Beautiful beasts, but Broch's right about them.
One of the local countryside rangers was discussing them a few years ago. He said that where mink could, they preferred water, they thrived along the river banks, but if otters came into the same area, the mink were driven onto the land, and on land they were much more vulnerable to other predators. Foxes, hawks, and humans, cars, etc., and their numbers fall.
I think it's the age old issue of human interference, but the otter numbers are certainly growing, and it's reported that the numbers of American mink have been falling for over 25 years now.