Gill Lice Study Update

The following information was prepared in coordination with Reid Calhoun, intern from Washington & Lee University, and Harrison Carter from Westminster College.

What is happening?

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.

What is the Henry’s Fork Foundation doing about it?

While gill lice may be benign in small numbers, the Henry’s Fork Foundation is concerned about the possibility of explosive population growth in the future and its effect on local salmonids. Because this discovery in the Henry’s Fork is so recent, we are still working to understand the breadth of this infestation and how best to deal with it. Thanks to a proactive angling community, it appears that we caught this issue fairly early and are working to responsibly manage this issue. Due to our water quality sonde network, we have years of data which can help us to better understand how this issue has come about. We are also using angler reporting to track where gill lice infestations are occurring and what proportion of the trout population is affected. Data from angler reports and our water quality network will guide us in coming up with a solution.

More specifically, after digging into scientific literature, we have identified three risk factors for gill lice proliferation – warm water, low dissolved oxygen, and crowding of fish into small areas. To minimize these risks, we need to ensure cool water, high dissolved oxygen, and higher flows to avoid fish crowding, especially during the spring spawning season. We already pursue these three objectives in our daily work. Through data collection over the past two years, we have learned that certain conditions in Island Park Reservoir keep outflow temperatures cooler. Although we have relatively little control over reservoir management, we can use this information during Drought Management Planning meetings. In terms of dissolved oxygen content, this year we worked with Fall River Rural Electric to meet its dissolved oxygen requirement for water coming out of the hydroelectric plant and have seen positive results. However, flow management continues to be difficult given the current drought cycle. Even though water supply was constrained and runoff was early this year, we worked with the U.S. Bureau of Reclamation and Fremont Madison Irrigation District to maximize flow as much as possible in April and May. Of course, we recognize that addressing flow management is important to addressing not only the issue of gill lice, but other issues of concern on the Henry’s Fork as well, and are working on more long-term solutions.

What have we learned so far?

Most of the angler reports we have received so far (1 July to 24 July) focus on the Last Chance and Harriman State Park (including Harriman East) reaches. These reports tell us that about 20% of rainbow trout caught in this reach host a gill lice infestation. Most of the trout infested in this reach have a light infestation, defined as less than 50% of gill edge area covered. Reports from elsewhere on the Henry’s Fork show that over 30% of rainbow trout are infested. However, this percentage may be high given the small sample size of seven trout.


Idaho Department of Fish and Game conducted a survey in Island Park Reservoir on 6 July and out of 25 rainbow trout inspected, 36% were infected. IDFG electrofishing surveys on the Osgood section of the lower Snake River near Idaho Falls have not encountered gill lice in the 314 total fish examined (brown trout, mountain whitefish, rainbow trout, Yellowstone cutthroat trout). Anglers fishing the South Fork have not reported any occurrence of gill lice and we have yet to receive reports from the Teton River.

How can you help?

Angler participation is important in helping us address this issue. If you fish on the Henry’s Fork, Teton River, South Fork Snake River, or any regional waters in or near the Henry’s Fork Watershed, including Montana, please check the trout you encounter for gill lice and report your finding. Data sheets can be picked up in Ashton at the HFF office and the Three Rivers Ranch fly shop or in Last Chance at Henry's Fork Anglers and TroutHunter. If you'd like more information or to submit your data online, please visit If you encounter a trout with gill lice or not, we would like to know about it! It's equally important to record the fish that DON'T have lice as well as those that do. This information will help us track the occurrence and trends in lice abundance through the summer.


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