Zombies in Our Abyss? Michigan Research Tracks Emaciated Trout

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Lake Superior

What happens when it looks like a high percentage of Lake Superior’s “fatty lake trout,” aka siscowet, turn up not fat at all? 

For researcher Shawn Sitar with the Marquette Research Station of the Michigan DNR, tiny red flags start waving … and he creates a catchy “Zombies in the Abyss” PowerPoint presentation for his colleagues that hits the press around the country.

While the tone of his presentation was somewhat tongue-in-cheek, the situation could indicate something serious, Shawn says. Exactly what that may be needs to be uncovered.

“For the last 10 years, we’ve been noticing a decline in the siscowet population. They are probably the most dominant form of lake trout in the Lake,” he says, estimating about 13 siscowet for every single lean lake trout.

Siscowet are “fat” for a reason. They have evolved to traverse the full column of Lake Superior waters, from its shallows through the 1,300 or so feet to its bottom – something deep-lake divers can appreciate. They generally hang out in the waters about 200 feet and deeper. Lean lake trout, meanwhile, tend to stay shallower than 250 feet. 

“The siscowet really moves like a yo-yo,” says Shawn. “To live in that deep habitat, they have to forage through the whole water column.

Shawn and his colleagues have been studying the fish that inhabit an area north of Munising, what he calls “Superior Maximus,” the deepest spot in Lake Superior (“And the entire Great Lakes!” notes Shawn), estimated at 1,300+ feet. Basically, four species of fish live that deep – the siscowet, the kiyi, deepwater sculpin and the burbot (perhaps the most versatile fish predator able to live in waters from rivers to the bottom of the Big Lake).

At Superior Maximus, the triennial fish surveys, which drop nets into the depths to collect specimen, have turned up a high percentage of emaciated siscowet in one area – about half of those collected in the most recent sampling. Samples in nearby deep-water areas have noted as many as 20% emaciated. Siscowet surveys elsewhere on the Lake, such as by Isle Royale and along the Minnesota North Shore, have not indicated those high percentages of sick siscowet. (This is contrary to a Mississippi news organization’s report on Shawn’s findings that declared  “Zombie Fish Take Over Lake Superior: Investigation Launched as Half of Catches Show Disturbing Signs.” Apparently catchy research marketing can create click bait.)

Species ebb and flow is not uncommon, even among Lake Superior fish, but this specific area with its abnormally high percentage of emaciated siscowet is raising concerns – a “yellow alert” – for a number of reasons. “We are not alarmed, but we are concerned. This may be a natural ecological phenomenon of a drop in food availability.”

For one thing, a skinny siscowet ultimately means a dead siscowet, according to Shawn. “High body fat is for buoyancy. When they lose their body fat, they become stuck on the bottom.” In fact, normal siscowet filets would actually float due to their high-fat content, he adds.

A siscowet stuck on the bottom likely will not find enough food to sustain it. Siscowet are opportunistic predators, and “to live in that deep habitat, they have to forage through the whole water column.” Diet analyses of lake trout have turned up such unique meal items as songbirds, bats, wrappers, bratwurst (found twice in Minnesota fish), mice and snakes (near Isle Royale) and barley, the kind being transported across Lake Superior in grain-carrying freighters. Fun fact: In the depths of the Lake where light cannot penetrate, siscowet hunt using a lateral line sensory system – special mechanical sensors on the side of fish that detect movement and vibrations.

Questions also arise as to whether whatever is causing these particular siscowets’  high rate of emaciation could affect other fish. Potential causes range from lack of food to a disease to contaminants such as mercury to sea lamprey attacks. Shawn suspects the latter is not the full cause of the emaciation issue, as applications of lampricides do continue to hold down that invasive species population and lamprey attach to both lean and fatty trout. “Initial looks at the data indicate that lamprey wounds on the zombies and healthy siscowets seem to be similar.”

Now that the issue has been identified, the cause will become the focus. “We’re going to go to Superior Maximus to research more,” Shawn says. “We want to make sure there is not a disease issue.” 

Lack of food, perhaps because some species are in decline, may be a concern. “The Great Lakes are really a low-production system,” he notes, adding that Lake Superior is like the Arctic in terms of food productivity because of how deep and cold it is.

“These are extreme environments with limited resources, and it doesn’t take much of a major change in the wrong direction to create situations such as famine.”

The research process also can be difficult. “Sampling the abyss of Lake Superior is not an easy feat,” says Shawn. “For example, it takes the RV Lake Char about two hours to reach Superior Maximus from the nearest port (Munising) and then it takes another two hours to lift the net from the bottom (1,300 feet) – a slow process because of the amount of stress and tension on the equipment onboard.  This has to be in calm weather.  Normally in near-shore, shallower areas, it usually takes about 30 minutes to lift a net.”

Researchers around the Lake are also collaborating, including those on the Kiyi, the U.S. Geological Survey’s research vessel based in Ashland, Wisconsin. Shawn is also involved with the Great Lakes Fishery Commission, another resource for networking research.

Whatever the results of the on-going studies, it’s likely that Shawn’s eye-catching sense of humor will generate news reports, as his “zombies” have this time.

He often works with students, Shawn says, and adding a few sci-fi references can help to grab attention. He admits to presentations titled “Lake Char Live Long and Prosper” and tossing the Starship Enterprise onto an opening presentation slide.

“Science demands integrity and quality, but not perpetual seriousness,” he points out. “You can take the science seriously without taking yourself too seriously.”