Occasionally fish are introduced into waters far from there native home, and they don't manage to spread as well as others. Snake head and sunfish are invasive, and therefore don't fi this category. In a small quantity of CT ponds an alien lurks: the rudd, a European species. The are invasive in some areas, but fail to spread from place to place around here. In fact, they are rarely seen or caught. I caught my first in a small pond near where I live, and I had no clue what it was. I remember my father catching a similar fish in Day Pond years ago. They are rare and interesting. I like that! I was lucky to find some schools in the same pond described above. I managed to get one to eat a little hair's ear nymph.