A small crocodile was among the exotic inhabitants found in the UK's waterways, a survey has revealed.
There was an unconfirmed crocodile sighting in Gloucestershire
Seals, dolphins and porpoise were also spotted, according to British Waterways' survey of the UK's 2,000 miles of canals and rivers.
But mallards, swans, moorhens and herons were the most common of the 60 species spotted in 3,700 sightings.
Rare wildlife seen included the endangered water vole, otter and the rarely seen kingfisher.
The crocodile was seen in Stroud water canal in Gloucestershire, but a waterways spokeswoman stressed it was an unconfirmed sighting.
TOP 10 WILDLIFE SIGHTINGS
Water vole 122
Grass snake 93
Source: British Waterways
"It could be someone's exotic pet, but we'll see if there are any more sightings."
The seals were seen in a sea lock in Scotland, and while British Waterways was aware of the occurrence in such places, it was rare for members of the public to spot them.
One in eight people reported the presence of mallards and swans, while one in 20 people observed kingfishers and water voles - the same proportion that spotted the non-native mink.
Other non-native species spotted included terrapins and Chinese mitten crabs.
"Some amazing wildlife has been spotted on our waterways and we want to encourage more people to visit our canals and rivers and enjoy the rich variety of animals and plants," British Waterways national ecologist Jonathan Brickland said.
"Whilst we are surprised to find a crocodile and osprey listed we are not ruling these out until further investigations take place."
He added that the survey would help the organisations "enhance the environment and maintain waterways that are rich in wildlife for all."
The survey involved sightings between May and October.