Henrys Lake Quarterly Newsletter - June 2020

Henrys Lake Quarterly Newsletter
June 2020





Many of you may have noticed that the hatchery has been closed to the public and our IDFG presence has been reduced around the lake in the last few months. In response to guidance from Idaho Department of Health and Welfare, all IDFG hatcheries remain closed across the state to limit the risk of transmitting the COVID-19 virus to both department staff and local residents until further notice. Our goal is to continue to serve our sportsmen and women while keeping safety at the forefront of our work.



Population Analysis

The Idaho Department of Fish and Game (IDFG) annually monitors fish populations in Henrys Lake using gill nets. Each year, IDFG sets gill nets as soon after ice-off as possible to obtain current information on the fish population in Henrys Lake. Due to reduced staff and changes in our work items in response to COVID-19, IDFG was unable to conduct a full gill net survey this year. However, an IDFG-supported University of Idaho graduate student is conducting research on Henrys Lake including using gill nets which allowed us to compare some gill net catch data from 2020 with previous years, although fewer gill nets were deployed this year, with a total of 30 gill nets versus the normal 50.

A capture rate of 14.9 trout per net was estimated for 2020. This is above both our management goal of 11 trout per net and our long-term average of 11.7 fish per net.

Henrys Lake Trt/net night 91-2020
Creative Commons Licence
Jenn Vincent, IDFG

Figure 1: The average number of trout by species per net night from 1991 to 2020. YCT: Yellowstone Cutthroat Trout, HYB: Yellowstone Cutthroat x Rainbow Hybrid Trout and BKT: Brook Trout.


The total number of trout captured during our 2020 gill net survey was the highest observed since our 2014 gill net survey. The breakdown of sizes of each species of trout captured is depicted below. The largest percentage of our catch was in the 14 - 18 inches in length for each species of trout. This indicates a large presence of age 2 and age 3 trout in the lake and points to a positive fishing forecast for this season and the future.


Henrys Lake Trout frequency 2020
Creative Commons Licence
Jenn Vincent, IDFG

Figure 2: The frequency of trout per total length (inches) per species. Yellowstone Cutthroat Trout (YCT), Hybrid Trout (HYB) and Brook Trout (BKT). The table shows the average length (inches) per species captured during the 2018, 2019 and 2020 gill net surveys.




Henrys Lake Graduate Study
by Darcy McCarrick University of Idaho

As mentioned above, University of Idaho personnel conducted spring gill net work this year as part of a study investigating Utah Chub and Yellowstone Cutthroat Trout interactions. The purpose of this year’s gillnetting effort was two-fold: (1) collect population monitoring data and (2) obtain data associated with the selectivity of gill nets. IDFG provided an update on the annual population monitoring and trends. I will briefly highlight the gill net selectivity. The selectivity portion of the effort was to gather information that will help managers better understand how well the nets sample different sizes and species of fish in the lake. To achieve these goals, a combination of IDFG standard gill nets and customized American Fisheries Society gill nets (customized AFS gill nets) were used. Customized AFS gill nets were 90’ long, 6’ deep, and had nine panels of randomly ordered mesh (i.e., ½”, ¾”, 1”, 1¼”, 1 ½”, 1¾”, 2”, 2¼”, 2½”). For comparison, IDFG gill nets were 150’ long and 6’ deep and had six panels of different sized mesh that were ordered smallest to largest (i.e., ¾”, 1”, 1½”, 1¼”, 2”, 2½”).

Hybrid Trout 2020
Creative Commons Licence
Darcy McCarrick

Darcy with the largest fish caught during the May 2020 survey was a hybrid trout that was 28.3 inches long and 8 pounds 10.7 ounces.

Preliminary results from 2020 gill netting efforts are summarized in the table below. Total number of fish is reported as well as the number of trout per 150’ of mesh and the number of Utah Chub Gila atraria per 150’ net (Table 1). Interestingly, we caught low numbers of Utah Chub in the ½” mesh. The ½” mesh size was added to the AFS experimental gill nets because it captured high numbers of Utah Chub in other systems. During our entire spring sample, only three Utah Chub were caught in the ½” mesh. On average, the customized AFS caught more trout per 150’ of mesh than the IDFG nets (Table 1). However, the pattern was reversed for Utah Chub (Table 1). A summer and fall gill net sample will be completed this year to continue the gill net selectivity assessment. The next gill netting event will begin July 6th and will last approximately two weeks.

Henrys Lake gill net summary 2020
Creative Commons Licence
Darcy McCarrick

Table 1. Results from the May 2020 gill net survey on Henrys Lake, Idaho. We conducted 30 net nights with Idaho Department of Fish and Game standard gill nets (IDFG standard) at six historic sites, 30 net nights with customized American Fisheries Society experimental gill nets (customized AFS) at six historic sites, and four net nights with customized AFS nets at four shoreline locations. The number of fish caught by species is reported: Yellowstone Cutthroat Trout (YCT), Rainbow x Yellowstone Cutthroat Trout hybrids (HYB), Brook Trout (BKT), and Utah Chub (UTC). The number of trout and Utah Chub per 150’ mesh are also reported.

Another ongoing component of the project is an evaluation of movement and habitat use of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and Utah Chub. This year we tagged 44 Yellowstone Cutthroat Trout and 45 Utah Chub using a combination of angling, trap nets, and electro-fishing. On average, Yellowstone Cutthroat Trout were 16.3 inches (SD = 1.9) long and varied between 12.4 and 22.1 inches. Utah Chub were 11.6 inches (SD = 1.3) long on average and varied between 9.6 and 14.5 inches. We will spend the remainder of the summer tracking fish movements and conducting habitat assessments. Look for more results and updates on this research in the next newsletter.





Soooo......how was the fishing on opening day weekend?

For the brave folks who were not discouraged by the weather on opening day brought in a number of large fish. For those who waited until the snow disappeared, you were all also very lucky. Fishing at Henrys lake this year has been quite fruitful and our anglers have been very happy with both the number and size of trout they are catching on the lake. It seems as though fishing on Henrys Lake this year will be one for the books as I am still hearing great fishing reports out there!

For opening weekend (May 23 and 24) 2020 we estimated a catch rate of 1.80 trout/hour. This catch rate is even higher than our estimate of 1.02 trout/hour for 2019 which itself was a great fishing year for our anglers.

An estimated total of 6877 angler hours were spent on Henrys Lake this past opening weekend which is higher than the 4965 angler hours estimated on opening weekend in 2016.

The largest hybrid checked in by our creel clerks was just over 29 inches in length.

Henrys Lake Creel Summary 2020
Creative Commons Licence
Jenn Vincent, IDFG





On June 24th and 25th 2020, approximately 250,000 sterile Hybrid Trout were stocked into Henrys Lake at 3 inches in length.

Wondering why our Hybrids are now being stocked in June? Check out the "Ask a Biologist" section below to find out.




Upcoming June to September 2020

  1. Riparian fences and fish screens: Our crews have been working vigilantly over the last few weeks installing and maintaining over 100 miles of electric and jack fences on the tributaries of Henrys Lake. These fences help to protect riparian areas and allow them to rehabilitate these important stream corridors. In addition, we maintain 11 fish screens around the lake which keep fish out of irrigation canals. Due to the excellent collaboration with multiple landowners around the lake we are able to protect important trout spawning and rearing grounds.
  2. Stocking: Approximately 1 million Yellowstone Cutthroat Trout and 150,000 sterile Brook Trout are scheduled to be stocked in Henrys Lake during the last week of September 2020.
  3. Graduate study: As mentioned above, a graduate student with the University of Idaho will be conducting another round of gill netting on the lake in early July. Her crew will also be tracking fish in the lake throughout the season.




Ask a Biologist
At the end of each newsletter we will have a section addressing any questions you may have. Please send any new questions to

Q: Why have you switched stocking the Hybrid Trout in the lake in June while the Yellowstone Cutthroat are still stocked in the fall?

A: In 2019 we switched our rearing facility for our Henrys Lake Hybrid Trout from the IDFG Mackay Fish Hatchery to the IDFG American Falls Fish Hatchery. The water temperatures at American Falls are warmer than those of Mackay. With increased water temperatures we are able to grow our fish at a faster rate. This way the trout are able to grow to 3 inches by June instead of 3 inches by September as done in the past. This allows these Hybrid Trout to have additional growth opportunities before their first winter which should increase survival rates. Over the past few years, water temperatures have stayed quite cool in Henrys Lake and have allowed us to minimize stocking temperature stress, allowing the stocked hybrids time to acclimate and grow throughout the summer and fall.

Henrys Lake Yellowstone Cutthroat Trout (YCT) are reared at the Mackay Fish Hatchery. Due to the large stocking request of around 1 million fish each year, Mackay is the best facility to accommodate the space needed for this request and the appropriate water temperatures ideal for this species. In their natural environment, YCT are reared at cooler temperatures. Although the colder temperatures result in slower growth rates. We monitor the temperatures of Henrys Lake in the fall prior to stocking to mitigate any loss of stocked trout due to temperature shock when stocked into the lake. By stocking the YCT at the end of September, there is adequate time for these fish to acclimate to the lake and grow before ice-on.


You can access the March 2020 edition of our newsletter here:

Our next quarterly newsletter will be sent out in September 2020 so stay tuned!