An adult specimen will attach itself to a larger fish and suck the blood and body fluids in a parasitic fashion. The one I saw had a round sucker mouth with circular rings of teeth. It looks somewhat like an eel, but had a dorsal fin, blackish brown body and grayish underbody. Lampreys either kill the host fish by draining it of fluids like a vampire, or leaving a wound that becomes infected.
The adults spend most of their time in the Great Lakes, but return to tributaries to spawn. They are thought by some to invade the lakes via Lake Huron and Erie through man made canals to the sea. Some experts believe the Great Lakes lampreys have been in the area since the Pleistocene Era, and are actually a native species.
Whether native or invasive, there is no argument that they are destructive to the basin. According to the Indiana DNR (Aquatic Invasive Species), U.S. and Canada are spending $8 million per year on control of the sea lamprey and another $12 million per year in the restoration of lake trout populations that were destroyed by lampreys. Most control methods are too costly to be effective in such a large area. These include barriers to keep the lamprey from returning to tributaries to spawn. Sterilization of male lampreys involves collection, sterilization and the return to spawning streams to compete with fertile males. This method reduces fertilized egg counts. Lampricide applied to larvae can be effective. Usually it takes a combination of techniques to control the population.
I hope I never catch another one.
Wound picture copied with permission from: http://biology.usgs.gov/s+t/SNT/noframe/gl129f04.htm