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Fishing WisconsinHeterosporis - yellow perch parasite

Identification || In Wisconsin || Further research || Controlling the spread

Heterosporis sp. is a newly identified parasite in Wisconsin, Minnesota and Ontario that infects fish muscle tissue. This infection does not seem to cause direct mortality, but when an infected fish dies, other fish may eat infected muscle or the infected muscle may break down, releasing spores into the water which are then acquired by other fish. In severely infected fish, almost 90% or more of the fillet is actually made up of the parasite's spores, rather than muscle tissue. There is no evidence that Heterosporis can infect people, however most may choose to discard infected fish based on changes in texture and quality of the fillet. For detailed information about the disease and its history in Wisconsin waters, please see the printable Heterosporis fact sheet [PDF].


The muscle (fillet) of infected fish appears white and opaque, almost as if the fish was already cooked or had freezer burn. The life cycle of the parasite has not been completely worked out, but the parasite's spores develop in muscle cells and cause the muscle tissue to degenerate. So far, fisheries biologists have not documented a decline in yellow perch abundance in lakes where Heterosporis is present.

Heterosporis in Wisconsin fish

Heterosporis sp. parasite - Areas of muscle infected with Heterosporis are opaque and white compared to the uninfected areas. Freezing the fish changes the appearance of infected muscle, so it is best to look for Heterosporis when fish are fresh (before they are frozen).

In Wisconsin

The parasite was first discovered in 2000 in yellow perch from the Eagle River chain of lakes in north central Wisconsin. Since that time, it has also been found:

  • in walleye and sculpin from Catfish Lake in the Eagle River chain (Wisconsin),
  • in yellow perch in Lac Vieux Desert (Vilas County, Wisconsin),
  • in yellow perch in Robinson Lake (Vilas/Forest Counties, Wisconsin),
  • in yellow perch, walleye, and one northern pike in several lakes in Minnesota, and
  • in yellow perch from the Bay of Quinte in Lake Ontario.

These are the first North American observations of Heterosporis. Until 2000, this genus of parasite had only been reported from aquarium species such as angel fish, bettas and cichlids, and the Japanese eel.

Further research

Dr. Dan Sutherland is a parasitologist at the University of Wisconsin - La Crosse and has been contracted by the Wisconsin DNR and Minnesota DNR to study this new parasite. In 2001, Dr. Sutherland exposed several species of fish to the parasite's spores (either suspended in water or fed pieces of infected muscle) to see if they were susceptible to being infected by the parasite. He found that rainbow trout, channel catfish, walleye and fat head minnows are all very good hosts for Heterosporis. Largemouth bass and blue gills could be infected, but the degree of infection was less severe. In 2002, he hopes to continue these studies with other fish species. He also hopes to learn whether Heterosporis spores can survive and pass through the digestive tract of fish eating birds such as herons, cormorants and loons. If this can occur, birds may be one vector that helps spread the parasite from lake to lake.

Controlling the spread

Heterosporis is a microsporidan parasite that is actually more closely related to fungi than to other common fish parasites such as yellow grub or black spot. Lab studies in Europe have shown other species of Heterosporis can survive up to a year in water in a refrigerator. However, as a group, microsporidans are negatively affected by dry conditions. Until other lab studies can be done to determine the best way to kill or inactivate the parasite's spores, Wisconsin DNR recommends the following to control the spread of this new parasite:

  • Do not throw infected fish back into a lake or other natural water body. Instead, place the fish in the garbage or bury them.
  • Thoroughly dry all equipment (outside of boats and trailers, nets, boots, etc.) when moving from one water body to another. Heterosporis can survive under moist conditions, but are vulnerable to dry conditions (dessication).
  • Drain all live wells and bilges away from lakes and rivers, on soil if possible so the water does not run into a natural water body. Because it is difficult to dry live wells and bilges completely, these areas can be disinfected with a bleach solution (one cup bleach in five gallons of water).

For more information on Heterosporsis, please contact Megan Finley or Bridget Baker

Last revised: Monday April 06 2015