Written by MNWR Aquatic Biologist Dom Bachman/Photos by Dom Bachman
Trout in the Blitzen River are contending with a newly arrived parasite: gill lice. These parasitic copepods look like clear grains of rice; female lice attach to the gills (or sometimes fins) of fish to suck their blood.
In 2025, they were discovered parasitizing fish in at least three lakes in the area: Fish lake, Chicohominy Reservoir, and Malheur Refuge’s Krumbo Reservoir. By 2026, they were discovered on Blitzen river trout, including planted triploid rainbow trout and native redband trout.
Refuge staff have observed some trout with parasites and others without. Of the captured trout examined this year, 32% had at least one gill louse. Average lice per infected trout was 3.18. The most gill lice found on a single trout was 36.
Below are the main questions we have for these copepods at Malheur NWR:
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What and where is the source of origin?
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Causal factor for spread
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Introduced – possibly from hatchery fish?
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Endemic – Has it always been here?
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What level of infection are we in?
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Reservoirs/Lakes – Krumbo Reservoir, Fish Lake, and possibly others not within the Blitzen River watershed
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Blitzen River
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How fast is the population of copepods spreading throughout the Refuge?
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Establish more robust surveys/monitoring
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Can we measure fish health in relation to parasite load?
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Refuge is currently documenting copepod number/fish and fish weight at this stage
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Gathering this baseline data will inform Refuge mitigation efforts as well as those of our partners in the area. From there we can then start talking about restoration projects, research development, possibly finding a biological control, etc. Fishermen should clean boats, waders, and fishing gear to help prevent the transfer animals from one place to another. If anyone sees them in a new place, please let us know.
Can you eat the fish? Yes, you can eat the fish. Can these parasites hurt humans? No.
For more about gill lice in Harney County, listen to this podcast featuring Dave Banks, Oregon Department of Fish and Wildife Malheur District Fish Biologist.
