Minks are really effective hunters. I think I remember reading about some anti-fur activists releasing hundreds of minks from a fur factory, and they caused havoc in the area. May have been in the UK. It was a while back.
The minks and fishers around where I live (Cayuga Lake in New York) are native. The bigger issue with wildlife around here is invasive species, especially plants. So much privet, bush honeysuckle, buckthorn, and multiflora rose. And many others.
The thing about predators, is that they tend to be spread thin. It’s the way they’re built. They are too competitive to form herds. Packs/prides, yes. Herds, no.
Having hundreds in one area is insane (unless it's for special occasions, like the Baja hammerhead convention). You only see that in video games and horror movies; not in nature.