Henrys Lake Newsletter – Summer 2024

Henrys Lake anglers, let’s get you caught up on recent happenings, the latest population survey, and much more! We’ve been busy this season, and there’s a lot to share. First off, a big “thank you” to the Henrys Lake Foundation for inviting me back to speak at their annual fundraising event again this year! As per usual, we had several engaging and constructive conversations related to the Henrys Lake fishery. Let’s dive in!

Annual Population Survey

To recap, IDFG conducts a gillnet survey on Henrys Lake each year to assess the current state of the fishery. We begin the survey immediately after ice-out, which occurred on May 1 this year (Figure 1), and was almost 3 weeks earlier than ice out last year in 2023! We set nets at six sites throughout the lake every night until we reach our goal of at least 50 net-nights of effort. 

In the 2024 gillnet survey, we caught 4.3 trout per net night (Figure 2), which is up from 2.9 trout per net night in 2023, but not significantly different (see confidence intervals). Individual species-specific trout catch rates are as follows: Yellowstone Cutthroat Trout (YCT) = 3.3 fish per net, hybrid trout (HYB; triploid Rainbow Trout X YCT) = 0.3 fish per net, and Brook Trout (BKT) = 0.6 fish per net (Figure 2). In this year’s survey results, there are signs for both encouragement and concern. Total trout catch rates appear to be moving back in the right direction, but we still have a long way to go. Particularly for our HYB trout, we have seen poor survival of recent year-classes, leading to declining abundance of HYB for the past few years. The bright side of poor recruitment (A.K.A. how many fish from each year class survive to the next year) is a trade-off in average size. This year, average total length of HYB was 22 inches (Figure 3), which is the largest average size on record for at least the last 20 years. In the coming years, we will be evaluating HYB stocking strategies to improve annual recruitment. 

The primary sign of encouragement I’d like to call attention to is a new year-class of YCT that has recruited to the fishery! Similar to HYB, recruitment of our stocked YCT over the last several years has been poor. While that led to some state-record sized fish, it is concerning from a population viability standpoint. This year, YCT from 10 – 13 inches (likely age-2 fish) made up the majority of the catch, and were a much-needed addition to the population. Most of the YCT in the 2023 gillnet survey were 17-21 inches, and were reaching the end of their lifespan. This strong young year-class of YCT should help prop up the Henrys Lake fishery for at least the next five years! Our BKT population is low as well, but showing moderate signs of increasing and an even distribution across all size classes. 

Genetics

In 2017, we began using some genetics tools to monitor a number of different population dynamics in the Henrys Lake fishery. One of these tools is known as “Parentage-Based Tagging” (PBT). By taking small fin clips from the adult fish we use for spawning each year (see picture below), we create a glossary of parental genetics that we can use for comparisons to the fish caught in our gillnet survey each year. Using this data, we can estimate things like: wild vs. hatchery origin, exact known age, recruitment, growth, survival/mortality, and much more! 

We don’t have the results back from the 2024 survey yet, but in 2023 9% of the YCT population was wild-origin, meaning 91% of the YCT were hatchery-origin fish, which emphasizes the importance of our hatchery supplemented program for the Henrys Lake fishery. Of those hatchery-origin fish, the oldest year-class in the population were born in 2017 (age 6; Figure 4). Of concern is the fact that the 2017 year-class was also the most abundant (Figure 4). Without a strong year-class of younger fish to prop up the population, we were due for even further declines in trout abundance. Fortunately, we saw good recruitment of the 2022 year-class in the 2024 gillnet survey. 

In 2023, we also used PBT to look at a subset of spawning YCT in Targhee Creek (n=31). Of those fish, 100% were hatchery-origin. As expected, the grand majority of spawners were from the strong 2017 year-class (Figure 5), with representation from the 2018 and 2019 year-classes as well. More to come on this subject in the future! This is only the beginning of our deep dive into tributary production dynamics. 

Current and Upcoming Projects

Creel Survey: This year (6/1/24 – 6/1/25), we are running a creel survey! Twice a week, creel clerks will be out interviewing anglers about their fishing trip that day. These data are vital for estimating total fishing effort, harvest rates, angler demographics, opinions, and much more! If you encounter one of our creel clerks, the survey should only take a couple of minutes and we greatly appreciate your participation. If you’re interested in getting involved, volunteering as a creel clerk is an easy way to do just that! Send me an email at Nathan.Tillotson@idfg.idaho.gov and we’ll get you trained up. 

Lower Targhee Creek: Over the past two years, IDFG, Henrys Lake Foundation, and other partners have been working with motivated landowners on lower Targhee Creek to plan, fund, and implement a large-scale restoration project. Targhee Creek provides incredibly valuable spawning habitat for wild YCT in Henrys Lake, as well as thermal and oxygen refuge during critical periods for fish survival. We’re breaking ground on Phase 1 of this project this year, which will include re-connecting a southern channel of Targhee Creek that was historically cut off for agricultural purposes. This phase will provide ~1.5 miles of additional spawning habitat for YCT in lower Targhee Creek, as well as improve hyporheic flow, groundwater recharge, wetland function, and expand oxygen/thermal refugia for trout in Henrys Lake. This project would not have been possible without generous contributions from both the landowner and the Henrys Lake Foundation, so they deserve a huge shout out here!

Fish Stocking: In June, we stocked 201,012 sterile hybrid trout in Henrys Lake. In September we plan to stock ~1.2 million YCT fingerlings, as well as ~100,000 BKT. Approximately 70,000 of our YCT fingerlings have been set aside for tributary stocking (Targhee Creek, Duck Creek, Howard Creek, and Timber Creek). These fish will be used as part of our tributary stocking evaluation, where we will monitor comparative recruitment between lake-stocked fish and tributary-stocked fish. We won’t be able to see the results of this study until the fish stocked this fall get big enough to be caught by our gillnets (likely 2026), so stay tuned! 

Electric Fences: Back in June, we installed miles of electric fence along the riparian areas of several of the Henrys Lake tributaries (Howard Creek, Targhee Creek, Duck Creek, and Kelly Creek). These fences protect the stream and riparian areas from habitat degradation. Throughout the summer, our IDFG Henrys Lake seasonal crew continually maintains these fences to ensure they remain “hot” while livestock are in the area. At the end of the season when cattle are moved elsewhere, we take down the fences for the winter to allow wildlife to freely move around the lake and tributaries. 

Fish Screen Maintenance: IDFG manages and maintains several “fish screens” located on irrigation diversions drawing water from Henrys Lake tributaries (Howard Creek, Targhee Creek, and Duck Creek). These fish screens prevent fish from being lost from tributaries, and instead, return them to the creek where they came from via underground pipes. During the runoff season, sediment builds up in the fish screens that must be dug out weekly to ensure proper function of the screens. 

Ask a Biologist

At the end of each newsletter, I like to have an “Ask a Biologist” section where I can help answer/explain a timely and important question I receive about Henrys Lake or the surrounding watershed. Please send me any relevant questions you may have! My email is Nathan.Tillotson@idfg.idaho.gov.

Question: What’s that big yellow buoy out in the middle of the lake for?

Answer: 

Water quality monitoring! In June, we deployed a water quality monitoring sensor in the middle of Henrys Lake under a large yellow buoy (pictured below). This sensor will monitor things like water temperature, oxygen concentration, algal blooms, nutrients, and much more! These data will help us identify factors important for trout survival in Henrys Lake, and help inform our stocking practices.