An aggressive fish more suited to tropical waters has been discovered in the River Witham near Lincoln.
The fisherman sent his photo to the Angler's Mail
The Environment Agency has confirmed a photo sent to them by angler Andy Alder was a giant snakehead.
It is believed the snakehead, which is native to Africa and Asia, would not survive long in the UK.
The fish is a predator and can grow up to one metre (3ft) long, but the agency said the cold conditions would probably prove fatal to it.
Mr Alder, who was not sure what type of fish it was, took it home but disposed of it after a few days.
Angler's Mail editor Tim Knight said: "Strange things do get caught in this country - as sometimes people dump them out of their fish tanks."
The snakehead eats other fish and frogs and can survive out of water for up to three days.
Rita Penman, of the Environment Agency's Anglian region, said: "It may have been smuggled into the country for an aquarium and then released.
"We are taking it seriously and would urge people not to put non-native species into our waters - because of the risk of spreading disease.
"If in doubt, please contact Environment Agency."
She said they were contacted a week ago about a two-foot long giant snakehead - and confirmed the photo showed the channa miscropeltes species.
Mrs Penman said it is not absolutely certain the fish was caught in the river as they had only the photo to go by.
The fish needs to live in water between 25C-28C to survive, but some other snakehead species are adaptable to colder conditions.
The fish cannot be legally imported into the UK.