Fighting the Red Swamp Crayfish: How a Popular Pet Became an Enemy

10.01.2025 | by HUN-REN

Non-indigenous species, particularly invasive animal and plant species, are an increasing concern worldwide. In Hungary, for instance, there are more than 30 non-native species of decapod crustaceans compared to only three indigenous ones. Researchers from the HUN-REN Centre for Ecological Research, in collaboration with international and Hungarian colleagues—with the involvement of university students—are studying the status and impact of decapod crustacean species in the Carpathian Basin, with a special focus on Procambarus clarkii, commonly known as the red swamp crayfish.

In Central Europe, including Hungary, there is no longer a habitat where alien species do not occur. The biggest issue is that these non-indigenous species can cause a wide range of changes, from displacing native species to causing damage in agriculture, forestry, nature and the environment, as well as fisheries and water management.

Several non-indigenous species of decapod crustaceans have successfully established and spread in Hungarian waters, while only three native species are present (Astacus astacus, Pontastacus leptodactylus, and Austropotamobius torrentium). The remaining populations of all three species have declined significantly and are now endangered, which is why they are protected by law. In addition to the indigenous decapod species, researchers have recorded the presence of more than 30 non-native species in Hungarian waters – a number that, unfortunately, places Hungary among the world’s leading countries in this respect.

CER - Procambarus clarkii — Photographer: Weipert András

Researchers from the HUN-REN Centre for Ecological Research (HUN-REN CER), in collaboration with international and Hungarian colleagues—with the involvement of university students—are studying the status and impact of decapod crustacean species in the Carpathian Basin, with a special focus on Procambarus clarkii. The aim of the project is to determine the species’ actual distribution, assess its impacts, and develop the most effective control methods for existing populations.

Procambarus clarkii (red swamp crayfish)

is native to the southern United States and northern Mexico. It has been introduced to numerous countries across the Americas, Africa, and Eurasia, becoming one of the most common decapod crustaceans worldwide due to both accidental and deliberate introductions. In Hungary, its emergence and successful establishment are most likely the result of illegal releases by less knowledgeable hobbyists.

To help prevent the further spread of the species, HUN-REN CER asks the public to take a photo if they encounter a decapod crayfish and send it, along with the location where it was found, to weiperth@urbanecology.hu.

Procambarus clarkii prefers slow-flowing and stagnant freshwater bodies such as swamps but can also thrive in periodically drying drainage ditches and canals. This means it is not only found in natural habitats but is also well adapted to urban environments. Research since 2015 has confirmed the mass occurrence of the species in numerous water inflows across Budapest, as well as in inflows and side branches of the central Danube basin across Hungary. The species has also been identified in Demjén, Gyula, Miskolctapolca, and, in the autumn of 2024, in the Péti-Séd basin and Lake Balaton. In the winter of 2024, it was additionally recorded in Lake Velence and one of its inflows. The red swamp crayfish is both territorial and aggressive. It not only competes with native species but also with other non-indigenous crustaceans for food. Moreover, it directly harms the native fauna of the habitats it invades by preying on smaller animals such as dragonflies, crustaceans, fish, and amphibians. By consuming aquatic vegetation, it triggers algal blooms, contributing to a decline in water quality, according to the researchers.

Researchers at HUN-REN have confirmed that populations of Procambarus clarkii in Budapest and its surroundings are active vectors of crayfish plague. In addition to this, the species harbours other pathogens that significantly deplete indigenous decapod populations, as well as fish and amphibians. Recent research has also identified pathogens harmful to fish and amphibians in specimens of the red swamp crayfish. Due to its exceptional substrate-burrowing abilities, it can weaken flood protection embankments, ornamental pond linings, sealed surface and underground sewer networks, and other infrastructure, potentially leading to significant economic damage.

"The red swamp crayfish does not spare the natural or built environment wherever it sets its claws, which is why the reduction and management of its populations in Hungary can only be achieved through the cooperation of multiple sectors," say the researchers.

Both national and international examples show that the public—especially hobbyists—play a crucial role and bear the greatest responsibility in preventing the further spread of Procambarus clarkii. This is supported by the latest research from HUN-REN ecologists, which indicates that Hungary has the highest genetic diversity of the species in Europe. In other words, nearly every population has a distinct origin. "The problems caused by the red swamp crayfish provide a strong argument against introducing any aquarium plant or animal species into our natural waters or private ponds, such as fish ponds or garden ponds," emphasise the HUN-REN CER researchers.