Invasive silver carp have invaded nearly every part of the Mississippi River already, devouring plankton and leaving native fish like walleye, perch and whitefish to go hungry.

That’s what brings scientist Jim Lamer and his crew out on an electrofishing boat on the Illinois River on a warm summer day. There are more silver carp here than just about anywhere in the world. Lamer runs the Illinois River Biological Station in Havana, Ill., which is about 200 miles south of Chicago, just north of where the river meets the Mississippi near St. Louis.

“They’re voracious eaters,” said Lamer, “and they’re eating the base of the food web.”