Nutrias, also known as beaver rats, are an invasive species in Germany and the European Union. The rodents, which originally come from South America, have no natural enemies here, which means that they can multiply rapidly. In many bodies of water in Germany, the nutria population has grown to such an extent that the animals are causing major damage and endangering native animal and plant species.
Beaver rats have become established at the Rheinauensee in Bonn in recent years. Numerous animals also live in the Rheinaue floodplain in a very small area - around 60 animals were counted in a survey conducted by the Lower Nature Conservation Authority in 2021. The rodents find optimal conditions in the Rheinaue floodplain: The winters are mild, they have no natural enemies and they always find enough food - especially because some people regularly distribute food despite the explicit ban. On average, nutria give birth to two litters of usually five young each year. In the Rheinaue floodplain, however, the Lower Nature Conservation Authority has already counted three reproductive cycles with more than five young per litter for some individuals.
Damage to the ecosystem - danger for the newly restored Rhine floodplain lake
A particular problem in the Rheinaue is that the nutrias eat away the useful aquatic plants in the Rheinauensee. The so-called macroalgae were specially planted as part of the complex and cost-intensive lake restoration and play an important role in the stability of the sensitive ecosystem and in cleaning the water. The large nutria population therefore jeopardizes the success of the lake restoration and thus the habitat for numerous animal and plant species. The population of the protected pond mussel, which is also on the rodents' food list, has already been decimated considerably. In addition, the nutria are destroying the bank reeds, which are important for native biodiversity. These will be introduced into the Rheinauensee in May this year.
The rodents also eat away tree bark, which can lead to the death of individual trees. Nutrias dig through the banks to build their burrows and cause a great deal of damage. If the animals continue to multiply, there is a risk that they will migrate and spread to sensitive nature conservation areas such as the Siegaue or endanger flood protection in other parts of the city.
Documented damage caused by Nutrias prior to the restoration of the Rheinauensee.
Hunters should keep the nutria population under control
In view of the impending ecological consequences, the city of Bonn, like other local authorities, is obliged to contain the large nutria population and prevent the animals from spreading further. This is stipulated by the European directive on invasive species (Art. 19 of EU Regulation 1143/2014) and the Federal Nature Conservation Act (§40).
In addition to the feeding ban, the use of hunters should also help to sustainably contain the nutria population in the Rheinaue floodplain. According to the State Agency for Nature, Environment and Consumer Protection (LANUV) and various experts with whom the Lower Nature Conservation Authority of the City of Bonn is in constant contact, hunting nutria is the only effective method that will lead to success and thus prevent further damage.
The city has commissioned experienced and qualified hunters for the operation in the Rheinauenpark. In accordance with the requirements of the Animal Protection Act and the Federal Hunting Act, the nutria are caught in live traps that immediately alert the authorized hunters by means of a sensor. In future, the animals will be transferred from the trap to an interception basket and then shot directly on site. The hunters fire a controlled "trap shot" towards the ground - all hunting safety rules are taken into account. The animals are thus spared stressful transportation in the trap basket.
To avoid disturbing visitors to the Rheinaue Park, the hunters are only deployed during periods when the Rheinaue is not very busy.
Education and enforcement of the feeding ban
The city is also continuing to focus on education and enforcing the feeding ban, which has been in place throughout the city since 2019. New signs have been put up at the Rheinauensee informing people about the negative consequences of feeding and the problems caused by invasive species.
The Lower Nature Conservation Authority urgently appeals to all citizens to adhere to the ban and not to feed the nutrias: "Feeding may be well-intentioned, but it doesn't make sense in terms of animal welfare. The often one-sided food supply, such as bread, can also lead to vitamin deficiency in wild animals and thus to diseases. Some citizens come to the Rheinaue every day to feed the animals. As a result, the animals become accustomed to the presence of humans. The population continues to grow and the damage to the ecosystem intensifies."
The city's public order service monitors the feeding ban. Anyone who feeds waterfowl or nutria must expect a warning fine of 35 euros the first time if small amounts of food are distributed. If feeding is repeated or larger quantities of food are distributed, the warning or fine can be up to 1,000 euros.
Monitoring starts in March
From March, the city has commissioned the expert office "Lanaplan", which has already drawn up the remediation concept for the Rheinauensee, to carry out extensive monitoring. The water experts will record the current nutria population and also document any new damage caused after the lake restoration. The results will be used to evaluate the effectiveness of the measures taken so far and determine the next steps. The aim is to significantly reduce the nutria population and control it in the long term so that the lake can develop well after the restoration.