@CWDancer Make sure you explore for any opening of 1" , reports show they can squeeze thru those, climb as well as a cat, and dig as well as a dog. 1/2" hardware cloth buried around the perimeter under those rocks would be a good idea. If the steel is inside a fence then they might be able to climb above and enter, if outside do ya think they jump as well? I have witnessed squirrels jump about 3 feet up a tree trunk and bet those minks can too.
Do set some traps at night to see if you can catch more - of course ones that cannot harm kitty.
Congrats to the great hunter CAT - could I borrow her? I have an excess of squirrels and chipmunks I would love to lose.
Good luck and hope the mink issue doesn't pose any loses, and the Cat keeps hunting.