A reader made a great comment and 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. It is a better argument than it sounds, it used to be roughly the scientific consensus, and the modeling done since 2019 has largely dismantled it.
Here is what the research actually found, including the parts that cut against the alarm.
The reader's argument was the consensus once
Lake Michigan is not the Illinois River. The river pools where silver carp reach nuisance density are warm, shallow, murky and loaded with nutrients. Lake Michigan is cold, deep, and — offshore — startlingly clear, because trillions of quagga mussels filter the water column and strip out the algae that a filter-feeding fish would otherwise eat. Early bioenergetics work took that at face value and concluded there was not enough food in open water to support a bigheaded carp population. That is the argument the reader made, and for about a decade it was the mainstream one.
What changed: diet flexibility and the water below one meter
A 2019 study in Freshwater Biology led by Peter Alsip, working with NOAA's Great Lakes Environmental Research Laboratory and the University of Michigan, rebuilt the model with two things the earlier ones left out.
The first is that bigheaded carp do not eat only plankton. They also eat detritus — dead organic matter, including the fecal pellets the quagga mussels themselves produce. Accounting for that flexible diet increased the volume of suitable habitat for bighead carp by a factor of 4.6.
The second is depth. Earlier models looked at surface water. Lake Michigan develops a deep chlorophyll layer each summer, roughly 100 feet down, where algae concentrate below the warm surface layer. Once the model looked there, bighead carp could find survivable habitat across the lake's entire 22,000-square-mile surface at certain times of year.
The highest-quality habitat was not spread evenly. It clustered near river mouths — the Milwaukee and the St. Joseph — and in the nutrient-rich stretches of Green Bay, where the model found conditions good enough year-round to support spawning. That pattern matters more than the headline number, and we’ll come back to it.
The mussel barrier is real, and it is temporary
The follow-up, published in Biological Invasions in 2020 by the same group, asked the obvious next question: do the quagga mussels function as a living barrier?
Partly. Under 1980s conditions — higher phosphorus loading, no mussels — the model had bighead carp growing two to five times faster than they would today. The mussels genuinely did make the lake a harder place to be a filter feeder.
But the authors concluded that protection is "limited in time and space," and warming erodes it. Warmer springs mean more feeding activity, and earlier, longer summer stratification separates the carp in the upper water column from the mussels on the bottom. In their simulations, warming roughly doubled suitable spring habitat for bighead carp and increased silver carp spring habitat by about a third. Lake Michigan's surface has been warming at roughly half a degree Fahrenheit per decade.
Surviving is not the same as establishing
This is the distinction that gets lost, and it is where the reader's skepticism still has real ground to stand on. A fish that can survive in a lake has not established there. Establishment requires successful spawning, and silver carp spawn in flowing water — eggs are semi-buoyant and have to stay suspended in current long enough to hatch, or they settle out and die.
For years the working assumption was that this required a very long river, longer than most Great Lakes tributaries. USGS work revised that down hard. Eggs settle out at mean velocities somewhere in the range of 5.9 to 9.8 inches per second, and river reaches as short as 16 miles may give eggs enough time to develop to hatching. Four tributaries have been flagged as having suitable spawning and egg-transport conditions: the Milwaukee and the St. Joseph on Lake Michigan, and the Maumee and Sandusky on Lake Erie.
Those are the same river mouths the habitat model identified as the best feeding water. That overlap is the actual case for concern — not the open lake.
How many fish would it take?
A separate 2020 modeling study looked at Saginaw Bay on Lake Huron, a productive embayment comparable in character to Green Bay. Under favorable recruitment conditions, as few as ten bigheaded carp were enough to establish a population. Under pessimistic survival assumptions, it took more than 100,000. That range is not a hedge — it is the honest state of the science, and it’s why forecasts of this invasion vary so widely.
That study also produced a result nobody promotes: while plankton and plankton-eating fish declined as carp density rose, modeled walleye actually benefited. We are not going to pretend that finding does not exist because it is inconvenient to the alarm.
What is still genuinely open
Three things. Whether spawning would actually succeed in those tributaries under real conditions rather than modeled ones — grass carp have been confirmed spawning in a Great Lakes tributary, but bigheaded carp have not. Whether the productive embayments alone could sustain a population without the open lake. And whether the carp themselves ever get there, which is the question the Brandon Road project and the juvenile fish found 80 miles downstream are actually about.
What it means for the water you fish
If it happened, the first effect would not be jumping fish. It would be a thinner forage base in the productive nearshore and bay water, because that is where the plankton is and where the carp would be. Green Bay's walleye and yellow perch fisheries sit directly on top of the habitat the models rate highest.
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 — a sensitive medium-light spinning stick like our Jig Head Minnow Rod paired with a smooth-drag reel — are the same ones that will work next year. If you want the full range, the rest of our rod lineup covers the other presentations on that water.
The honest summary is this: the reader was right that the old science said the lake could not feed them, and wrong that the science still says it. What replaced it is not certainty in the other direction — it is a narrower, more specific worry about a handful of river mouths and one very productive bay.