The Baltic sturgeon is one of the most endangered animal species worldwide - all the more important is the reintroduction program for these fish that is ongoing on the Oder. But the fish die-off that has been rampant since the end of July 2022 has also affected them: not only a good thousand sturgeons in the river were killed, but also around 20,000 young animals in two breeding stations on the Oder. Whether there is still hope for the sturgeon population in the Oder now depends on the recovery of the river.
Since the end of July, tons of dead fish have washed up on the banks of the Oder. According to initial findings, the environmental catastrophe on the river that flows from the Czech Republic via Poland and Germany to the Baltic Sea appears to be the result of the unusually high salinity of the Oder. These apparently favored the growth of the toxic microalgae Prymnesium parvum. This releases a strong toxin that is life-threatening, especially for fish, mussels and amphibians. But where the high salt content of the river, which is probably responsible for the fish kill, comes from, is still unclear.
Sturgeon: An ancient, critically endangered species
The poisoning of the Oder is also a species-threatening catastrophe for the sturgeon, which has been threatened with extinction for a long time, because the sturgeons are on the red list of the World Conservation Organization IUCN and are currently the group of animals most threatened with extinction worldwide. In order to save these ancient, rare fish, there is a program to reintroduce the Baltic sturgeon in the Oder. But what about the sturgeon after this important habitat was hit by a man-made environmental disaster? Could any of the fish even survive?
Jörn Gessner, a scientist at the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB), knows best about the fate of the sturgeons in the Oder. He has been coordinating the resettlement program for the Baltic sturgeon since 1996 and has released around 3.5 million young animals to the Oder since 2007 with many partners. That may sound like a considerable amount, but it takes a long time before the animals can provide for their offspring themselves, because sturgeons can live up to 100 years, sometimes even older. They spend the first three years of their life in the river before migrating to the Baltic Sea, where they grow up and finally return to their home river to spawn at the age of 14 to 16 years.
Killed more than 20,000 young sturgeons and a thousand larger specimens
Gessner and his colleagues estimate that most of the more than 1,000 young sturgeons that were released into the river in the spring of this year fell victim to the disaster. According to the current state of knowledge, the fish die-off also affected sturgeons that were up to three years old and up to 90 centimeters in size. They were discovered in the Lower Oder Valley - on the way to the Baltic Sea, which they never reached. "It is currently not possible to reliably estimate how many sturgeons are still swimming in the Oder or have died," says Gessner. At least the older sturgeons, released more than three years ago, probably survived the catastrophe. Because they had already emigrated to the Baltic Sea and were therefore probably spared the algae toxin.
However, the fish kill did not only hit the sturgeons that were already swimming in the river. Two breeding facilities with a total of around 20,000 young sturgeons were also hit by the disaster. Hardly any sturgeon survived there. “The animals that died were only a month old and were actually supposed to be put into the Oder in the fall to help build up self-sustaining stocks there in the future,” explains Gessner. Both systems are fed with river water so that the animals can get used to their later habitat and its normal water quality and one day find their way to their natural spawning grounds. And so, on August 10th and 11th, 2022, the wave of poison reached them. "If the Polish authorities had warned early on, these fish could probably have been saved," Gessner criticizes.
Uncertain future
It is currently unclear how many sturgeons have survived in total. "We now hope that only a small part of the animals from the overall program will be affected," says Gessner. Smaller fish in particular sometimes stay in the side arms of the Zwischenoderland, where they may have survived. Especially in the Szczecin Lagoon, which has so far been largely spared from the fish kill, young animals are likely to cavort in search of food. However, Gessner and his colleagues cannot yet estimate whether and when the resettlement program on the Oder can be continued. Because even if the growth of the toxic algae decreases, residues of the toxin remain in the river, which must first be broken down before the Oder can be further colonized. In addition, the snail and mussel populations are also severely affected by the environmental catastrophe and the food supply for the young sturgeon is significantly restricted.
Whether and how quickly the sturgeons can be resettled in the Oder also depends heavily on how quickly the aquatic ecosystem recovers. Scientists therefore urgently advise to refrain from anything that puts additional strain on the Oder and its species community. Above all, this includes the controversial Oder expansion project, which the Federal Republic and the Republic of Poland decided in 2015 and which plans to narrow the river and strengthen the banks - a process that would lead to the loss of valuable habitats and which is already precarious could aggravate the situation of the sturgeon.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB)