This story is a partnership between Grist and WBEZ, a public radio station serving the Chicago metropolitan region.

The silver carp is big, unwieldy, and requires Joe Greendyk to use both his hands to measure it before tossing the fish overboard into the Illinois River. The nearly 2-foot-long invasive fish, which now overruns the river, has become the centerpiece of a state-run monitoring program to rein in its exploding numbers.

“They’re pretty slimy and pretty strong,” Greendyk, a seasonal fisheries technician with the Illinois Natural History Survey, or INHS, said with his hands caked in fish slime. “So if you don’t grip them right, they’re pretty hard to control.”

For decades, local, state, and federal officials have worried that the voracious filter feeder, which ca

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