Today the St. Marys River produces more parasitic Sea Lampreys than all Great Lakes tributaries combined. Sea lampreys attach to fish and feed on the fish's body fluids.
Some fish survive a sea lamprey attack, but often with life- threatening wound s.
Prior to the mid-1970s, the St. Marys River had been an inhospitable place for sea lampreys to live and reproduce. Water quality and habitat improvements during the past couple of decades have turned the river into a producer of hundreds of thousands of sea lampreys annually. The river's tremendous size and flow volume prohibit effective sea lamprey control using conventional methods. Sea lampreys produced in the St. Marys River migrate into Lake Huron and northern Lake Michigan. Sea lampreys attach to fish with a suction cup mouth and rasp through the fish's scales and skin with a sharp tongue.