Green Bay's whitefish refuge anchors Great Lakes fishery as stocks show strain

Southern Green Bay — once dismissed as too polluted to support commercial fishing — has become the last significant refuge for Lake Michigan whitefish, an iconic species that has nearly vanished from the open lake after invasive mussels stripped away its food supply. Commercial harvests in the bay’s Wisconsin waters surged from fewer than 100,000 pounds in 2000 to more than 800,000 pounds in 2024, even as catches from the wider lake collapsed. But population models now show whitefish stocks in the bay have shrunk by roughly half from their mid-2010s peak, and scientists warn that climate-driven ice loss threatens the species’ last stronghold in the lower Great Lakes.

Green Bay’s unlikely revival has offered biologists a template for recovery efforts throughout the Great Lakes, but scientists and commercial fishers say the bay itself may be entering a period of decline — raising the question of whether the conditions that saved whitefish here can survive warming winters and sustained fishing pressure.

Southern Green Bay has become the last significant whitefish refuge in the lower Great Lakes, drawing thousands of ice anglers and anchoring commercial fishing operations that once depended almost entirely on Lake Michigan’s open waters. The shallow, narrow bay in northwestern Lake Michigan — long contaminated by paper-mill discharges and farm runoff — now supports the fishery that the wider lake can no longer provide.

The numbers mark a dramatic reversal. Wisconsin’s commercial harvests from Lake Michigan’s open waters fell from more than 1.6 million pounds in 2000 to fewer than 200,000 pounds in 2024, according to reporting by Bridge Michigan distributed through the Associated Press. Over the same period, Green Bay harvests in Wisconsin waters rose from fewer than 100,000 pounds to more than 800,000. Michigan has seen a comparable pivot: its total commercial whitefish harvests from Lake Michigan have fallen roughly 70 percent since 2009 to about 1.2 million pounds annually, with the bay now accounting for more than half of what remains.

“A very conservative approach is going to be necessary,” said Todd Stuth, a commercial fisherman who chairs Wisconsin’s state commercial fishing board. “Because it’s our last stronghold. If that goes away, what do we have?”

How a polluted bay became a refuge

The collapse in the open lake traces to invasive zebra and quagga mussels, which arrived from Eastern Europe and spread across the Great Lakes over several decades. The mussels are prolific filter feeders that monopolize the plankton and nutrients that whitefish depend on. Whitefish populations in Lakes Michigan and Huron have largely collapsed as a result.

Green Bay’s shallow, nutrient-loaded waters told a different story. Decades of agricultural runoff and industrial discharge left the bay with more nutrients than the mussels could consume, creating conditions hospitable to the fish even as the open lake lost them.

Cleanup efforts played a parallel role. A restoration project cleared more than 6 million yards of sediment laced with polychlorinated biphenyls from the Fox River — the bay’s largest tributary — and from lower Green Bay itself. Phosphorus concentrations near the river’s mouth have declined by a third over 40 years, according to Scott Hansen, senior fisheries biologist with the Wisconsin Department of Natural Resources, though they remain above recommended levels.

“Pelicans are back, and the bird population seems to be thriving,” said Sarah Bartlett, a water resources specialist with the Green Bay Metropolitan Sewerage District. “And now we have this world-class fishery.”

Scientists are not certain exactly when or why whitefish reestablished in the bay. Hansen’s theory is that fish wandered in from Lake Michigan as conditions improved, found the shallow waters hospitable, and stayed. By the early 2010s, population models suggested the bay held tens of millions of them.

Signs of strain

Despite the bay’s success, the most recent population models have raised alarms. Stocks have shrunk by roughly half from their mid-2010s peak, according to models using data from Wisconsin DNR surveys and commercial and recreational harvest records. Adult fish appear healthy, but fewer offspring have been surviving to adulthood, and scientists have not identified the cause.

Climate change has introduced an additional threat. Whitefish lay their eggs on ice-covered reefs in late fall, where a protective layer of ice shields the eggs from wave damage and synchronizes hatching with the spring plankton bloom on which larvae feed. A recent string of mild winters has left some reefs without reliable ice cover, exposing eggs to physical stress or causing premature hatches that leave larvae without adequate food.

“Whitefish are a cold-water species, and we know that’s not where the trends are going,” Hansen said.

Wisconsin has not lowered Green Bay’s annual quota of 2.28 million pounds, split evenly between the commercial and sport fisheries. Commercial boats are restricted to fish larger than 17 inches; recreational anglers may keep 10 fish per day of any size. In a recent presentation to the state’s Natural Resources Board, Hansen said current trends warrant closer monitoring and potentially lower harvest limits.

Commercial fishing guide JJ Malvitz, whose business depends on the bay’s whitefish, said voluntary cuts are overdue. He favors lowering the recreational daily limit to five fish and reducing commercial quotas. “I don’t want to be standing on the shore in five years saying ‘remember when,’” Malvitz said.

Stuth said he is not ready to accept tighter limits but acknowledged that if population models continue to show decline, cuts could become necessary.

Lessons for Michigan and beyond

Scientists studying the bay believe its characteristics could inform recovery efforts throughout the Great Lakes. Matt Herbert, a senior conservation scientist with the Nature Conservancy in Michigan, said biologists are testing whether Michigan whitefish can be coaxed into spawning in rivers that connect to nutrient-rich embayments such as Lake Charlevoix. The goal is to give young fish a chance to fatten up in hospitable shallow waters before migrating into the mussel-infested open lake.

“Having places they are doing well gives us context for the places that they aren’t doing well,” Herbert said. “It helps us to figure out, how can we intervene?”

Dan Isermann, a fish biologist with the US Geological Survey, said tracking spawning locations and habitat characteristics in Green Bay could yield information applicable to recovery efforts throughout the region.

Some researchers have raised the possibility of adding nutrients to the main lake basins to help whitefish compete with mussels. Herbert said the idea faces significant obstacles: the lakes are far too large to treat at meaningful scale, and excess nutrients carry risks including oxygen-depleted dead zones and harmful algal blooms.

For now, Green Bay remains what Stuth and others call the last stronghold — a refuge that materialized through a combination of environmental cleanup, ecological accident, and a still-poorly-understood set of conditions that have not been replicated elsewhere in the lower Great Lakes.