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Fish Parasites

Thousands of copepod species are fish parasites; this is one of them. Photo by Jeff Gunderson

Thousands of copepod species are fish parasites; this is one of them.

Chances are if you go to a restaurant that serves sushi, the raw fish in front of you will have been frozen or thoroughly inspected to ensure you donít end up with a parasitic infection. Donít be alarmed; fish and their parasites are as commonplace as vegetables and their insects. You might not want to, but you can eat most fish parasites without coming to harm. Still, you should cook, hot smoke, or freeze the fish you catch to ensure their parasites are well and truly dead before eating them.

Parasites can be killed by thoroughly cooking or hot smoking the fish to 140 degrees Fahrenheit or by freezing the fish flesh at 0 degrees for 48 hours.

Tapeworms are one of the fish parasites that can infect humans who have consumed un- or undercooked fish and even sometimes newly pickled fish and ceviche. Because they are easy to see, "grubs" (more formally known as "flukes") are the most commonly reported parasite living on fish. Like many parasites, their life histories are fascinating. The most common flukes in Minnesota fish are:

  • Yellow grub (Spends time in the mouths of herons, aquatic snails, and fish.)

  • White grub (Found in the liver, heart, and other internal organs of sunfish as numerous white "specks.")

  • Black grub (Spends time in the intestines of kingfishers, aquatic snails and fish.)

Fish can also develop fungal infections that look like white hairs on the body. If you catch a fish that has discolored flesh, tumor-like lumps, or has something akin to a rash, it could be infected with a virus; as a general rule, if it looks bad, donít eat it. Removed tainted sections of the fish before preparation.

Lymphocystis is a virus that causes lumpy growths on fish, particularly in walleye. Photo by Jeff Gunderson

Lymphocystis is a virus that causes lumpy growths on fish, particularly in walleye.

You can do your part to help prevent fish from getting infected. Fish secrete a mucous coating over their entire bodies that wards off fungal, viral and bacterial infections. If the coating is damaged, the fish will be more susceptible to infection. Anglers can help prevent this by taking extra care when returning fish to the water. When you catch a fish you plan on releasing, remove the hook while the fish is still in the water or wet your hands before handling the fish. You should also release the fish as gently and quickly as possible after removing the hook. While you should always clean, drain, and dry your boat after going fishing, you can also do other things to make sure diseases and parasites wonít cross over into the next lake you visit. Dispose of your unwanted baitfish and fish parts in the trash, donít harvest fish for bait from an infested lake, and donít move water between waterbodies.

For more on the parasites of freshwater fish, visit Minnesota Sea Grant online at: www.seagrant.umn.edu/fisheries/parasites.


By Emily Kolodge
May 2013

Return to May 2013 Seiche



This page last modified on March 23, 2017     © 1996 – 2017 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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