Invasive fish pose irreversible threat to Lithuania’s ecosystems

A scientist has warned that irresponsibly released invasive fish species can cause irreversible damage to Lithuania’s natural water bodies, LRT.lt reports.

Dr. Justas Dainys of the Nature Research Centre emphasised that non-native species, particularly prohibited invasive ones, can devastate ecosystems when released from private ponds into natural waters.

“People might find it interesting to get an unusual fish, like a small catfish—it seems fun, but eventually, that little catfish grows into a demon that eats everything around it,” Dainys said.

Invasive fish enter Lithuania’s waters solely due to human error. The black bullhead catfish, listed in the EU’s 2022 alert list of concerning invasive alien species, was already observed dominating a quarry pond near Klaipėda years ago. Recently, a Facebook group discussion emerged about eliminating a fish resembling the black bullhead—also known as the North American catfish—from a private pond. Some suggested catching and releasing it into the wild.

Dainys strongly advises against this, as the environmental harm could be irreversible. He noted that cases of unusual fish overrunning private water bodies are not rare, with humans often to blame for not researching the species they purchase.

One problematic species is the topmouth gudgeon, or rotan, which has spread from private ponds and aquariums into natural waters. It thrives in small bodies of water with few predators, often consuming all other fish, invertebrates, and even frogs, leaving little to no life behind except its own population, which eventually resorts to cannibalism.

When asked how to control such fish in private ponds, Dainys suggests introducing predatory species like pike, which may help reduce numbers but won’t eliminate them entirely. The best solution, he stresses, is careful consideration before purchasing any fish.

“How to eradicate them completely? I don’t have a cure. Probably no one does,” he said.

Dainys highlighted that people often buy exotic fish only to later regret it and release them into natural waters. He recalled an incident where guppies, typically aquarium fish, were found in the Neris River. Investigations traced them back to a person who had raised them in an aquarium before releasing them. While guppies have little chance of surviving in the wild, their potential impact on the ecosystem would have been severe.

He also mentioned cases of piranha-like fish being caught near Kėdainiai and in Lampėdžiai Lake in Kaunas. The accidentally introduced round goby in the Baltic Sea has already become a major ecological problem, contributing to the decline of waterbirds such as ducks by depleting mussel populations, whose biomass near the coast has dropped tenfold.