NET WORTH Using gill net surveys to study the health of Michigan’s fish populations
By Victor Skinner 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
12 • may 30, 2022 • Northern Express Weekly
