Saving the crucian carp

The rounded dorsal fin is a good identifying feature of the crucian carp. Unlike the common carp, crucian carp lack barbels. Photo: Rostislav Štefánek

A master of survival...

The natural habitat of the crucian carp (Carassius carassius) consisted of still or slow-flowing waters, oxbow lakes and vegetated ponds in floodplains. These are unstable environments where few other fish species can survive—yet the crucian carp is capable of enduring both short-term freezing and months in water with little or no oxygen. In such conditions, it develops a dwarf form and switches to anaerobic metabolism, producing oxygen from its own body fat. In nutrient-rich waters, especially where predatory fish are present, it instead develops the so-called “deep-bodied form”, whose shape makes it harder for predators to swallow.

Crucian carp—dwarf form. Juvenile crucian carp have a distinctive dark spot at the base of the tail fin. Photo: Rostislav Štefánek

... on the verge of extinction

With the agricultural transformation of the landscape, the crucian carp lost most of its natural habitats and survived mainly in small ponds, fire reservoirs and flooded quarries. However, it soon began to decline there as well, due to the introduction of the common carp (Cyprinus carpio) and the invasive Prussian carp (Carassius gibelio). Both species compete with the crucian carp for food and can also interbreed with it. The Prussian carp has an additional advantage—it can reproduce by so-called gynogenesis, a form of asexual reproduction in which eggs develop after being stimulated by sperm from another species, without the transfer of genetic material.

Never release Prussian carp, goldfish, Christmas carp or any other fish species into the wild—you could cause irreversible harm!

The non-native Prussian carp. It differs from the crucian carp mainly by having a straight or slightly concave dorsal fin. Photo: Rostislav Štefánek

Save the Crucian Carp Project

The idea to help save the crucian carp appeared in 2020, when Prague Zoo was seeking a meaningful use of the former millrace in the lower part of its grounds. The channel first had to be drained, limed, and planted with aquatic and riparian vegetation to create suitable spawning sites.

Thanks to experts from the Biological Centre of the Czech Academy of Sciences and the Czech Angler’s Union, we obtained crucian carp that now form back-up populations in several reservoirs at Prague Zoo—such as the pond in the Children’s Zoo and tanks at the zoo’s breeding facility in Dolní Dobřejov. These serve as insurance populations for reintroductions into the wild. In November 2022, the pond called U Kamenného stolu, located in Prague-Vinoř, became the first site to be stocked with crucian carp of pure Elbe River lineage—the original Czech form, now vanishing from our waters. The genetic origin of these fish is verified by our colleagues from the Czech University of Life Sciences.

Curator Petr Velenský at the pond in the Children’s Zoo during the capture of crucian carp for release. Photo: Petr Hamerník, Prague Zoo

In June 2023, the project moved from its pilot phase to a fully-fledged conservation programme when genetically tested crucian carp were ceremonially released into a revitalised pond in Přerov nad Labem, a small town east of Prague. This was followed by releases at more than twenty other sites, from the town of Kamýk nad Vltavou in the Central Bohemian Region and the town of Albrechtice nad Vltavou in the South Bohemian Region to the retention reservoir in Prague’s Motol district and the three Chodovec reservoirs in the Záběhlice district of Prague. By the end of 2024, these waters were home to tens of thousands of genetically pure crucian carp—a sustainable base for further reintroductions.

Members of the public can also help select suitable sites for reintroduction. More information can be found at zachrankarase.cz.

Juvenile crucian carp before release into the pond U Kamenného stolu in Prague’s Vinoř district. Photo: Miroslav Bobek, Prague Zoo