After the fish die-off in the Oder, experts are initially giving the all-clear for animal land-dwellers: so far only cold-blooded animal species seem to have been affected by the disaster. So far, birds and mammals have been spared. However, there are a number of obstacles to the river’s recovery.
Whether kingfisher, white-tailed eagle or otter: a whole range of rare and protected species are native to the Oder region. According to experts, the massive fish kill has so far not posed any danger to many. “According to current knowledge, only cold-blooded animal species are affected, such as fish, mussels and snails,” said Dirk Treichel, head of the Lower Oder Valley National Park. The so-called cold-blooded animals include fish, amphibians and reptiles, but not birds and mammals.
According to Treichel, in addition to hundreds of tons of dead fish that have already been recovered along the Oder, there are now also huge carpets of dead water snails near the shore. Many dead mussels also float to the surface of the water. “We assume that otters have now also eaten carcasses or dying fish. But there are no finds of dead specimens,” said Treichel. “Ornithologists have found two dead cormorants and five dead ducks. However, it is not apparent that there is a causal connection to the fish kills,” added Christian Wolter from the Berlin Leibniz Institute for Freshwater Ecology and Inland Fisheries.
Last week, Wolter and colleagues pointed out the strong growth of the algae species Prymnesium parvum, which can form a poison that can kill fish. “This poison was detected in significant concentrations in the Oder, which could have caused this death,” said Wolter. The mass development of the alga was only possible due to a high salt concentration in the river water. “Colleagues of mine found out that it is common salt. It is well suited for the development of algae,” says Wolter. He assumes that it is a waste product from the industry. “But I don’t know which process produces such amounts of common salt and why you have to get rid of them when the water is low.”
There have already been various other cases of mass fish deaths caused by the algae poison worldwide. “Similar disasters have not been reported to affect warm-blooded animals as well,” Wolter said. But the Oder disaster could reduce supply for species like herons and cormorants that hunt live fish. “However, these species can also escape to surrounding waters without having to change their approach area significantly,” says Wolter. Sea eagles would also eat dead fish. “They have no problem with that.”
Until the massive fish kill, the Oder was considered a very fishy body of water. “Around 50 species have been identified in the Oder, including such special ones as loach, asp and mud-cheeked cat,” said Treichel. The aquatic ecologist Wolter assumes that some of the fish survived. For them, however, the lack of oxygen, which will still occur when the algae break down, could be dangerous.
He assesses the chances for a recovery of the river as good. “The Oder benefits from the fact that it has no barrier to the sea. Fish can migrate in and out without our intervention. Fish will have avoided the damaging wave and can migrate back afterwards,” says Wolter. “Fish have a high reproductive potential. I reckon that in two to three years we will only be able to recognize fish mortality from the age pyramid,” says the water ecologist. Then there will be mainly young and relatively small fish – which, however, will be a problem for the fisheries.
A great danger for the Oder from his point of view: the planned expansion for shipping. “This is absolutely not compatible with a recovery of the river. With the Oder disaster we learned once again that we have to try to prevent low water situations and not let them arise in the first place,” says Wolter. The expansion is also not economically comprehensible and the small number of ships is disproportionate to the effort.
The director general of the Senckenberg Nature Research Society, Klement Tockner, also warned against an expansion: “This is catastrophic for the Oder.” The construction of a deeper fairway will cause the groundwater level in the adjacent areas to drop. Further dams are also planned. All of this leads to a massive deterioration in the ecological status, according to Tockner. “You upgrade the river for a single function at the expense of many valuable services. The river provides recreation, is a valuable ecosystem and retains water in the landscape – at low tide and at high tide. All of this will suffer with an expansion and this can lead to significantly greater damage.”
(This article was first published on Thursday, August 25, 2022.)
