Dozens of gold-coloured fish have been spotted swimming in the River Frome.
The shoal of fish are currently located at the weir next to the footbridge at Crowther's Avenue, Yate.
River and coastal engineering specialist, David Gaskin, confirmed the species as goldfish, normally kept as household pets.
Section 14 of the Wildlife and Countryside Act 1981 makes it illegal for anyone to introduce non-native fish into the wild without prior consent.
David said:
"Depositing non-native fish into a river is highly illegal and can lead to prosecution if a person is caught.
"It is a biosecurity risk, goldfish can carry infections such as KHV (koi herpes virus) and that has up to a 100% mortality rate in other members of the cyprinid family in the river.
"Basically non-native fish can upset the natural eco-system of the river and cause major problems.
"Also, due to their inconspicuous colour, a goldfish is more likely to be preyed upon in a river, as they are an easy target for predators such as pike, cormorants and grebes"
"Carelessly putting goldfish in a river could be quite consequential."
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