After more than a decade, University of Minnesota researchers have renewed studies on a malicious fish parasite that slowly dissolves muscle cells, ruining the fillet.
The parasite, called Heterosporis, is commonly found in north-central Minnesota and northeastern Wisconsin, where it has befuddled anglers who catch fish and find they have a freezer-burnt appearance. Yellow perch are some of the most widely affected by the disease, which turns the fillet opaque white instead of translucent and is often observed only after it has been cleaned.
“It’s essentially liquefying them,” said Nick Phelps, a U researcher and assistant professor. “Our hypothesis is that as the muscle gets destroyed, their endurance goes down. And a fish’s life depends on endurance to catch prey and escape predation. If they can’t do that, they’ll be pulled from the population.”
In severe cases, the fish will appear to curve inward due to muscle loss.
Walleye and northern pike also have tested positive for the parasite, increasing the tally to 15 affected species.
“There’s nothing more beloved in Minnesota than the walleye,” said Phelps, who hopes to create an evidence-based management program to thwart the spread of the disease, now found in 16 Wisconsin lakes.
But what does this parasite do to anglers who might eat the decimated fish?
People are not thought to be susceptible to the microscopic parasite, but experts say its spread would be detrimental to Minnesota’s aquatic ecosystems — where 26 state lakes have become infected since 1990.
Heterosporis was first discovered that year in Leech Lake. Angler reports show that more than one-third of the yellow perch have become contaminated there in the past 25 years, Phelps said.
The Minnesota Aquatic Invasive Species Research Center identified the parasite as a high-priority invasive pathogen during last year’s needs assessment. Phelps’ team at the university has been surveying fish in Leech and Cass lakes since July to determine how prevalent the infected fish population has become.
Little is known about how the parasite moves from lake to lake, but health officials recommend that infected fish or fillets be thrown away or buried, not put back in the water.
Researchers are trying to raise awareness with anglers, many of whom don’t know what to do with the abnormal-looking fillets that are mushy to the touch.
Phelps said the disease will affect stakeholders beyond typical consumers and fishermen, including environmentalists and the aquaculture — or fish farm — industry.
“This one reaches everybody,” he said.