Each year, IDFG conducts a gillnet survey on Henrys Lake to assess the current state of the fishery. We begin the survey immediately after ice-out, which didn’t take place until mid-May this year! We set nets at six sites throughout the lake every night until we reach our goal of at least 50 net-nights of effort. IDFG manages Henrys Lake for a target population size that results in a gillnet capture rate of 11 trout per net.
Figure 1. Average number of trout captured per net night during our annual Henrys Lake gillnet survey from 1991 – 2022. The dotted line indicates the management goal of 11 trout per net and the dashed line indicates the long-term average of 11.5 trout per net.
This year, our gillnet survey resulted in a capture rate of 5.6 trout per net. Although this estimate is considerably lower than the 2020 and 2021 survey estimates, it is still within the range of estimates from the 2017 – 2019 surveys (Figure 1). A smaller population size isn’t always bad news, though. Fewer fish in a waterbody can sometimes lead to increased growth and average size. In this case, average size of all three trout species in Henrys Lake increased from 2021 (Table 1).
Table 1. Average total length of Yellowstone Cutthroat Trout, hybrid trout, and Brook Trout captured during the annual Henrys Lake gillnet survey in 2019, 2020, 2021, and 2022.
Most notably, average size of hybrid trout (HYB) was over 20 inches and increased by 4.4 inches since last year! You can also observe the distributions of fish sizes captured in our survey in Figure 2. Yellowstone Cutthroat Trout (YCT) and Brook trout (BKT) size distribution was normal. We can see multiple year-classes present in the length-frequency graphs (Figure 2) despite increasing average size for both species (Table 1).
Figure 2. Length-frequency histograms of Yellowstone Cutthroat Trout (upper left), hybrid trout (upper right), Brook Trout (lower left) and Utah Chub (lower right) captured during our annual gillnet survey on Henrys Lake.
Utah Chub were particularly abundant in this year’s survey (31.8 fish per net), and were more abundant than they have been since 2015. A recent graduate study (read more about it here) investigated the potential effects of Utah Chub on Yellowstone Cutthroat Trout in Henrys Lake, and the results (as of 2020) suggested that Utah Chub were not having population-level effects on Yellowstone Cutthroat Trout due to lack of spatial-distribution and resource-selection overlap. However, IDFG will continue to monitor Utah Chub abundance and responses of Yellowstone Cutthroat Trout populations. Utah Chub also serve as a food source for some of the larger trout in Henrys Lake. For example this 27.8 inch hybrid (pictured below) had three 8+ inch Utah Chub in its stomach when we captured her during our survey.
Current and Upcoming Projects
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 has to be dug out weekly to ensure proper function of the screens. This August, IDFG dug up and repaired about 100 ft of a broken and clogged underground fish return pipe on Targhee Creek diversion fish screen. The project went well and the fish should have a much easier path back to Targhee Creek now!
Henrys Lake Tributary Surveys
Natural reproduction of native Yellowstone Cutthroat Trout in the Henrys Lake tributaries has been low in recent years. In order to learn more about trout production in Henrys Lake tributaries, IDFG is increasing our monitoring and conducting in-depth habitat surveys to learn more about the relative quantity and quality of spawning habitat. We are conducting backpack electrofishing surveys to assess fish communities and collecting genetic samples from Yellowstone Cutthroat Trout to determine fry captured in tributaries are of wild or hatchery parental origin. We began these surveys in August, and will continue to conduct them well into the fall.
In June, we stocked 157,070 sterile hybrid trout in Henrys Lake! They appear to be surviving and doing well so far, as we’ve encountered several of them in the 3-4 inch range during our tributary backpack electrofishing surveys. In early September, we will stock approximately 800,000 Yellowstone Cutthroat Trout fingerlings and 100,000 Brook Trout fingerlings!
Year-long and Memorial Weekend Creel Survey
We’ve put a lot of effort into conducting a year-long creel survey in 2022, and there are only a few months left! Each week, technicians and/or volunteers are out interviewing anglers so we can learn more about catch rates, harvest rates, fish sizes that are caught, and much more. We also conducted creel surveys both days of Memorial Day weekend (the harvest season opener). It was quite cold, but the fishing was excellent! Stay tuned for an in-depth dive into what all we found after the year-long survey ends!
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.
In the heat of summer when water temperatures are high, where can we go on the lake to catch fish?
It’s true – the bite can certainly slow in the heat of summer. Bank fishing becomes difficult and it’s harder to find fish off-shore. There are a couple of key reasons for that! Fish move off the bank in some areas after the spawn in search of more oxygenated water as temperatures rise. Fortunately, we now know a bit more about seasonal movement of Yellowstone Cutthroat Trout in Henrys Lake. In her thesis research, Darcy McCarrick found that in the heat of summer Yellowstone Cutthroat Trout sought thermal refuge in and near the mouths of the major tributaries (read more about Darcy’s study here, and look at some really handy maps of seasonal movement). Other than the tributaries, trout can find thermal refuge in deep water spring upwellings present throughout the lake. These springs are harder to find and can produce fish less consistently, though. As it becomes more difficult for trout to find thermal refuge in the heat of summer, fish will also self-regulate when they decide to expend the energy to feed. When it’s hot, trout may feed less frequently, and focus their feeding efforts during key times of the day when temperatures are lowest (late evening and early morning). Regardless, summer fishing on Henrys Lake can still be good if you know when and where to target!