Using gill net surveys to study the health of Michigan’s fish populations
By Victor Skinner | May 28, 2022
Pat O’Neill and Kris Snyder are cruising the Lake Michigan shoreline, stopping overnight at a half-dozen popular fishing ports along the way.
They started out in St. Joseph but are surely to Arcadia by now, more than halfway to their final stop in Charlevoix. From April through June, the duo haul in thousands of fish of all kinds into their state-owned research vessel, the Steelhead, where they meticulously document the number, length, weight, and other biological data. They also take specific samples from certain fish: stomachs, bone structures, and whatever else is requested from a handful of universities.
“We’re working for a lot of different people when we’re out here,” says O’Neill, a Michigan Department of Natural Resources employee and captain of the Steelhead. This year, it’s Grand Valley State University, Purdue University, and Michigan State University.
The Science of Surveys
At a typical stop in South Haven in mid-May, O’Neill and Snyder, a DNR technician, took in about 180 fish, including 50 or so lake trout, about the same number of whitefish, dozens of perch, walleye, suckers, gobies, steelhead, and several other species.
They set about 2,000 feet of specially designed gill nets each day—at depths of 30 feet, 50 feet, 100 feet, and 150 feet—then spend the night aboard the Steelhead before retrieving their catch the next day and repeating the process again further north. In 2021, researchers set more than 100,000 feet of net and collected more than 5,000 fish.
Since 1997, the Michigan DNR has collaborated with the U.S. Geological Survey (USGS) and Native American tribes to conduct the spring gill net surveys, which provide the most comprehensive picture of the status of adult Lake Michigan fish populations.
Using a standardized process, researchers from each agency survey different areas of the lake to assess recreationally, commercially, and ecologically important fish populations with a focus on lake trout, burbot, lake whitefish, and yellow perch in Michigan waters. The information collected is used to inform ongoing research and management for multiple species.
“We can determine survival rates, what kinds of food they’re eating, [and the] age structure of the population,” says Dave Clapp, DNR research station manager in Charlevoix. “It all helps inform stocking decisions.”
The hard numbers collected on the water are difficult to come by and are not published publicly in a central location. Instead, the data is fed into computer models to monitor trends in specific populations. The resulting reports are presented to and published by the Great Lakes Fisheries Commission (GLFC), a collaboration consisting of representatives from eight states, the Province of Ontario, and U.S. tribes that work together to manage the Great Lakes.
Creating the Big Picture
A lag time to process the information means the reports are typically based on data from a year or two prior, but taken with a variety of other research like forage fish surveys, bottom trawls, creel surveys, and acoustic telemetry work, the reports help to paint a picture of how the lake is evolving, and how the fish are evolving with it. The gill net data is also incorporated in reports to justify federal funding and to help inform recommended harvest objectives negotiated between the state and Native American tribes.
In the broadest sense, the data shows Lake Michigan is diversifying, moving from a waterbody dominated by chinook and Coho salmon to one that offers a more mixed bag. While “it really depends on the part of the lake you’re looking at,” Clapp says, there are clear trends emerging lakewide, driven largely by invasive species like zebra and quagga mussels and round gobies.
Alewives, a dominant forage fish, have steadily declined in recent decades, and so have the salmon that depend on them for survival. But that change has been a boon for the lake trout the state and federal government have worked hard to restore since they were nearly wiped out more than 50 years ago.
By the Numbers
After decades of stocking lake trout, the fish are now naturally reproducing in different areas of Lake Michigan, with rates of up to 50 percent wild fish in the southern basin and percentages of the same at a measurable 10 percent or more in the north end of the lake.
Researchers clip the adipose fin of all fish stocked in Lake Michigan, and the presence of a growing number of unclipped lake trout in gill net surveys has prompted the DNR to eliminate lake trout stocking entirely in southern Lake Michigan and to more closely monitor their numbers further north.
“Now we’re actually starting to see more wild fish, they’re starting to reproduce out there in the lake,” Clapp says. “They’re on a bit of an upswing.”
Other species are cyclical and spotty.
Yellow perch, for example, don’t number what they once did in the 1980s and '90s, though the population continues to lure anglers in both Lake Michigan and Grand Traverse Bay.
“Generally perch numbers are down a little bit, but relatively stable,” Clapp says.
Watching the Whitefish
The species of most concern for both the state and tribes in the region is whitefish, which have dwindled as quagga mussels have slowly choked out spawning habitat.
“It’s just been a steady decline,” says Jay Wesley, DNR Lake Michigan basin coordinator and GLFC representative. “They’ve been on the decline for 10-15 years now, and that seems to be tied pretty tight to quagga mussels and the effects they’ve had on the lake.”
Clapp says the situation is troublesome because “[whitefish] fuel a commercial fishery that fuels the restaurants up here.”
The DNR is currently working on tagging whitefish in Grand Traverse Bay to learn more about what’s driving the dynamic, while the Grand Traverse Band of Ottawa and Chippewa Indians are also monitoring the situation at several sites in the bay: Lee Point, Old Mission, the Grand Traverse shoal, and an East Bay reef north of Elk Rapids.
The tribe’s commercial whitefish harvest in the bay and northern Lake Michigan peaked in 1996 at 620,000 pounds, but in the last three years has averaged only 10,451 pounds, says Erik Olsen, lead tribal biologist for the Great Lakes fisheries. Commercial whitefish harvest for all tribes was typically between 4-5 million pounds until 2012, but now averages around 1 million pounds, Olsen says.
“It’s basically gone down 75 percent,” he explains. “I know state [commercial anglers] fishing out of Leland haven’t reached their quota in quite a few years. This is an environmental shift, not an overfishing shift.”
Quagga mussels have filled in spawning areas, leaving eggs vulnerable, while green algae blooms and the invasive rusty crayfish have also played into the dynamic. The algae “covers up a lot of those shoals [where whitefish spawn], and you have rusty crayfish going in there and eating eggs,” Olsen says.
Perhaps the biggest issue for whitefish, however, is the loss of diporeia, a small, shrimp-like crustacean that have been filtered out by mussels. “They’re almost gone,” Olsen says. “They were a high energy food whitefish could eat. Now you’re finding fish with crushed up mussels in some of their stomachs.”
And while whitefish are struggling, another native fish known as cisco or herring is growing in number in several places in Grand Traverse Bay, according to Olsen.
“One thing we have seen is there’s a lot more herring in Grand Traverse Bay than there has been in my time,” Olsen said. “In the last 10 years they’ve really popped up. You can now charter and go herring fishing out of Elk Rapids,” he says.
The cisco charters are one of several ways the fishing industry is changing in northern Michigan, says Mark Breederland, Michigan Sea Grant educator for the region. For decades, charter captains and most big lake recreational anglers targeted primarily salmon, with little interest for anything else, but perceptions of the perfect catch are shifting, he says.
“My sense is there’s a lot broader acceptance to diversify the fishing opportunities,” he says. “Most of the charter captains I run into … are an opportunistic business looking to match the fishery.”