A reader pushed back on our invasive carp piece with the argument that gets made every time the subject comes up: Lake Michigan is a plankton desert, silver carp are plankton eaters, so the whole thing is a billion dollars spent on a fish that would starve anyway.
We went and read the modelling. He is half right — and the half he is right about is the half most coverage of this gets wrong, including our first pass at it.
The trap: answering a silver carp question with bighead carp data
Bighead carp and silver carp get bundled together as "bigheaded carp" in headlines, in press releases, and in a fair amount of the coverage. They are not interchangeable here. The most-quoted finding from the 2019 Lake Michigan modelling — that suitable habitat exists across the whole lake at certain times of year — is a bighead result. It does not transfer.
Here is what the same study, led by Peter Alsip with NOAA's Great Lakes Environmental Research Laboratory and the University of Michigan, said about silver carp, verbatim from the abstract:
"For silver carp, these considerations extended suitable habitat within Green Bay and in some tributary-influenced nearshore areas, but offshore areas remained predominantly unsuitable in all feeding scenarios."
All feeding scenarios. The model ran six of them, letting the fish eat phytoplankton, zooplankton and mussel-derived detritus anywhere in the water column. Silver carp still could not make a living offshore. So on the open lake, the reader is simply correct, and the research agrees with him.
The counterintuitive part: silver carp are the harder case
The natural assumption is that silver carp should cope better with a starved lake than bighead, because they feed lower on the food web. Their gill rakers are fused into a sponge-like filtering pad rather than the comb of unfused rakers a bighead has, with two to three times the filtering surface relative to respiratory surface. They are built to strain finer particles out of more water.
Every Lake Michigan bioenergetics study finds the opposite. Bighead carp are modelled to grow faster; silver carp are the species that struggles. Anderson and colleagues put silver carp's modelled annual weight change in Lake Michigan somewhere between a 9 percent loss and a 23 percent gain, against 68 to 177 percent for bighead. Cooke and Hill found silver carp's daily maintenance requirement slightly higher than bighead's at the same temperature.
The Alsip authors are careful about why, and we will be too: they note the gap "probably reflect[s] differences in energy density and mass of the specific fishes we used in our model," while adding that where the two species coexist in the wild, bighead do grow faster — so the direction of the result matches the field. Either way, nobody should be arguing silver carp are better positioned to survive oligotrophy. No Great Lakes study supports that.
The plankton desert is real, and the numbers are worse than most people think
Offshore phytoplankton abundance and productivity fell 66 to 87 percent between the mid-1980s and the late 2000s. The spring diatom bloom did not shrink; it disappeared. Mid-stratification primary production dropped 22 percent. Mysis, the deepwater shrimp near the base of the offshore food web, fell 70 percent at the 110-meter station.
Trillions of quagga mussels did that. If you have wondered why the water off Milwaukee looks like a swimming pool now, that is the answer, and it is the same reason a filter-feeding fish would go hungry out there.
There is one wrinkle the reader would not expect. The mussels are also food — bigheaded carp will eat mussel biodeposits, the fecal pellets and pseudofeces that settle out beneath a mussel bed. Letting the model use detritus and the full water column roughly doubled the suitable habitat available to silver carp. It just doubled it from very little, in a few specific places, and none of them offshore.
Where the reader's argument stops working
Establishment never required the open lake.
The follow-up study, in Biological Invasions in 2020, put silver carp habitat as "completely absent in the spring and confined to Green Bay in June–October" under present-day conditions. That is a real answer, not a null one. Green Bay is 1,600 square miles of warm, shallow, river-fed, nutrient-loaded water that behaves nothing like the lake it opens into. Add Michigan's drowned river mouths and the tributary plumes and you have the entire habitat picture for silver carp in Lake Michigan.
That is a modelling result, so the obvious objection is that models are not fish. But a 2024 survey went out and sampled nearshore zooplankton communities directly, and found growth-supporting conditions "restricted to a handful of sites in Green Bay and Michigan drowned river mouths" — the same short list, arrived at from field data rather than simulation.
The same 2020 study found phosphorus loading was the single most influential driver of the lake's suitability, and estimated that under 1980s conditions — more phosphorus, no mussels — the fish could have grown an additional 8 to 40 percent a year. Warming shifts it back, but unevenly and not much for this species: in the warm scenario bighead gained up to 32 additional growing days a year. Silver carp gained two to four.
Surviving is not establishing
A fish that can feed somewhere has not established there. Establishment needs successful spawning, and silver carp spawn in current — the eggs are semi-buoyant and have to stay suspended long enough to hatch, or they settle out and die.
The old assumption was that this took a very long river, more than 100 kilometers, which ruled out most Great Lakes tributaries. USGS work revised it down hard after measuring how fast the eggs actually sink: reaches as short as 25 kilometers, about 16 miles, may give eggs enough time. Four tributaries were flagged as having suitable spawning and egg-transport conditions — the Milwaukee and St. Joseph on Lake Michigan, the Maumee and Sandusky on Lake Erie.
Worth being straight about a limit in that work: it treats "Asian carps" as a group and does not analyze silver carp separately. There is no silver-carp-specific Great Lakes tributary spawning assessment. What we can say is that the rivers flagged as spawning-capable are the same water the habitat models rate as the best feeding, and that overlap — not the open lake — is the actual case for concern.
How many fish would it take?
A 2020 study modelled establishment in Saginaw Bay on Lake Huron, a productive embayment that behaves much like Green Bay, and this one did separate the species. For silver carp: under optimistic assumptions about first-year survival, as few as ten founding fish gave roughly a 10 percent chance of establishment, and a hundred or more made it close to certain. Under pessimistic survival assumptions it took more than 100,000. Equilibrium biomass, where they did establish, ran 230 to 320 kilograms per hectare.
That spread from ten to a hundred thousand is not hedging. It is the honest state of the science, and it is why credible forecasts of this invasion disagree so violently.
Two results from that study that nobody promotes: modelled walleye actually benefited in some scenarios, and round goby abundance collapsed where silver carp survival was high. We are not going to leave out the walleye finding because it is inconvenient to the alarm.
And the strongest available support for the reader's position is real peer-reviewed work: a 2021 food-web analysis projected low or negative population growth for bigheaded carp in the oligotrophic main basins of Lakes Michigan and Huron, ending at 0.5 to 1.1 times the introduced biomass, with negligible effects on most food-web groups. That study does not separate silver from bighead — but it is the honest counterweight, and it points the reader's way.
What changed this August
On August 16 and 17, Illinois DNR crews found several hundred juvenile silver carp, four to six inches long, in the Marseilles Pool of the Illinois River — about 80 miles from Lake Michigan. Adults had been found that far upriver before. Fish that young had not, in sixteen years of monitoring. It implies a spawning event in the upper river or one of its tributaries, attributed to summer flood pulses and water temperature.
The timing was awkward. Construction on the Brandon Road Interbasin Project — roughly a billion dollars of layered deterrents at a lock and dam near Joliet, cost-shared by the federal government, Illinois and Michigan — had been halted by an Army Corps stop-work order in late July. It was lifted on September 3 and construction resumed.
For the record on the lake itself: there is still no established population of silver carp in any Great Lake. A single silver carp eDNA hit came back from 220 samples taken in the St. Joseph River in June 2023, and follow-up sampling found nothing further. A positive eDNA sample is not a fish; the DNA can arrive on a boat trailer.
What is still genuinely open
Whether spawning would actually succeed in those tributaries under real conditions rather than modelled ones — grass carp have been confirmed spawning in a Great Lakes tributary, bigheaded carp have not. Whether Green Bay and the river mouths alone could sustain a population without the open lake. And whether the fish ever get there at all.
It is worth knowing what the far side of that question looks like, because other watersheds are already past it. Maryland has resorted to paying up to $1,500 per blue catfish charter trip to pull an established invasive out of the Chesapeake, and snakehead bowfishing on the same water is what management looks like once prevention has failed and anglers become the removal program. Prevention is cheap only by comparison.
What it means for the water you fish
If it happened, the first effect would not be jumping fish on the open lake. It would be a thinner forage base in exactly the productive nearshore and bay water where the plankton still is — because that is the only place these fish can be. Green Bay's walleye and yellow perch fisheries sit directly on top of the habitat the models rate highest, including the shallow fall walleye bite that most of that water's best fishing depends on.
None of which changes what anyone does this fall. Green Bay is fishing the way it always has, and the jigging and live-bait presentations that work there are not going anywhere next season either. Based on the specifications, construction and our experience with similar gear, a sensitive medium-light spinning stick like our Jig Head Minnow Rod paired with a smooth-drag reel is the setup that suits that job. If you want the full range, the rest of our rod lineup covers the other presentations on that water.
The honest summary
The reader was right that offshore Lake Michigan cannot feed silver carp, and the modelling backs him on it in every scenario tested. He was wrong that this settles the question, because nothing about establishment requires the open lake. The real answer is narrower and more specific than either side of the usual argument: not "silver carp would starve," but "silver carp would be confined to the productive edges of a very large lake" — Green Bay, a handful of drowned river mouths, roughly June through October.
Those edges are also where the spawning rivers are. And this August, several hundred fish young enough to have hatched upriver turned up 80 miles from the lake.