Gill Lice Study Update #2

In June 2016, the Idaho Department of Fish and Game (IDFG) and Henry’s Fork Foundation (HFF) received reports of the presence of gill lice on trout caught within the Henry’s Fork River system. Shortly after, a collaborative effort between HFF and IDFG was underway and we asked anglers and guides to help collect data on the prevalence, severity, and distribution of the parasite within the Upper Snake River region.

Christina Morrisett, our former Research Assistant, wrote an informative piece on gill lice and provided an initial update of the study at the end of July ( Since her update, the sample size of the study has doubled. Here is a quick summary of the data from July 1 through September 1.


Survey overview

  • Total number of surveys: 49

  • Breakdown of surveys by location:

    • 33 from Harriman Ranch

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

    • 2 from Robinson Creek

    • 2 from South Fork Snake River

    • 1 from Fall River

  • Total number of fish sampled: 181

  • Results by species:

    • 141 rainbow trout sampled, gill lice observed in 37.

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

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

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

    • 7 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 September 1, 2016, 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).



With the exception of one rainbow trout sampled on Robinson Creek, gill lice have only been reported on rainbow trout within the Henry’s Fork River. Of the rainbow trout with gill lice, under 10% were classified as having “heavy lice”. It is difficult to draw inference at a finer scale (river reach specific) with our current data set, but this study is ongoing and we hope to be able to answer river reach specific questions in the future. Unfortunately, the HFF won’t pay me to hook-and-line sample all day so we continue to ask for the assistance of anglers and guides to collect data and fill out surveys.

We send many thanks to all the guides and anglers who took time to help us collect  data thus far; we couldn’t have done this without them!

For information on how you can help, click here (Gill lice survey)

For background information on gill lice, click here (

If you have questions, comments, or concerns, feel free to contact me at or 208-652-3567