Nature
PUBLISHED
Scientists have been exploring the deepest point of Lake Superior in search of what could be driving an undesirable transformation in the ecosystem’s top predators. Known as “zombie fish”, these siscowet lake trout are raising eyebrows among experts due to their extreme emaciation.
Siscowet occupy some of the deepest and coldest waters of Lake Superior. As such, they need to have a generous layer of fat to survive. It’s with some concern, then, that scientists have noticed an increasing number of gaunt "zombie fish" with about a third of the fat content of healthy fish.
In 2015, just 3 percent of siscowet caught during a survey of the deepest parts of Lake Superior were zombie fish. That number has since risen, reaching 54 percent in one 2025 catch at Superior Maximus (though it's worth noting this comprised only 15 fish and doesn't necessarily reflect the wider population of the lake).
“While this trend is cause for further investigation, it’s not cause for alarm,” said fisheries research biologist Shawn Sitar in a Department of Natural Resources press release. “Some social media content has drifted from the facts. I’ve seen posts that said things like 50% of fish in Lake Superior are ‘zombies,’ which is absolutely not the case.”
Living at depths of 396 meters (1,300 feet), siscowet are typically robust fish. They survive where no light reaches and are a top predator in an ecosystem where there's a lot of competition for limited resources.
It’s possible the zombie fish are the result of a natural ecological event. The delicate balance of deep-water ecosystems means it only takes one resource to be depleted to cause widespread starvation, and living in such extremes is very energetically expensive.
It could also be a disease outbreak, which the new expedition, which took place in June, hoped to investigate and – ideally – rule out. The expedition plan included collecting fish for thorough testing to investigate the possibility of some kind of contamination, such as a toxin.
According to MPR News, the zombie fish have significantly higher levels of mercury compared with healthy fish. However, we don’t yet know if this is related.
In the past, invasive lampreys have decimated the numbers of trout in Lake Superior. As such, they’re also under suspicion although – as Sitar told MPR – the lack of scars on zombie fish would suggest lampreys are probably not the culprits.
The researchers hope the expedition to Superior Maximus will settle the mystery once and for all, once the results from the surveying and testing are in. The mission included a dive to the deepest point in the Great Lakes – somewhere nobody has explored for 40 years.
While we wait for answers, you can watch a livestream of the footage with insights from Sitar above.
