Over the past few weeks anglers have been reporting “black spots” found in the fillets of rainbow trout caught from H.C. Kuhr Reservoir, located south of Chinook.

Black spot, or neascus, is a parasite that spends a portion of its life cycle in fish. The parasite’s eggs are released into the water by fish-eating birds. The eggs develop into intermediate stages of the parasite in snails. The free-swimming parasites then penetrate the muscles of fish and become enclosed in cysts.

Black pigmentation is deposited onto these cysts, hence the name black spot. Infected fish are consumed by birds, and the life cycle starts all over again. Adult worms also can be found in fish-eating birds.

“Black spot occasionally occurs on the Hi-Line,” said Cody Nagel, a Havre-based biologist with Montana Fish, Wildlife & Parks. “Fish tend to become more susceptible to the disease when water temperatures are high for a prolonged period of time and they’re stressed.”

Many species of fish can become hosts to the black spot parasite. On the Hi-Line the most common carriers tend to be fathead minnows, yellow perch and rainbow trout.

“Although it doesn’t look appetizing, the fillet is safe to eat if it is thoroughly cooked,” Nagel explained. Currently, there are no practical means to control this parasite and the bodies of water it may affect.

Logo courtesy of the Montana Fish, Wildlife and Parks

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