Shoals of marauding goldfish are starving out all rivals in the ponds and lakes of Munich, officials say.

The former pets are being released from private ponds into the city’s public waters by their owners, the council has said.

It warns that, while they are innocuous pets, goldfish are highly successful breeders which eat everything in sight, depriving the other pond dwellers.

This includes small life forms in the pond, and other fishes’ eggs.

The problem has got so bad, Munich Council has promised to fine (in German) anyone caught releasing their pets this way.

“The expulsion of ornamental fish and other purchased animals from the garden pond into the free water landscape is therefore prohibited by law and can be penalised with fines,” a statement released on its website warns, adding that if a person’s pond becomes overcrowded “the animal owner is responsible for finding a new home for his ornamental fish”.

Munich is not the only place to suffer from marauding goldfish. Just last week, a man in the Netherlands was ordered to recapture 80 goldfish he had released into canals in Veenendaal over fears of what they may do to the native species.

And in Australia, the problem of how to control the large population of goldfish which had been released and were now living in the Vasse River spawned a year-long study by researchers at Murdoch University.

The study found the former pets not only cover huge distances, but can also grow to large sizes – one goldfish found by researchers in the Vasse River weighed 1.9kg (4.1lbs).

More concerning, goldfish can also harm water quality, introduce disease, disturb habitats and compete with native species, threatening their survival.

Researcher Dr Stephen Beatty told the BBC: “Once established, self-sustaining populations of alien freshwater fishes often thrive and can spread into new regions, which is having a fundamental ecological impact and are major drivers of the decline of aquatic fauna,”