A male otter will often have a linear territory along a waterway of about 40km
There have been confirmed sightings of an otter at Attenborough Nature Reserve in Nottinghamshire.
Photos of the mammal swimming in one of the reserve's ponds were captured by local naturalist Alan Knightly.
Tim Sexton, from the reserve, said: "It's really exciting. We've only ever found footprints and spraints before."
According to Erin McDaid, of the Nottinghamshire Wildlife Trust, these may be the first ever pictures of an otter taken in the county.
The creature has been seen on a number of occasions, both in and out of the water, including crossing the footpath on the reserve from the Main Pond to the Delta Pond.
A holt was constructed by rangers at the reserve in 2008 to try and encourage one of the mammals to make the reserve their home.
Since that time there have been many false sightings which have turned out to be the non-native mink.
But things are looking up for the otter with the
Environment Agency stating that it has made a remarkable comeback
from the brink of extinction and a British Waterways survey finding that sightings are up 194% this year on last.
"They are becoming more common as the waterways are becoming cleaner. We're protecting areas at nature reserves for the otters and giving them a better chance of survival," said Mr Sexton.