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Meet Mr. Walleye

image of Dennis (Denny) Schupp holding a mounted walleye

Minnesota has produced millions of pounds of walleye -- and a world-renowned walleye fisheries researcher.

By Tom Dickson

It was an ugly crowd.

The fishing at Mille Lacs Lake that summer of 1982 had been exceptionally lousy. Anglers were calling the famed walleye fishery "Home of the Quarter-Pounder." No matter where or how they fished, they caught mostly 10-inchers -- throwback fish.

In the Isle High School gymnasium that fall evening sat more than 400 resort owners, charter boat operators, fishing guides, and anglers. They'd gathered to demand answers from the Department of Natural Resources. Among those representing the agency was Dennis (Denny) Schupp, a nationally recognized fisheries research scientist.

"I remember when we walked into the room, people turned to look at us and they actually started booing," says Schupp, 75, who retired from the DNR in 2001 and now lives near Pequot Lakes.

Schupp was nonplussed. He'd faced crowds of angry anglers before, though none as numerous and vocal.

"I knew why they were upset," he says. "It was my job to explain why they were catching so many small fish, and why the fishing was almost certain to improve within a few years."

During his 45-year career with DNR Fisheries, Schupp stood in the center of some of Minnesota's biggest walleye management controversies: Mille Lacs in 1957, Lake Osakis in 1962, Leech Lake in the mid-1960s, Lake of the Woods in the late 1960s, then Mille Lacs again in the early 1980s through the mid-1990s storm surrounding the 1837 treaty.

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"I was essentially used as a troubleshooter on the big walleye lakes," Schupp says. During that time, Schupp developed a reputation as a consummate scientist, committed to finding and interpreting the best biological information, then using that data to drive fisheries management decisions.

"Denny understands walleyes and walleye populations as well as anyone ever has," says Peter Colby, retired walleye research chief with the Ontario Ministry of Natural Resources. "Anyone who knows anything about walleye management recognizes his name and enormous contributions to the field."

Fishing pressure

Schupp grew up in Glencoe and graduated from the University of Minnesota in 1955, the first in his family to earn a college degree. He quickly landed a job with the then-called Minnesota Department of Conservation as a creel census biologist, but within a year he was drafted into the Navy. After a two-year stint serving on an aircraft carrier in the Asian Pacific, Schupp returned to Minnesota and in 1957 picked up again with the department, which sent him into the middle of a walleye management trouble spot at Mille Lacs. There, anglers were blaming the DNR for a recent spate of poor walleye fishing.

Still new to the field, Schupp couldn't provide answers; but he did pioneer a method for measuring fishing pressure. At the time biologists had no way of knowing how many anglers were fishing a lake like Mille Lacs because it was too big for them to go out and count all boats on the water, the method used for smaller lakes. So Schupp and a colleague figured out a formula by which they counted the number of boats landing along various 1- to 3-mile stretches of shoreline and then extrapolated that number for an estimate of how many boats were on the entire 200-square-mile lake. That information, combined with traditional creel surveys of anglers coming off the lake, provided the first estimates of how many fish were being harvested from Mille Lacs each year.

"That seems like pretty basic stuff today," Schupp says, "but in the 1950s, we didn't have a clue what was happening in the large lakes."

In 1962 Schupp was sent to Glenwood. Anglers were up in arms about the poor walleye fishing at Lake Osakis. Schupp investigated and learned that naturally spawned walleye eggs were dying due to a lack of dissolved oxygen on the spawning beds, likely caused by agricultural runoff from surrounding farm fields and leaky lakeside septic systems. (Runoff added nutrients, which increased algae and depleted dissolved oxygen that walleye eggs needed to survive.)

"We worked with the Osakis community to stock walleye fry, which led to a walleye population recovery that continues to this day," he says.

More pressure

In 1965 the DNR sent Schupp to another walleye pressure cooker.

"Leech Lake that year was another 'dead sea' situation," Schupp recalls. "People were furious at the department, demanding we build a hatchery on the lake. I was sent up there to figure out what was going on with the walleye population and why anglers weren't catching fish, especially in midsummer."

During the next five years, Schupp conducted an unprecedented 1,000 trawl hauls (a trawl is a large net dragged by a boat along a lake's bottom) to better understand the walleye and prey fish population structure. One of his most important findings was the cause of the legendary midsummer fishing slump -- a period from mid-July to late August when anglers at Leech and other lakes catch few walleyes.

Schupp learned that the walleye bite was tied directly to the size and abundance of that season's crop of perch and other prey fish. In the spring walleyes were hungry due to the rigors of spawning, but the newly hatched prey fish were still too small to eat, making anglers' offerings appealing. But in midsummer the young-of-the-year perch were abundant and had become the perfect size for walleye consumption, causing them to ignore lures and bait. "That was a huge revelation," says Schupp.

Next, the DNR sent Schupp farther north into the heart of another walleye management controversy. "My job was to work with University of Minnesota researchers to conduct a three-year study to see how commercial fishing was affecting the Lake of the Woods walleye population," Schupp says. The researchers found that commercial gill netting was severely altering the size structure of the lake's walleye population.

"Over three years, the biggest walleye we found was just 23 inches long," Schupp says. "There were few big fish left in the lake, just a lot of little ones." Schupp grew to respect the commercial gill-netters for their hard work and knowledge of the lake. And he believed commercial fishing would have disappeared on its own by the mid-1990s, due to a restriction on licenses begun by the state decades earlier. However, the state bought out the Lake of the Woods commercial fishing interests in the 1980s and ended gill netting there.

Head of year-class

After working four years as regional fisheries supervisor in Bemidji ("I soon realized I much rather preferred working with fish than pushing money around."), and then turning down an offer to become the DNR Fisheries research chief ("I had no stomach for moving to the Twin Cities."), Schupp found himself back at Mille Lacs. This time, facing the hostile crowd at the high school gymnasium in 1982, he had answers.

Having spent the previous 25 years studying the highly prized, marble-eyed state fish, Schupp had learned that good or bad walleye fishing years are often determined largely by walleye population fluctuations, which are based on the strength and weaknesses of specific year-classes -- the generation of fish that hatch in a given year.

Strong walleye year-classes occur when the water gets steadily warmer each day and fosters the growth of zooplankton eaten by newly hatched walleyes, as well as the growth of young-of-the-year perch which walleyes eat later. This is more likely to happen when walleyes spawn later than normal.

A weak year-class is caused when spring lake water temporarily warms sooner than average -- tricking walleyes into spawning ahead of schedule. If a cold snap moves through and cools water temperatures, zooplankton production is stifled and many newly hatched walleye and perch fry die.

Schupp explained to the crowd that a strong year-class causes no noticeable change in fishing for a few years, because the walleyes are still too small to bite lures or bait. But after three years, the 10-inch fish begin to show up on the end of anglers' lines. When such a class of youngsters is particularly abundant, as on Mille Lacs in 1982, small fish outnumber larger, older fish even more than usual and dominate the angler catch.

The DNR can do nothing about a strong or weak year-class, says Schupp. But it can -- and now regularly does -- monitor year-class strength and tell anglers what they might expect to catch a few years later.

"That night at Mille Lacs was when we first really started explaining the concept of year-classes, and it didn't take long for people to get it," Schupp says. "We explained that those 10-inchers were the enormous year-class of 1979, and that in a few years all those fish would be keepers.

"That's exactly what happened. The fishing at Mille Lacs over the next several years was just fantastic, and people were very happy," Schupp says.

Power of science

Schupp has made many other fisheries science contributions. He and Jack Wingate, head of DNR Fisheries research and Schupp's supervisor for 19 years, compiled decades of fish population sampling techniques -- developed by Schupp and others -- into a comprehensive guide that has now become the DNR's bible for managing Winnibigoshish, Cass, Mille Lacs, and Minnesota's other seven large walleye lakes, which account for more than 40 percent of the state's total walleye harvest.

In the 1990s the walleye monitoring methods Schupp developed provided essential biological data used by federal courts, including the U.S. Supreme Court. When several Indian bands sued the state of Minnesota over their rights to hunt and fish in territory ceded under the 1837 treaty, the Supreme Court granted the bands the right to spear and gill net walleye on Mille Lacs. But the Supreme Court ruling set the harvest at a level, based on the Schupp and Wingate protocols for surveying large lake game fish populations, that would not damage the Mille Lacs walleye fishery.

In that same decade, Schupp developed a lake classification system that built upon one devised in the 1950s by DNR Fisheries legend John Moyle. The system organizes all 4,500 Minnesota game fish lakes into 43 separate categories based on size, depth, fertility, water clarity, fish communities, and other factors. Every lake has a unique mix of ecological characteristics that make it productive for some fish species but not others -- and that make its fish populations respond to harvest regulations differently than populations in other fisheries. That's why "one size fits all" fishing regulations often weren't helping improve fishing on many waters.

"Without Denny's system, we wouldn't be able to do individual lake management and improve fishing quality near to the extent that we have," says Ron Payer, DNR Fisheries chief.

Schupp also authored a scientific paper theorizing that atmospheric ash from the 1991 Mount Pinatubo volcano eruption in the Philippines was to blame for the poor walleye fishing throughout Minnesota in the 1990s. Climatologists say the ash from the massive explosion drifted east and blocked sunlight from reaching much of North America and Europe, causing the cold, wet summers of 1991 and 1992. Because the cool summers didn't create good conditions for the production of walleye fry or perch fry, the walleye year-classes those two years suffered.

Schupp says, "People blamed the DNR stocking program, but the major factor was Mount Pinatubo."

Colby, the Ontario walleye guru, agrees. "There's no doubt in my mind that Denny is absolutely right about that," he says.

Sharing the catch

His remarkable scientific achievements notwithstanding, Schupp "has a great penchant for people," says Payer. "He really values relationships with his co-workers and the public, and works at maintaining those relationships."

One of those is with outdoors writer Joe Fellegy Jr., who was a fishing guide on Mille Lacs for more than 20 years. "Denny has long been an invaluable mentor and friend," says Fellegy, author of Lake Mille Lacs: Thirty Years on the Big Lake. "He's always been willing to share his scientific studies; is always ready to share and trade insights and anecdotes."

Junior biologists have long appreciated Schupp's willingness to help. "Here was this legendary guy doing some of the most groundbreaking walleye research in North America, and he always had time for us young guys coming into his office with what he might have felt were some pretty trivial questions," says Tim Goeman, now DNR regional fisheries supervisor at Grand Rapids. But Schupp says, "There are no trivial questions, in my opinion."

Schupp's office, adds Goeman, was legendary for paper clutter. "Along every wall, it was stacked with reports, clippings, folders, you name it," he says. "The thing is, you could ask him for some technical paper published in, say, 1974; and he'd think for a few moments, then reach over and pull it out of some 5-foot-tall stack. Your jaw would just drop."

These days, Schupp remains fascinated by Minnesota fish populations, which he ponders from his lakeshore cabin. The retired biologist continues to discuss fisheries science via e-mail with peers, mentors, and the next generation of research biologists. An emeritus of sorts, Schupp says he enjoys these online technical chats and the ponderings that come from studying the interrelations of fish, their habitat, and the anglers who pursue them. Though he no longer writes reports, his electronic missives still reflect the tone of a man who cares deeply about the fate of Minnesota fish, the quality of its waters, and the importance of good science.

Tom Dickson, is a freelancer writer in Helena, Mont., who went fishing with legendary walleye biologist Denny Schupp several years ago. They fished for Denny's favorite fish-bass and bluegills.

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