R.W.S. They eat snails (slugs) small rodents....The balance of nature. If you were starting an aquatic project, with small fish, they would be of concern if allowed to venture in that water.
When I was young, my Romanian grandfather and I found one near the well. After I was allowed to hold it, he told me to let it go, that they were good. Grandpa understood the ground and worked it all his life. So, in respect to the great knowledge of my grandfather, who had no formal education, I would let them go....But that's just me. They can clear out a new litter of mice in no time. Mrs. R.W.S. might start naming them....