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When something looks fishy - Yellow and Black Spots On Fish

Throughout the year, North Dakota Game and Fish Department staff field an array of calls and questions on oddities in the fish and wildlife world. Just the deer season alone results in many calls about rank meat, abnormal growths or sick looking deer.

As spring and summer fishing gets into full gear, inquiries turn to the water. before I explain some of the more common concerns, understand that fish, similar to wildlife and people, can become ill. One odd fish caught, or dead fish observed washed up on a shoreline, probably doesn’t need to be reported.

However, instances of multiple abnormal or dead fish should be reported to Game and Fish. It may be a normal occurrence, but an assessment of the situation is needed to  determine if it was caused by some type of environmental disruption.

Here’s a summary of the more common fish ailments that anglers might encounter.

Lymphocystis or the appearance of fish “warts,” is one of North Dakota’s more commonly reported viral diseases in fish. The “warts” are actually pools of viral cells. After the disease runs its course, usually in 3-4 weeks, cells slough off, releasing viral articles into the water. Because these particles can infect other fish of the same species, it’s best to keep (not release) fish with lymphocystis, unless keeping them would be illegal.

Although a fish with lymphocystis is unsightly, the disease doesn’t infect the flesh, but it’s typically not fatal to the fish, and is not known to transmit to humans. Thus, the flesh of the fish is all right to eat as long as it’s cleaned, prepared and cooked properly.

Bacteria are common in North Dakota waters, and when conditions are right, columnaris and aeromonas  bacteria can cause infections in numerous fish species. Columnaris appears as a grayish-white patch on the fish’s skin. If a columnaris infection persists, the patch may develop into a bloody, reddish lesion and the fish may eventually die.

Aeromonas often appears as a reddish lesion, and is difficult to distinguish from an advanced columnaris infection. These bacteria are always present in the environment, and only cause problems when fish become stressed, such as after spawning or during periods of low water flows and high water temperatures.

In recent years, columnaris and aeromonas bacteria have likely caused catfish and carp die-offs on the Red River.

If you catch a fish with a bacterial infection on the skin, the flesh of the fish should be edible when cleaned, prepared and cooked properly.

A fungal infection may appear as gray-white mats on the skin. Fungal infections can occur when fish are stressed or injured, especially when their slime coating is damaged. The fungus can progress to the point where it covers, and eventually kills, the fish.

Since the mucous (slime coat) is a fish’s first defense against fungal or bacterial infections, anglers should take special care to not damage any fish you plan to release. If you harvest a fish with a fungal infection, the flesh should be edible if cleaned, prepared and cooked as usual.

In walleye, sandy flesh (myofi brogranuloma) is a form of muscle degeneration often compared to muscular dystrophy in humans because it has similar symptoms. An affected fish shows no external symptoms or abnormal behavior. However, the fillets will have a rough, sandy texture that resembles freezer burn. The flesh may range from slightly discolored to yellowish-brown.

Although there is no known link between in fish and muscular dystrophy in humans, the cause of sandy flesh is not known and it’s recommended that you do not eat an infected fish.

Doug Leier is a biologist with the Game and Fish Department. He can be reached by


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