Reading magistrates have charged a man after he illegally released more than a dozen goldfish into the River Thames.
Derek May of Reading in Berkshire was given a six-month conditional discharge and charged 60 costs after a member of the public spotted him dumping the fish and contacted the Environment Agency.
May pleaded guilty and was charged under Section 14 of the Wildlife and Countryside Act and Section 30 of the Salmon and Freshwater Fishery Act, which make it illegal to introduce non-native fish into the wild, and to introduce fish without consent.
May told magistrates at Reading Magistrates' Court that he was left with 15 to 20 goldfish that remained brown, and rather than rehoming the fish responsibly, he dumped them into the River Thames at Scours Lane in Reading.
Environment Agency crime officer Dennis Welling said: "Although some people may think this is a harmless act, introducing non-native fish into the wild can have serious implications for our native fish populations.
"They carry disease, out-compete native species, and breed so prolifically that they change the natural balance of the ecosystem.
"Although the humble goldfish has been part of our society for 400 years, it is still a non-native species that should be enjoyed in garden ponds and fish tanks, rather than damaging native fish stocks in our lakes, streams and rivers.
"Where the evidence is available we will support our police colleagues in prosecuting those who fail to protect our environment for future generations."
If May commits a similar offence, May will be sentenced on both the original charge and the new charge.