Lake Superior Lake Trout: Recovery and the “Zombie Fish” Mystery
Lake Superior supports robust native lake trout populations, from the lean lake trout familiar to many anglers to deepwater siscowet adapted to some of the deepest habitat in the Great Lakes. Now researchers are investigating an unusual group of siscowet so thin they have been nicknamed “zombie fish.”
The broad story of Lake Superior lake trout is encouraging. In 2024, the Lake Superior Committee announced that lake trout were fully restored in most of Lake Superior after nearly 70 years of rehabilitation work. Current lakewide assessments continue to describe lake trout populations as robust, with stable adult abundance and recruitment.
But that success does not mean every lake trout in every part of Lake Superior is experiencing the same conditions. In the deepest Michigan waters, scientists have found increasing numbers of severely emaciated siscowet lake trout.
Michigan Department of Natural Resources fisheries research biologist Shawn Sitar has a memorable description for them:
“They actually kind of look like zombies.”
Those “zombie fish” are now the focus of an ongoing investigation—and the subject of Sitar’s latest conversation with Walt Lindala and Frida Waara on the Lake Superior Podcast.
In This Article You’ll Discover
Lake Trout in Lake Superior Today
If the phrase “zombie fish” sounds alarming, the first thing to understand is that it does not describe Lake Superior’s lake trout population as a whole.
The Great Lakes Fishery Commission’s current Lake Superior assessment says the lake continues to support robust lake trout populations, including the original suite of lean, siscowet, humper and redfin forms. Adult abundance and recruitment are considered stable, and several important lake trout indicators are meeting management targets.
Sitar makes the same point in the new podcast episode:
“Overall, Lake Superior is in a really good place. We have abundant populations of fish. We have a lot of biodiversity also.”
He also notes that lean lake trout—the form most closely associated with nearshore waters and human harvest—have reached recovered, or rehabilitated, status.
The recovery story is the big picture. The mystery begins when scientists go much deeper: the unusual siscowet now being studied are a localized puzzle within a much larger and generally healthy lake trout population.
What Kinds of Lake Trout Live in Lake Superior?
“Lake trout” can sound like a single kind of fish, but Lake Superior tells a more interesting story.
The Great Lakes Fishery Commission recognizes four lake trout forms still present in Lake Superior: lean, siscowet, humper and redfin. These forms use different parts of the lake and help illustrate how lake trout have adapted to one of the world’s largest and deepest freshwater environments.
Lean lake trout dominate nearshore waters. They are the form most familiar to many recreational anglers an
d are important to commercial and subsistence fisheries.
Siscowet lake trout dominate offshore waters. They are adapted to colder, deeper environments and naturally carry much more body fat than lean lake trout.
Humper and redfin lake trout persist in more specialized offshore habitats, including areas around Isle Royale and elsewhere in Lake Superior.
The distinction matters when discussing the zombie-fish mystery. The affected fish are not representative of every form of lake trout in Lake Superior. They are siscowet living in an extreme deepwater environment.
What Is a Siscowet Lake Trout?
A siscowet is a native deepwater form of lake trout. It is especially well suited to Lake Superior’s offshore habitat, where temperatures are cold, light disappears, food is sparse and depths can exceed 1,000 feet.
Sitar explains in the podcast that healthy siscowet are normally stout fish with high body-fat content. Their high fat content helps give them more neutral buoyancy, making it easier for them to move vertically through the water column while searching for food.
The contrast with the emaciated fish researchers are finding could hardly be greater. Sitar describes some affected fish as having heads comparable in size to healthy siscowet but bodies that narrow dramatically behind them.
Siscowet can occupy extraordinarily deep water. Michigan DNR researchers have sampled them at Superior Maximus, a deepwater research location about 1,320 feet below the surface north of Munising. The U.S. Environmental Protection Agency lists Lake Superior’s maximum depth at about 1,332 feet, making it the deepest of the Great Lakes.
That deepwater use is not new to the current investigation. In a peer-reviewed 2008 study led by Sitar, researchers documented siscowet near Lake Superior’s greatest recorded depths and examined how their size, diet and sea lamprey wounding varied across depth zones.
That depth helps explain both why studying these fish is difficult and why their food web can behave very differently from the nearshore Lake Superior environment most people know.
What Are Lake Superior’s “Zombie Fish”?
The nickname refers to severely emaciated siscowet lake trout found during deepwater fisheries surveys in Michigan waters of Lake Superior.
The fish are not a newly discovered species, and the term does not mean scientists have identified a contagious “zombie” disease. It is an informal description of fish that look extraordinarily thin compared with healthy siscowet.
The Michigan DNR’s 2026 investigation update shows why the phenomenon attracted attention. Across the agency’s deepwater surveys, the incidence of highly emaciated siscowet rose from about 3% in 2015 to about 20% in 2024 and 2025.
See a “zombie” siscowet beside a typical siscowet → Michigan DNR’s side-by-side photo shows the striking difference between an emaciated fish and a siscowet in normal condition.
The numbers were higher at Superior Maximus itself. In that research location, 11 siscowet caught in 2024—37% of the sample—were classified as “zombies.” In 2025, 15 fish, or 54% of that sample, were classified that way.
Those figures come from the multiagency, lakewide triennial deepwater survey, which gives researchers a long-running basis for comparing fish condition at Superior Maximus rather than relying on a single unusual catch.
Those figures should not be generalized to the entire lake. They do not mean that half of all fish in Lake Superior are emaciated, or even that half of all Lake Superior lake trout are affected. They describe small research samples from one extreme deepwater location.
Sitar frames the finding as an unusual, localized condition that deserves investigation rather than a lakewide emergency.
Sitar also presented these findings to the Great Lakes Fishery Commission in his 2026 presentation, Zombies in the Abyss: Conditional Anomalies in Deepwater Siscowet.
Why Are the Zombie Fish So Thin?
Researchers have answered one part of the question: the fish are starving.
“The simplest thing is that they’re starving.”
What remains unresolved is what is driving the starvation.
Sitar’s first explanation is ecological. Offshore and deepwater parts of Lake Superior are naturally much less productive than nearshore areas. In the podcast, he compares the deep lake to other extreme ecosystems where relatively few species live close to the limits of available food.
If an important prey resource declines or shifts—even subtly—the effects on a predator living at the top of that sparse food web could be significant. Federal researchers who survey prey fish have been helping examine the data at a finer scale, but Sitar says there has not yet been one obvious prey decline that completely explains the condition.
Could Food Availability Be the Cause?
Food limitation remains a leading explanation. The Michigan DNR describes the affected habitat as an extremely low-productivity environment where relatively small ecosystem shifts can affect food availability.
The laboratory findings confirm that the affected fish are starving, but the underlying cause remains unknown. Researchers are examining food availability, disease or pathogens, contaminants, and the possibility that several factors may be interacting.
Could Disease Be Involved?
Disease is another possibility researchers have actively tested.
In June 2026, Michigan DNR researchers and fish pathologists from Michigan State University traveled aboard the Lake Char to Superior Maximus to collect healthy and emaciated siscowet for comparison.
By the time of Sitar’s Lake Superior Podcast interview, he said the initial laboratory observations and autopsies had clinically verified that the affected fish were starving, while pathogen testing was still being worked up.
The condition has not been established as a contagious disease. Michigan DNR has specifically said the fish are not thought to be spreading a communicable disease among one another.
“We’re not alarmed, we’re concerned.”
Could Contaminants Be Involved?
Sitar says researchers have also considered contaminants.
Sitar says previous testing found elevated mercury levels in both healthy and emaciated deepwater siscowet. Because elevated mercury was present in both groups, he says researchers do not currently think mercury itself explains why some fish become so thin.
Other contaminants and possible interactions among multiple stressors can still be investigated. Scientists have not yet identified one confirmed underlying cause for the starvation.
NPLSF has also explored efforts to protect Lake Superior water quality, including the legacy of industrial contaminants and newer pollution concerns.
Is This Happening Throughout Lake Superior?
No evidence presented by Sitar or the Michigan DNR suggests that the high concentration of zombie-like siscowet is occurring throughout Lake Superior.
The condition has been documented primarily in very deep Michigan waters, from roughly 600 feet to about 1,320 feet, with the most striking percentages recorded around Superior Maximus.
Sitar says other researchers—including Tribal, federal and Canadian partners—have been watching for the same pattern elsewhere and have not found similarly high concentrations.
Researchers do occasionally find unhealthy or thin individual fish in routine surveys. What makes this case different is the repeated concentration of severely emaciated siscowet in a particular deepwater area.
That is also why cooperation around Lake Superior matters. Through the Lake Superior Committee and the Great Lakes Fishery Commission, state, provincial, Tribal and federal scientists can compare surveys across the lake instead of interpreting one research location in isolation.
Is Lake Superior’s Fishery in Trouble?
The current evidence does not support describing Lake Superior’s fishery as being in broad trouble because of the zombie-fish finding. Sitar says Lake Superior remains healthy overall, while current fisheries assessments describe lake trout populations as robust.
Instead, the evidence points to a localized condition affecting some deepwater siscowet that researchers are still trying to understand. Detecting that kind of change is exactly what long-term monitoring is designed to do.
Lake Superior Lake Trout Fishing: What Does This Mean for Anglers?
For people interested in Lake Superior lake trout fishing, the zombie-fish finding does not mean Lake Superior’s recreational lake trout fishery is disappearing.
Sitar notes that most anglers and commercial fishers focus on lean lake trout and lake whitefish rather than the deepwater siscowet at the center of the current investigation.
For anglers, where lake trout are found—and how they are caught—changes with the season.
The Minnesota DNR’s Lake Superior fishing guidance notes that lake trout are often closer to shore and higher in the water column during colder months, then move offshore and deeper as nearshore water warms. Common approaches include trolling spoons or stickbaits when fish are higher in the water column, using downriggers to reach deeper summer fish, and jigging over underwater structure. Conditions on Lake Superior can change quickly, so depth, presentation and location often matter more than any single lure or tactic.
There is also an important conservation consideration when releasing fish. Research led by Sitar on post-release mortality found that mortality of angler-caught Lake Superior lake trout increased substantially as surface-water temperatures warmed. Minnesota DNR continues to remind anglers that post-release mortality may be around 43% when surface temperatures are about 50°F or warmer, so repeatedly catching and releasing lake trout during warm periods can have consequences even when the fish swim away.
Rules differ by jurisdiction and management area. Wisconsin, for example, as of 2026 manages Lake Superior lake trout with a harvest quota and monitors recreational harvest during the season. Anglers can check the current Wisconsin Lake Superior lake trout harvest and season information, the current Michigan fishing regulations, or the current Minnesota fishing regulations before fishing.
Are Lake Superior Lake Trout Good to Eat?
Lake trout have long been important food fish around Lake Superior, but consumption guidance depends on the form of lake trout, where it was caught and current health recommendations. Lean lake trout are the form most familiar to many anglers and fish consumers, while siscowet are much fattier deepwater fish. Sitar notes that historical accounts described siscowet as desirable food fish, especially when preserved by salting and smoking.
Modern consumption decisions should be based on current health guidance rather than whether a fish looks normal. Michigan’s 2026 Eat Safe Fish guidance for Lake Superior places siscowet in its “Limited” category because of PCBs and toxaphene.
How Deep Can Lake Superior Lake Trout Live?
Lake Superior lake trout occupy an extraordinary range of depths, and siscowet are the specialists at the deep end of that range.
The Michigan DNR has documented fish at Superior Maximus at about 1,320 feet—more than a quarter-mile beneath the lake surface.
For Sitar, that vertical distance is part of what makes offshore work feel like an expedition. Researchers may already be many miles from shore while also trying to understand an ecosystem hundreds or more than a thousand feet beneath the vessel.
The deepest offshore habitats require specialized vessels, equipment, favorable weather and substantial planning, which helps explain why so much remains to be learned about them.
Sitar’s deepwater work long predates the current zombie-fish investigation.
His first Lake Superior Podcast appearance in 2022 focused on an unusually deep “egg hunt” off the north side of Isle Royale. In a 2022 Michigan DNR account written by Sitar, the research team reported confirming lake trout spawning at depths greater than 400 feet—the deepest documented lake trout spawning in the Great Lakes. Related peer-reviewed research around Isle Royale found that siscowet reproduce at least during both spring and fall, while lean, humper and redfin lake trout reproduce during the fall.
The Hidden Below expedition to Superior Maximus offers a rare look at that environment. It used an underwater vehicle to explore the deepest part of the Great Lakes.
How Scientists Monitor Lake Superior Fish
Sitar is the science officer aboard the Michigan DNR research vessel Lake Char, which operates from the Marquette Fisheries Research Station.
Since entering service in 2007, the vessel has allowed researchers to reach remote and offshore locations including Isle Royale, Stannard Rock, Klondike Reef and Superior Maximus. The Marquette Fisheries Research Station’s Lake Superior program uses these surveys to monitor fish populations and provide information for management.
Researchers return to established areas and sample fish over time, building datasets that can reveal changes in abundance, recruitment, fish condition and other indicators. That work has also documented the oldest known lake trout recorded in the Great Lakes—a 62-year-old fish documented by Sitar and colleagues.
The monitoring is collaborative. The Great Lakes Fishery Commission helps connect state, provincial, Tribal, federal and university researchers. In the new episode, Sitar describes that network as especially valuable when something unusual appears because teams around the lake can compare what they are seeing.
In his earlier Lake Superior Podcast appearance, Sitar compared fisheries researchers to gauges on an engine. His job, he explained, is to census populations at different times of year, compare them with biological benchmarks and watch indicators such as sea lamprey wounds.
The same long-term approach is what allowed the zombie-fish pattern to stand out. Without years of comparable surveys, scientists would have far less context for deciding whether a group of thin fish represented a meaningful change.
Researchers are also watching environmental conditions such as climate and ice cover and considering how changes elsewhere in the ecosystem may affect cold-water fish indirectly. NPLSF has also covered how record-breaking warm weather is affecting Lake Superior.
How Lake Superior Lake Trout Recovered
The strange deepwater finding is easier to understand in the context of how closely scientists have watched lake trout for decades. Today’s mystery sits on top of a much longer recovery story.
Why did restoring lake trout matter so much? In his earlier podcast conversation, Sitar described lake trout as the “top boss” of Great Lakes deepwater food webs. A 2024 peer-reviewed synthesis led by Sitar similarly identifies lake trout as the keystone predator in Great Lakes deepwater areas and concludes that their recovery in Lake Superior restored important ecosystem function while supporting valued fisheries.
By the middle of the twentieth century, intense commercial fishing had heavily reduced lake trout populations. The spread of invasive sea lamprey added another major source of mortality. Across much of the Great Lakes, native lake trout populations collapsed.
Lake Superior retained native lake trout, giving fisheries managers a foundation for recovery. The United States and Canada created the Great Lakes Fishery Commission in 1955, and agencies ultimately relied on three major tools: sea lamprey suppression, fishing controls and hatchery stocking.
Sitar explains that three-part approach in his earlier Lake Superior Podcast episode:
“The first thing was to suppress sea lampreys, which continues to this day. The second component was to control fishing. The third thing was to use hatcheries.”
Nearly 70 years of coordinated work produced a major fisheries-management milestone. In 2024, the Lake Superior Committee declared lake trout fully recovered in most of Lake Superior.
Recovery does not mean the work is finished. Continued sea lamprey control remains important, and Sitar and his coauthors emphasize the need for ongoing interagency cooperation, effective management and long-term monitoring even after restoration goals have been reached.
For the full recovery story, listen to Sitar’s earlier Lake Superior Podcast episode, The Lake Trout Resurgence in Lake Superior: A Conservation Success Story.
Listen to the Full Shawn Sitar Conversation
The “zombie fish” are only part of the conversation. Sitar also takes Walt Lindala and Frida Waara into the broader work of researching Lake Superior’s offshore and deepwater environments.
They discuss what it is like to work beyond sight of land, why deepwater surveys resemble expeditions, how agencies cooperate across borders, and what scientists are learning from some of the lake’s least accessible habitats.
For more conversations about the science, history, wildlife and people of the lake, explore more episodes of the Lake Superior Podcast.
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About Shawn Sitar
Shawn Sitar is a fisheries research biologist with the Michigan Department of Natural Resources at the Marquette Fisheries Research Station and serves as science officer aboard the RV Lake Char. His work includes long-term monitoring of Lake Superior fish populations in both nearshore and offshore waters.
About The Lake Superior Podcast and NPLSF
The Lake Superior Podcast is produced by the National Parks of Lake Superior Foundation (NPLSF), the official nonprofit friends group for Lake Superior’s five national parks: Apostle Islands National Lakeshore, Grand Portage National Monument, Isle Royale National Park, Keweenaw National Historical Park, and Pictured Rocks National Lakeshore.
Funded by grants and private donations, NPLSF projects help preserve, interpret, and celebrate the parks and stories surrounding the Great Lakes’ greatest waters.
For more information or to support our work, contact us at info@nplsf.org.
