Heat fuels dangerous surge of giant carp in Vienna's Danube

The Danube River in Vienna has seen a rapid spread of Asian-origin giant carp, driven by climate change and unusually high temperatures, according to the German outlet Bild. The fish can grow up to 1.5 meters long and weigh as much as 50 kilograms.

The carp were deliberately introduced to the Danube in the 1970s and 1980s to help control excessive weed and algae growth. For decades, the cool Central European climate kept the species from spawning widely, but conditions have now become favorable, with the river's water temperature in Vienna reaching 23–25 degrees Celsius in summer.

Risks to the ecosystem and to people

Experts warn of an impending population boom, since a single female can lay around five million eggs per year. The giant carp feed by filtering large volumes of plankton through wide-open mouths, depriving native European fish species and their young of food. Scientists warn of a scenario similar to the Mississippi River in the United States, where the invasive species now makes up as much as 90% of total fish biomass in some stretches.

The carp also pose a direct danger to people. Startled by the noise of passing boats or jet skis, they can leap out of the water en masse to heights of up to three meters, and a body weighing dozens of kilograms can cause serious injuries. This is a particular concern in Vienna's New Danube channel, a popular spot for rowers and swimmers.

The scale of the problem was illustrated by an incident at the Untere Lobau nature park, where falling water levels and oxygen depletion caused a mass fish die-off, with rescuers finding that a significant share of the dead fish were Asian carp.

Vienna's city administration has so far failed to find an effective solution. Netting has been ruled out as impractical in the 21-kilometer-long, up to six-meter-deep waterway, since the fish can easily evade nets, while conventional angling does not work because the carp feed only on plankton and ignore bait.