Can piranhas survive in North American lakes?

From hobby to nuisance: imported exotic species

The Erft is home to tropical flora and fauna

Even in the relatively warm water of the Erft, a small river west of Cologne, exotic species are observed again and again. In the past few years, several small and larger piranhas have attracted particular attention, although they have never been able to survive for long.

The now established foreign animal species - called neozoa - include the subtropical annelid worm and the migrating mussel.

The Erft also offers a suitable home for aquatic plants from foreign countries and continents. For example, the American waterweed and the tropical duckweed were able to adapt to our climate over many years.

One concern now is that many introduced plants - called neophytes - could use the Erft as a starting point for further spreading to other rivers or lakes.

Dominant aquatic plants

Many aquatic plants introduced from Asia, North and Central America can now survive here because our winters are becoming milder.

The introduced species often have no natural enemies in rivers and lakes and can spread rapidly. In the case of aquatic plants, this can go so far that the species spectrum is completely turned upside down.

For example, divers have found that only three species of introduced aquatic plants - two species of waterweed and the conifer - can dominate large areas of lakes. It takes many generations before these plants are included in the ecological cycle and serve as food for the native animals.

Until then, they just take up space and threaten our nature. Even the smallest bits of a plant can cause it to spread in a body of water. The conifer does this particularly well because it can thrive both above and below water.

New species threaten native animals

Some of the aquarium fish species that used to be particularly popular, such as the sunfish and the dwarf catfish, have now established themselves in our lakes. In the F├╝hlinger See near Cologne there are already whole schools of these perches that are spreading ever further.

The same applies to the slotted turtles, which are colorful and cute and sold as young animals for aquariums. However, these turtles can grow to be up to 30 centimeters tall and sometimes even aggressive.

Stable populations of these reptiles are increasingly encountered in local lakes. They threaten the few remaining populations of the critically endangered European pond turtle.

However, one of the most threatening immigrants is the North American bullfrog. It becomes so big and voracious that it has become a threat to the rare native amphibians in many parts of Germany - on the one hand as a food competitor and on the other hand as a predator.

When fully grown, the bullfrog is an omnivore, which not only eats amphibians, frogs and fish but also freshly hatched ducklings, mice and snakes.

Better to give away than to suspend

If you want to get rid of your animals and plants, you have numerous other options. Buyers can often be found on the Internet, for example, who have a place for the exotic ones.

Caution is required, especially with aquatic plants from foreign countries. Under no circumstances should you use them in your garden pond. It is all too easy for waterfowl that visit the pond to get small stalk sections caught in the plumage and be carried into other bodies of water.

For this reason, you should not move plants from ponds that contain introduced species into foreign ponds and bodies of water. They belong in the trash (if it is incinerated) so that they don't spread any further. Even with tropical aquatic plants, you should be on the safe side.