Unsightly Fish Diseases Don’t Always Make Fillets Unfit to Eat

   Few conversations are more futile than trying to persuade someone to eat a fish that’s scarred by a virus, parasite, bacteria or other nasties.

   The Wisconsin Department of Natural Resources recognizes that challenge, which is why it includes this explanation on its “Fish Diseases” (https://dnr.wisconsin.gov/topic/Fishing/fishhealth) web page: “Even though we are reporting on fish disease in Wisconsin, we encourage you to continue to enjoy your fishing experience. … You can enjoy catching and eating your fish as long as you fully cook them before eating.”

   The DNR, of course, is dutybound to inform anglers about fish diseases they might see while catching everything from bluegills to muskies. Most fish the DNR manages get caught, unhooked and cleaned by everyday anglers like you, me, friends and neighbors. Experienced anglers calmly assess fish wounds, and don’t react as if their grandkid’s crappie is the world’s first cold-blooded creature with anthrax. We see most fish injuries, lesions, blisters or ulcerations, slice them away, and drop them into the garbage.

   Besides, when we eat fish in restaurants or buy it at a store, we realize all those issues have long been removed by people we’ll never meet. So, we might get a little testy when loved ones think some stranger oceans away is a better fish inspector than we are.

   We’re also happy to help beginners or inexperienced anglers identify unusual marks on fish. In most cases, it’s a common blight that’s long plagued local fish. Two common culprits are blackspot (neascus) and yellow grubs (clinostomum), both parasites. With a little foresight, filleting and flicking, we routinely prevent both from becoming a distraction in the boat or on the plate.

Panfish like crappies, bluegills and perch sometimes carry unsightly parasites like blackspot or yellow grubs, which are harmless if their fillets are fully cooked.

— Patrick Durkin photo

   As its name suggests, the blackspot parasite creates tiny black dots on the skins of many fish species, but it’s seen mostly on bluegills, crappies and other panfish. The black dots are small cysts with a tiny worm inside. When skinning infected fish, you remove most of the blackspots. But in heavily infected fish, many blackspots can remain in the flesh.

   Advice: If anyone asks why their fish has all those pepper dots, say “harmless parasite.” If you say “tiny, encapsulated worm,” you’re in for a long day. Just fillet the fish, remove its skin, batter the fillet and thoroughly fry it. The finished product will taste fine, especially with lemon and tartar sauce.

   Yellow grubs are usually easier to deal with, unless your fish is loaded with them. These harmless grubs resemble those found in golden-rod galls some anglers collect for icefishing bait. Although I’ve found yellow grubs most often in the flesh of yellow perch caught in shallow, weedy waters from northern Wisconsin to Ontario, I occasionally find them in bluegills, crappies, pumpkinseeds, and largemouth and smallmouth bass.

   Advice: If fishing with someone squeamish, sequester yourself when cleaning suspect perch. Once alone and piling up fillets, simply flick out the grubs with your fillet knife and drop all evidence in the trash.

   Yes, you can leave grubs in the meat and fry them into oblivion, but what if a prissy friend spots a grub’s outline while you’re battering the fillets? Again, you’re in for a long day.

A disease called black crappie sarcoma has been present in the Madison lakes and waters in Polk and Barron counties in northwestern Wisconsin. It’s known to infect only black crappies. —Wisconsin DNR photos

   Among calmer folks, you can explain that both of these parasites require birds, snails and fish as hosts to complete their lifecycles. And because you’re a responsible angler, you bury or properly dispose of all bones, skins and innards from your catch to end these parasites’ time on Earth. You never throw the waste and remnants into the water or brush to extend the parasites' lifecycle.

   Still other fish diseases can look more gruesome, and involve bleeding, bulging eyes, and skin ulcers or blisters — on fish, not you. Other signs are frayed fins, ragged gills, and white fungus-like blotches. If the scars are caused by a columnaris bacteria, VHS (viral hemorrhagic septicemia), or lymphosarcoma, you can clean the fish and safely eat it as long as you fully cook the meat.

   Still, those diseases can be fatal to some fish species. Lymphosarcoma, for example, is a transmissible virus-caused cancer that affects muskellunge and northern pike. Its tumors often regress in pike, allowing them to survive. The disease is usually more severe in muskies, and can eventually kill them.

   Meanwhile, black crappie sarcoma continues to afflict this fish species with its raised, red, sometimes bloody sores on a black crappie’s fins, operculum or mid-body skin. The operculum is that white, wrinkly trapdoor in the back of a fish’s throat that separates its mouth from its stomach and interior organs.

The sore on this bluegill, which is healing, was likely caused by a columnaris bacteria during late spring. — Walt Larsen photo

   The DNR recommends not eating fish with black crappie sarcoma because science hasn’t determined how it affects humans. This disease, likely caused by an “underlying virus,” was first documented in the 1980s in Madison’s lakes and dozens of waters in Polk and Barron counties. Cases have also been found in Adams, Pierce and St. Croix counties, and it’s suspected to be in Wood, Shawano, Columbia and Winnebago counties. Prevalence rates varied from 1% to as much as 30% in lakes where it’s been found.

   Kyle Broadway, the DNR’s fisheries manager for Polk and Barron counties, said there’s “nothing really new on the black crappie sarcoma front.” Broadway said the agency doesn’t have current infection rates for lakes in those counties, but continues to collect and test samples. This sarcoma has only been found in black crappies, and typically only in larger fish. Similar sores on bluegills are usually caused by the columnaris bacteria.

   As with other fish diseases, there’s little or nothing agency biologists can do to control or manage black crappie sarcoma. Further, it’s also difficult to tell one fish lesion from another without a test. All that’s known is the DNR has no evidence of black crappie sarcoma infecting other fish species. When in doubt, euthanize the fish, dispose of the carcass in your garbage, or bury it away from water.

   And just remember: Even if you dispose of a fish, it still counts toward your daily bag limit.

Hmm. Something tells me you just winced.