Published July 18, 2009, 01:00 PM

Black spots on area fish aren’t harmful

“What are all of those black spots on this fish”? The guide clients’ faces contorted in disgust as they examined the small sunfish dangling from the end of the fishing line at arms length.

By: Jason Durham, Park Rapids Enterprise

“What are all of those black spots on this fish?” The guide clients’ faces contorted in disgust as they examined the small sunfish dangling from the end of the fishing line at arms length.

If you fish at all, you’ve probably encountered one of these speckled specimens, their scaly skin rough with tiny black dots.

Called Neascus or black grub, this tiny parasite is released into the water, in egg form, by fish-eating birds. Those eggs hatch and the parasite grows inside of snails.

The Neascus parasite matures and then swims freely, attaching to fish and penetrating into their muscle, eventually forming a cyst that looks like a small black spot.

Birds eat the fish and the life cycle simply begins once again.

The guide client, reluctant to grab the spot-covered bluegill to remove the hook, inquired, “Can you eat fish with these spots”?

According to the Minnesota Department of Natural Resources, fish fillets with Neascus present are not harmful to humans, so long as the fish is well cooked.

Another client in the boat added his presumptions regarding the parasite. “I heard that the spots are more common in fish that live in very clear lakes, not lakes that have some color or stain.”

In reality, water clarity has little correlation with the prevalence of Neascus, though certain bodies of water have more fish with the parasite.

Doug Kingsley, Park Rapids Area Fisheries Supervisor, says lakes with greater numbers of fish eating birds combined with a strong population of snails harbor more of the parasites simply due to those two “hosts” which are integral to Neascus’ life cycle. “Eutrophic (fertile) lakes sometimes have more fish with the black grub (Neascus) simply because there is more vegetation for snails and greater natural habitat along the shoreline for fish eating birds,” says Kingsley.

He also adds that fish that are in poor condition or stressed are at greater risk for acquiring Neascus. Although the parasite is not fatal, it could further contribute to the poor condition of a fish.

Fortunately for anglers and those who dine upon freshwater fish, the parasite usually burrows just beneath the skin, so those flaky white fillets everyone desires are either not affected or the Neascus can be easily removed with the tip of the fillet knife. But again, if prepared properly, the parasite is not harmful to humans and does not affect the look or flavor of a fish meal.

Though all species of freshwater fish can host the Neascus, it is most commonly found in bluegill. However, it’s not uncommon to catch an occasional perch, bass or northern pike with the black spots. Yet, it is rarely seen on muskie.

In other words, while filleting fish, don’t worry about the spots and speckles, but simply look forward to entertaining your guests at your next fish fry.

For more information on fish diseases, check out the Minnesota DNR Web site at www.dnr.state.mn.us/

fish_diseases/index.html.

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