A UK mammal facing extinction is thriving two years after being reintroduced to a Hertfordshire river.
Due to habitat loss, they had last been seen there in 1987.
The animals' occupancy in the Ver Valley has, on average, more than doubled each year, the trust said.
The trust said water voles were once abundant in rivers throughout Hertfordshire but populations declined by over 90% in the last 50 years due to habitat loss and because they are being preyed upon by American Mink, an invasive, non-native species
It added the voles are the fastest declining mammal in the country and face extinction.
The voles were reintroduced to a stretch of the River Ver after extensive restoration work to improve the area.
Since the release, the trust's water vole officer, Josh Kalms, has headed up a team of volunteers who have been trained to look for signs of water vole occupation, including latrines and feeding sites.
The first data collected in 2022 reported a 238% increase in range along the river and its watercourses, while this year's findings show the occupancy in the Ver Valley, has, on average, more than doubled each year.
Mr Kalms said water voles are a "key species" for maintaining "healthy wetland ecosystems" and it was "wonderful" to see how well they are doing on the River Ver.
"They are mini ecosystem engineers with their burrowing and feeding helping our river banks and wetlands stay in good condition," he said.
"Water voles are now occupying almost all of the River Ver between St Albans and Redbourn, with some travelling an additional 4.8km upstream.
"This year we counted more water vole field signs than the previous year and I'm thrilled to be able to share this amazing success story."
The trust added it is aiming to have water voles back in every river in Hertfordshire by 2030, subject to funding and suitable locations.