A large school of pseudorasbora parva, known variously as the topmouth gudgeon or stone moroko, was discovered in a stream near Aarhus by a team of researchers from natural history museum Statens Naturhistoriske Museum.
The benign looking fish rarely grows to more than 10 centimetres long, but they are ferocious and often deadly to other, much larger, fish.
“They attack again and again and sever the backbone of their victims,” Henrik Carl, leader of the museum’s fisheries project, told Politiken newspaper. “We had a six centimetre long one in an aquarium and it killed two 12 centimetre long carp in less than a day.”
The topmouth gudgeon is also a member of the carp family and females spawn up to 14 times a season. The rapid growth in the population endangers the natural balance of streams and lakes.
"There are places in Greece where schools of this fish have almost wiped out all of the other species,” said Carl. “It is considered an invasive species throughout Europe.”
The fish originally comes from Asia and was first introduced into European waters in Romania sometime in the 1960s. It has also been sited in northern Jutland. Carl didn’t think the most recent school hailed from either of those sources.
“It is more likely they originated in someone’s pond,” he said. “They are sold as ornamental fish at garden centers all over the country.”
Carl said the fish could do a “troubling” amount of damage to Danish waters.
“Even though they are pretty, their temperament makes them an unsuitable as a garden pond fish and they are unwelcome in our environment.”