The otter has made a remarkable comeback from the brink of extinction, the Environment Agency has said.
Otters almost disappeared from England in the 1970s after pesticides routinely used three decades ago brought their numbers to near extinction levels.
Now many of those chemicals have been banned and the creatures are present once again in rivers across England.
In many watercourses in the south-west and along the River Wye otter numbers are at maximum capacity.
Their numbers are being limited not by pollution but their own territorial behaviour.
That recovery is rapidly being matched elsewhere, and otters are now found in every English county except Kent.
There are also healthy populations in Northumbria, Cumbria, Wessex and the Upper Severn.
BBC rural affairs correspondent Jeremy Cooke says it is a remarkable tale of survival that is encouraging news for conservationists.
Paul Raven, head of conservation and ecology at the Environment Agency, said: "The otter is at the top of the food chain, and as such is an important indicator of the health of English rivers.
"The recovery of otters from near-extinction shows how far we've come in controlling pollution and improving water quality."
The agency predicts the species will fully recover in numbers across England in less than 20 years.