What's a lamprey?

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Two sea lampreys parasitize a lake trout.

The sea lamprey is a predatory fish that attaches to a  host fish and feeds on the blood and body fluids. A single sea lamprey kills 40 or more pounds of fish in its life as a parasite. It is native to the Atlantic Ocean and existed throughout the St. Lawrence Waterway  and Lake Ontario but was prevented from moving up into the Upper Great Lakes by Niagara Falls. The canal system used to provide boat transport through the eastern states and ultimately, the construction of the Welland Canal in the late 1800s to provide a shipping route bypass around Niagara Falls, allowed sea lampreys to gain access to the Great Lakes. By the mid-1900s sea lampreys had colonized all of the the Upper Great Lakes.

http://www.fws.gov/midwest/fisheries/sea-lamprey.html