The number that has Great Lakes agencies moving is 80. That is how many river miles separate Lake Michigan from the Marseilles Pool of the Illinois River, where Illinois Department of Natural Resources crews removed several hundred juvenile silver carp during routine monitoring last Thursday and Friday.
Adults have been caught that far north before. Juveniles have not, and that distinction is the whole story. Silver carp lay their eggs upstream and let them drift, so four-to-six-inch fish in the Marseilles Pool point to a spawning event somewhere above it — closer still to the lake.
The IDNR pointed to this summer's conditions in explaining how it happened. "Flood pulses and water temperatures this summer created conditions that have been conducive to carp spawning," the department said. "Crews are now ramping up additional monitoring in the upper river and tributaries to gather more information that will allow them to assess the scope and significance of the discovery."
Biologists call a carp population established when multiple life stages share the same water. By that test, the lower 240 miles of the Illinois, from Peoria down to the Mississippi, has been carp country for years. Above Peoria, the juveniles were the missing piece. Last week they turned up.
The response is already underway: intensive electrofishing in juvenile habitat, extra surveys in the Dresden Island Pool some 50 miles from the lake, microchemistry work on the captured fish to trace their origin, and contract commercial fishing for adults resuming in September once the water cools enough to make it worthwhile.
What has turned a monitoring result into a political fight is what happened three weeks earlier. In late July the Army Corps of Engineers issued a stop work order on the Brandon Road Interbasin Project at Joliet — the electric barrier, bubble curtain and engineered channel meant to stop carp entering Lake Michigan via the Des Plaines. The Corps cited an administrative review.
Joel Brammeier of the Alliance for the Great Lakes wants the shovels back in the ground.
"Every day we don't have shovels in the ground at the Brandon Road project puts us a day closer to seeing invasive carp cause irreversible harm to the Great Lakes ecosystem and economy that our region depends on," said Brammeier, the group's president and CEO. "With carp spawning in the upper river, we don't have any more time for administrative delay. The Army Corps must allow the construction teams to get back to work now — otherwise, years of bipartisan collaboration and investment to stop the carp will have been for nothing."
Illinois Governor JB Pritzker, already at odds with Washington over the project's funding, put it more sharply when the pause was announced.
"It's clear the Trump administration is literally attacking fresh water, clean water, drinking water for the Great Lakes," Pritzker said.
Strip out the politics and the angling stake is simple enough. Karen Murchie, director of Fresh Water Research at Shedd Aquarium, described what these fish do to a system.
"These invasive carp are eating machines," Murchie said. "Two of the species are plankton eaters, and they will outcompete some of our native fish, (including) our walleye, perch, and lake whitefish."
Bighead, silver, black and grass carp all arrived in the southern United States in the 1970s as a biological fix for algae, weeds and parasites in aquaculture and wastewater ponds. Flooding in the 1990s put them in the Mississippi, and they have been heading north since.
There is a case for calm. The leading edge of adult carp has held at roughly 50 miles from Lake Michigan, and the multi-agency protocols that trigger on a detection like this one are functioning as designed. IDNR, the Illinois Natural History Survey, the U.S. Fish and Wildlife Service, the Army Corps and the U.S. Geological Survey are all now working the upper river. The open question is whether the permanent barrier those protocols were meant to buy time for ever gets finished.