Fish kill in Oder river - it is what I have feared

As a background, I have studied temperature responses of fish since 1980 and have written the book “Introduction to Aquatic Toxicology”, which was published by Elsevier in 2014, so both temperature- and pollution-related problems are within my expertise.

This summer has been intolerably warm in Central and Southern Europe. Fish kills are frequent in small, shallow lakes during heat waves in Finland, because of both reduced oxygen levels and increased temperatures. Fish kills don’t usually occur in rivers, because they have higher oxygen levels and remain cooler than small lakes. The species composition of fishes in the rivers also differs from that in the small lakes: rivers have more species, which require cooler water and higher oxygen levels. Thus, in the case of the Oder fish kill, one would need to evaluate, if all species have died equally or if species with lowest thermal maxima are overrepresented among the dead fish. If the latter is the case, then it is likely that we are experiencing the first climate change-related mass mortality of fish in Central Europe.

As the reason for the Oder fish kill, one has almost unequivocally concentrated on chemical pollution, although there have not so far been definitive measurements showing that this would have been the case. This is by all means possible, as dredging has been done in the river, which usually liberates contaminants which have been hidden in the sediment. However, even in this case an equally worrisome problem is that drought has caused the flow of the river to decrease markedly: any liberated pollutants have much higher concentrations than would be the case without drought. Also, virtually all pollutants are more toxic to fish at high than at low temperature. This being the case, even if pollutants are the cause of fish deaths, their level need not have increased. Because of the temperature increase, the toxicant level is lethal, even though it would not have been that at a lower temperature. This could be the case even if contaminant concentrations have increased.

In conclusion, the Oder fish kill is either directly caused by climate change, or temperature increase has contributed to the pollutant-induced fish kill. If serious climate actions are not done soon, I am afraid that similar events are observed regularly throughout the globe.