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Gill Lice Detected in Rainbow Trout in Virginia

An image of a fish with gill lice; which are highlighted via a green circle

Gill lice, highlighted here inside the circle.

Fisheries biologists with the Virginia Department of Wildlife Resources (DWR) recently identified gill lice on rainbow trout in Blue Springs Creek, a tributary to Cripple Creek, located in Smyth and Wythe Counties.

Gill lice are tiny, parasitic copepods, a type of zooplankton, that attach to a fish’s gills, mouth, and fins. The gill lice recently found in southwestern Virginia is likely Salmincola californiensis, a species that specifically uses rainbow trout and related species (cutthroat trout and Pacific salmon) as its host. Another species of gill lice, Salmincola edwardsii, only infects brook trout. DWR biologists collected gill lice samples and have submitted them to the U.S. Fish and Wildlife Service’s Lamar Health Center for identification. Since they were found on rainbow trout, however, they are likely Salmincola californiensis and pose little threat to native brook trout populations at this time.

A minor gill lice infection can generally be tolerated, but a heavy infection can have a negative impact on a fish’s ability to breathe. The degree of impact to a trout population can depend on the level of infection and the presence of any other stressors such as high water temperature or drought conditions.

An image of a fish with gill lice once again highlighted with a green oval

Gill lice, highlighted here inside the circle.

Gill lice can be unsightly and may make anglers reluctant to eat an infected trout. However, trout infected with gill lice are safe to eat as long as the fish is cooked properly.

Given the native distribution of Salmincola californiensis in the Pacific Northwest, its introduction to waters east of the Continental Divide has likely occurred concurrently with the introduction of rainbow trout and other host species to non-native waters. The specific source of the introduction to Virginia waters is unknown at this time. DWR staff are currently working to determine whether gill lice are present in other trout waters of the state or if this is an isolated occurrence. After the extent of gill lice infections has been assessed, DWR will develop a plan to limit the spread to uninfected waters and to reduce the impacts in waters where gill lice are found.

Anglers can assist DWR by reporting trout infected with gill lice to: Although gill lice are likely transferred through direct fish-to-fish contact, anglers are encouraged to clean and dry their fishing equipment after use, especially if they intend to travel to different waters. This is a good practice in helping to stop the spread of any aquatic invasive species.

Updated: March 23, 2022