I keep koi; the picture above is a few of my fish. Koi are a domesticated version of the common carp, Cyprinus carpio, which is a widespread invasive species. There are common carp in almost every lake and river in Wisconsin, and they cause significant problems in some areas due to their habit of stirring up the mud to look for food. Common carp are actually pretty good eating, although most people in the US consider them trash fish.
But there's a far more evil side to carp. They can be killers. A species called the silver carp has the bad habit of leaping out of the water when boats scare them. For most types of fish that wouldn't be a problem, but silver carp can grow to over forty pounds and jump ten feet high! Getting slammed by one when you're cruising at speed can be lethal. I don't know much about the first picture below, but the women in the second picture ended up with a broken jaw.
Silver carp are one of several large introduced species collectively referred to as Asian carp. Far more important than their attempts to assassinate fisherman is the damage they can do to our native aquatic ecosystems. Asian carp have spread throughout the Mississippi River basin and are now on the verge of getting onto the Great Lakes. The only thing keeping them out is an electric barrier on the Chicago Sanitary and Ship Canal. If that barrier fails, and it inevitably will, Asian carp will get into almost every major lake and river in the Eastern US.
