The parasites you are talking about are a species of leech. Currently there is a student at our office working on identifying the species and its interactions with the fish they attach to. I see these leeches attached to other species as well including perch, rockbass, etc. The fish that i've seen with the leeches don't seem to be any worse for the wear.
I haven't seen too many on the walleyes in our nets, although i did catch an eye today near metro that had quite a few on its tail, pec and pelvic fins.
Hopefully, i/we will know more soon about these little critters. I will be sure to post again when i have more info.
Jeremy