EU Water Framework Directive calls for a good status of water bodies and prescribes a ban on deterioration. A near natural, good status would strengthen the resilience of rivers and at the same time improve protection against floods and low water. We now need to implement the necessary measures as quickly as possible."

The rapid increase in salt concentrations in the Oder and sunlight created favourable conditions for the rapid growth of the algae species Prymnesium parvum. The German expert report revealed this was confirmed by numerous water samples and satellite images. The brackish water alga and its algal toxin Prymnesin were clearly identified in microscopic and molecular biological analyses. But, it is still unclear how the alga, which normally occurs in brackish salt water in coastal areas, found its way to the Oder. Excessively high salt content in water bodies is also an issue we are seeing in other rivers in Germany. The salinity of the Werra River has been too high for decades due to salt discharges from potash mining. However, the reason for the recent rapid and sharp rise in the salinity of the Oder will have to be clarified by the findings of the Polish investigations.

The authors of the German report examined numerous hypotheses for the cause of the fish die off. The experts concluded that the most probable cause of the fish die-off was a combination of high salinity and a massive proliferation in toxic brackish water alga, adding to the stresses on the River Oder already arising through climate change. Other causes proved unlikely. The Federal Institute of Hydrology (BfG), for instance, analysed water from the Oder for heavy metals and for more than 1,200 potentially harmful chemicals using non-targeted analysis. While the analysis identified many compounds in the Oder, none of these could have led to a mass die-off, at least not individually. Unusually high concentrations were also detected for some compounds which create favourable conditions for algal bloom caused by salt, but which cannot have triggered the fish die-off.

In the view of the expert group, to prevent long-term effects, further algal bloom must be averted in the Oder and other rivers. UBA Vice-President Lilian Busse commented: “We must ensure that brackish water algae do not spread in rivers like the Werra or Elbe. Furthermore, we need to scrutinise discharge permits for chemicals and saline water”. The non-targeted analysis used for the Oder is a very promising approach. It is currently being trialled in the River Rhine. The fish die-off in the Oder is also an opportunity to review and adapt the warning and contingency plans for our major rivers.

In future, restoring the Oder as a habitat for rare species and a source of ecosystem services for local people will be a vital task. That is why the Federal Environment Ministry is moving forward on the launch of a project under the Federal Nature Conservation Fund. This project will identify damage to the ecosystem, monitor the natural regeneration and lay the foundations for effective restoration measures.

With their specialist knowledge, people who fish in the Oder can play an important part in implementing the project, and those participating would be paid for their services. The Baltic sturgeon, now rarely seen, was also affected by the algal toxins. This species had already become extinct in Europe and a project was launched in 2006 to reintroduce it to the Oder. The success of this measure has now been jeopardised by the disaster in the Oder. To mitigate the damage caused, in the short term the BMUV plans to financially support this sturgeon project in Mecklenburg Western Pomerania and Brandenburg, in particular through restocking with juvenile sturgeons. The BMUV will continue to support and advance ongoing activities of the Länder and third-party initiatives on planning and implementing measures in the Oder through the Blue Belt federal programme (floodplains funding programme). Starting points for this include the "Blue Belt Feasibility Study - Pilot Oder", commissioned by the Brandenburg State Office for the Environment (LfU), and ideas on implementing measures put forward by environmental associations. Furthermore, we are reviewing the option of an action programme for the Oder with a focus on restoring the Oder riverscape.

In mid-August, Federal Minister Steffi Lemke and her Polish colleague Anna Moskwa set up a German-Polish expert group to establish the causes of the Oder disaster. The group was composed fifty-fifty of experts from Germany and Poland. The German delegation was headed by UBA, which also drew up the report published today. The Federal Institute of Hydrology (BfG) contributed substantially to the report. The German delegation also included representatives of various State Offices of the Länder. The Polish delegation commissioned a scientific report that was published at the same time as the German one. The publication of the reports wound up the activities of the German-Polish expert group.

30.09.2022 | Press release No. 133/22 | Water & Waste & Soil
Joint press release with the Umweltbundesamt

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