For the first time in 40 years otters are thriving over American mink along river banks, new figures show.
Otters have declined in Britain since the 1960s and 1970s
Mink are effective, aggressive hunters which have thrived in the British countryside since they were introduced from the United States by fur farmers and were later released into the wild when the fur trade declined.
By the mid-1990s there were thought to be about 100,000 of them across the country.
Meanwhile, during the 1960s and 1970s the number of otters crashed as the mammals fell victim to industrial waste and chemicals polluting rivers.
Zoologists at the University of Oxford have now found the reintroduction of otters along river banks has caused mink numbers to drop.
In some rare cases otters have even killed mink in direct competition.
The survival of the otters may be one indicator of improving water conditions.
One project carried by the university's Wildlife Conservation Research Unit in the upper Thames area - including rivers such as the Windrush - tracked mink numbers after otters were reintroduced there in 1999.
Almost immediately mink numbers fell by 75% and since then have only recovered slightly.
Similar results were found in south west England.