From what I’ve read, from various studies, releasing Beavers is a bit counter productive and arguably cruel, if there is a healthy local Otter and Fox population, = very soon no Beavers. (not discounting raptor predators- Eagles, Buzzards etc)
This is why they soon inevitably disappear.
The river system I’ve been a syndicate member in for many years, used to be plagued with Mink. Rarely didn’t see at least one on most trips, sometimes you would see a Sow with four to six kits in full view. About 8 years ago they reintroduced 8 pairs of Otter over the whole river system, within a few months, you never saw another Mink and haven’t done since.
It’s Well documented that the Otters have now virtually eliminated Mink in most areas. We still see the Otters occasionally and quite close up at dusk and there are often their tracks in the bankside silt. Foxes are also regular bankside companions.