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Gill Lice Study

Updated: Jun 2, 2021

The Idaho Department of Fish and Game and the Henry’s Fork Foundation launched a study in 2016 investigating gill lice in the Upper Snake River region after receiving reports of anglers observing gill lice in rainbow trout in the Henry’s Fork. Anglers submitted data on the trout they caught on the Henry's Fork and surrounding water bodies in 2016.

Background information

Prepared by Christina Morrisett, in coordination with Reid Calhoun, intern from Washington & Lee University, and Harrison Carter from Westminster College.

Gill lice have recently been found in the Henry’s Fork. Native to the region, this particular species of gill lice is likely Salmincola californiensis, a parasite that infects fish in the Salmonidae family, including rainbow trout, cutthroat trout, mountain whitefish, and kokanee salmon. Recently, anglers throughout North America have encountered gill lice more and more often. Because we are seeing warmer rivers due to climate change, gill lice have been able to reproduce more rapidly and efficiently. On the Henry’s Fork, river temperatures in early spring were some of the warmest on record. Paired with low flow below Island Park Dam during peak spawning season when fish are most crowded, gill lice populations proliferated and were able to establish a visible presence on the Henry’s Fork.

How do gill lice affect trout?

Mature female gill lice primarily attach to the gills of the fish, but can also be found in the mouth or near the fins. Upon attaching to their host, gill lice carve depressions in these soft tissues, making the fish more vulnerable to bacteria and fungi. In this way gill lice also weaken the salmonid’s ability to breathe, particularly when infestations are chronically high, which can impair fish behavior, growth, and ultimately survival. If water temperatures are too warm (above 68⁰F) or the river’s dissolved oxygen levels are too low (4 mg/L if temperature is below 55⁰F, 6 mg/L if temperature is above 55⁰F), fish become lethargic and weak – further exacerbating the effect of gill lice. Additionally, exercise or stress (i.e. a fight with an angler) in the presence of a heavy Salmincola infestation can further prevent the fish from obtaining sufficient oxygen, and potentially lead to death. While infestations can lead to death in some circumstances, most observations in the wild have documented more benign impacts, particularly when infestations are low.

More information

Prepared by Bryce Oldemeyer

What are gill lice?

Gill lice are a parasitic type of copepod, which is a large subclass of common marine and freshwater organisms related to shrimp and crabs. Although we have not definitively identified the species present in the Henry’s Fork, they are most likely Salmincola californiensis, which is native to streams west of the Continental Divide but has spread widely throughout the U.S. This species is known to infect rainbow trout, cutthroat trout, kokanee salmon, and other species of salmonids native to western North America. The lice have a simple life cycle that involves only one host—a salmonid fish. After hatching, the larval lice, called copepodids, swim freely in the water for a period of 2-17 days in search of a host. They usually attach to the gills of the host, after which they undergo several molts to become reproductively mature. After fertilizing the eggs of a female, the male detaches from the host. The female remains attached for the remainder of her life and can produce two broods. Eggs are contained in two sacs, which are about the size of small grains of rice and are readily visible on an infected fish. As with most cold-water aquatic organisms, growth and reproduction of lice ceases when water temperatures are too cold (less than about 40 degrees) and increases as the water warms, up to about 70 degrees.

How can gill lice affect trout?

Typical lice infestations are light—less than 10 individuals per fish. At this intensity, there is little long-term effect on the individual fish. However, the lice inhibit the fish’s ability to take up oxygen through its gills, so as the number of lice increases, the potential effect on the fish increases, especially in water with low dissolved oxygen or if the fish undergoes strenuous physical exertion. Very heavy infestations can reduce survival and reproductive ability of individual fish. At the population level, the additional stress of the parasite can allow non-susceptible fish species to gain a competitive advantage over the susceptible species. For example, a species of gill lice that infects brook trout but not brown trout has been suspected to have accelerated displacement of native brook trout in Wisconsin streams by nonnative brown trout. In both Wisconsin and Colorado, lice infestations have been linked to warming water temperatures. Although some biological and chemical treatments have been proven effective at reducing lice infestations in fish hatcheries, there is currently no feasible treatment for lice in wild trout populations, other than maintaining good streamflow, water temperature, and habitat conditions for wild trout.

Summary of data July 2016 - January 1, 2017

  • Total number of surveys: 78

  • Breakdown of surveys by location:

    • 41 from Harriman Ranch

    • 26 from Henry’s Fork (Box Canyon, St. Anthony, etc.)

    • 6 from South Fork Snake River

    • 2 from Robinson Creek

    • 2 from Henry’s Lake

    • 1 from Fall River

  • Total number of fish sampled: 242

  • Results by species:

    • 189 rainbow trout sampled, gill lice observed in 45.

    • 39 rainbow trout with light infestation (< 50% of the gill edge covered)

    • 6 rainbow trout with heavy infestation (> 50% of the gill edge covered)

    • 37 brown trout sampled, no presence of gill lice found

    • 16 cutthroat trout sampled, no presence of gill lice found

Figure 1. Number of Rainbow Trout sampled in the Upper Snake River region from July 1 through January 1, 2017, partitioned by Harriman Ranch reach (Harriman), all other reaches of the Henry’s Fork outside of Harriman Ranch (Henry’s Fork), and other rivers within the Upper Snake River region (Other)

Results Summary

Reports of gill lice are primarily isolated to Rainbow Trout in the Harriman Ranch reach. One third of Rainbow Trout sampled in the Harriman Ranch reach had gill lice present and four cases were classified as having a “heavy” infestation. The percentage of Rainbow Trout reported with gill lice in other reaches of the Henry’s Fork and Upper Snake River region declined to 4% during October through December (down from roughly 20% during July through September). It is difficult to say whether this decline is product of a seasonal change but we hope to be able to identify temporal trends in gill lice prevalence, severity, and distribution as this study progresses.


Gunn, C. (2012). SALMINCOLA IN COLORADO. Retrieved from

Hargis, L. N., Lepak, J. M., Vigil, E. M., & Gunn, C. (2014). Prevalence and intensity of the parasitic copepod (Salmincola californiensis) on Kokanee salmon (Oncorhynchus nerka) in a reservoir in Colorado. The Southwestern Naturalist, 59(1), 126-129. doi:10.1894/n06-jc-72.1

Vigil, E. M., Christianson, K. R., Lepak, J. M., & Williams, P. J. (2015). Temperature effects on hatching and viability of Juvenile Gill Lice,Salmincola californiensis. Journal of Fish Diseases, 39(7), 899-905. doi:10.1111/jfd.12422



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