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Henrys Lake Quarterly Newsletter - July 2021

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Henrys Lake Quarterly Newsletter
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July 2021

Population Analysis

The Idaho Department of Fish and Game (IDFG) annually monitors fish populations in Henrys Lake using gill nets. Our goal is to set 50 gill-nets as soon after ice-off as possible. This allows us to obtain information on the current state of the fishery. IDFG manages Henrys Lake for a capture rate of 11 trout per net night.

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Fisheries staff pull gill-nets into the boat at Henrys Lake 2021

 

A capture rate of 10.6 trout per net night was estimated for 2021. This is slightly below our management goal of 11 trout per net and our long-term average of 11.7 trout per net. The total number of trout captured per net night during our 2021 gill net survey was slightly lower than our 2020 survey of 14.9 trout per net night although the capture rate observed this year is above capture rates observed in the 5 years prior as shown in Figure 1 below.

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Figure 1: The average number of trout by species per net night from 1991 to 2021. YCT: Yellowstone Cutthroat Trout, HYB: Yellowstone Cutthroat x Rainbow Trout, and BKT: Brook Trout. The dotted line denotes our management goal of 11 trout per net and the dashed line denotes the long term average of 11.7 trout per net.

The breakdown of sizes of each species of trout captured is depicted below in Figure 2. Two distinct length cohorts of Yellowstone Cutthroat Trout (YCT) were observed. This indicates a large presence of age-2 and age-4 YCT with slightly lower numbers of age-3, and age-5 YCT currently inhabiting the lake. The average size of YCT in the population has been increasing over the last three years as the largest cohort of YCT in the population continue to grow. Hybrid Trout (HYB) exhibited a wide range of sizes from 8 to 22 inches in length and averaged 15.8 inches. Brook Trout (BKT) also exhibited a wide range of lengths from 6 to 20 inches with an average of 13.3 inches.

There is a large number of trout currently in the lake with a wide size range. As these fish continue to grow and more trout are stocked into the system there are plenty of fish for anglers to get a hold of and this all points to a positive fishing forecast for this fall, winter, and the future.

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Figure 2: The frequency of trout per total length (inches) per species captured during the 2021 gill-net survey. Yellowstone Cutthroat Trout (YCT), Hybrid Trout (HYB) and Brook Trout (BKT). The table shows the average length (inches) per species captured during the 2019, 2020, 2021 gill net surveys.

 

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Top photo: Fisheries Technician Logan Thomas holds a Brook Trout.
Bottom photos: Fisheries Technician Andrew Johnson (left) and Fisheries Biologist Jenn Vincent (right) hold a Hybrid Trout captured during our 2021 gill net survey.

 

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

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Opening weekend this year was certainly one for the books. Many anglers flocked to the lake to wait patiently in line for their turn to launch a boat. Bank anglers were also happy to grab a lawn chair, or waders to try their luck.

 

For opening weekend (May 29 and 30) 2021 we estimated a catch rate of 0.38 fish per hour which is significantly lower than our estimated 2020 catch rate of 1.03 trout/hour and our 2019 estimate of 1.06 trout/hour.

An estimated total of 1527 angler hours were spent on Henrys Lake this past opening weekend which is slightly lower than the 1610 angler hours estimated for the 2020 opening weekend. We estimated that a total of 587 trout were captured with 266 of those trout harvested by anglers over the entire weekend.

The largest hybrid checked in by our creel clerks was 30 inches in length. Below you can see the size distribution of each species harvested by anglers we interviewed.

Table 1. Lengths of trout checked in to creel clerks during opening weekend 2021 where BKT=Brook Trout, HYB=Hybrid Trout, and YCT=Yellowstone Cutthroat Trout.

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Stocking

On June 17, 2021 approximately 131,197 sterile Hybrid Trout were stocked into Henrys Lake.

During the last week of September we will stock our Yellowstone Cutthroat Trout and sterile Brook Trout.

 

Upcoming July to October 2021

  1. Riparian fences and fish screens: Our crews have been working vigilantly throughout June 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 aid in the rehabilitation of 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. We will continue to maintain and monitor these fences and screens on a weekly basis throughout the summer and fall.
  2. Fry trapping: In June, we installed traps in each of the major tributaries to Henrys Lake. These traps will be consistently monitored throughout the season to obtain valuable information on Yellowstone Cutthroat Trout spawning activities. This includes the successfulness of the spawn by tributary, fry survival, fry emergence timing, and the parental origin of fry (wild or hatchery).
  3. Backpack shocking: Every few weeks throughout July and August we will be backpack shocking the main tributaries of Henrys Lake in order to monitor the successfulness of Henrys Lake YCT spawn in tandem with our fry trapping efforts.
  4. Stocking: Approximately 1 million Yellowstone Cutthroat Trout and 100,000 sterile Brook Trout are scheduled to be stocked in Henrys Lake during the last week of September 2021.

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 jenn.vincent@idfg.idaho.gov


Q: With the rising air and water temperatures is IDFG planning to close or implement any fishing regulations in the upper snake?

A: IDFG, along with any people around the state are concerned about the effects this summer with have on fish and we are committed to monitoring key populations including Henrys Lake, and will implement fishing regulations if needed. However, IDFG does not want to prematurely restrict fishing opportunities unless those restrictions would prevent adverse impacts to our fisheries at the population level.

Trout populations experience high levels (30-60%) of natural mortality annually which generally ranges from 30-60%. Fishing related mortality associated with catch-and-release angling can increase with increasing water temperatures although multiple studies have indicated that catch-and-release mortality in warm water is minor and does not rise to a level that become insurmountable to the population.

Furthermore, wild trout are very resilient and have evolved behavioral mechanisms to survive increasing water temperatures. This includes becoming less active and migrating to deeper colder sections of water, migrating to cooler tributary waters, and seeking refuge at cool-water springs.

We are committed to monitoring our Idaho water bodies, but at this we are not implementing any fishing restrictions due to water temperatures in the upper snake region.

Check out the news release below for more information
http://idfg.idaho.gov/press/summer-heat-will-cause-some-trout-mortality-halting-fishing-unlikely-improve-situation

You can access the April 2021 edition of our newsletter here:
https://mailchi.mp/a60dd9f1683e/henrys-lake-newsletter-april-2021

Our next quarterly newsletter will be sent out in October 2021 so stay tuned!

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