Packman, I beleive you are thinking of sea lampreys, which have infested the great lakes and are partially responsible for a decline in the lake trout fishery there. If you do a little research you can find out a ton of info about that situation.
However there are several species of lamprey that are native to the inland lakes and streams of north america, I know of three species where I live in Iowa. The only one I've ever found was dead in a trout stream, but I've never caught a fish with a lamprey attached. Also, not all species of lamprey are parasitic, some of them are scavengers. They may or may not kill the fish as far as i know, but the ones that do survive will have a scar in their flesh. Over all they probably don't hurt fishing at all. BTW lapreys are primitive fish, not leeches. They have no jaw bone but they use their mouth as a suction cup to attach themselves and then they use their toothy tongue to rip bits of food off their hosts. Kinda gross i know but hey, who said mother nature was clean? people have this image of nature as this clean, well oiled machine, but nature is actually more like a bloody chaos.
PS cool picture