Henrys Lake Quarterly Newsletter - April 2021

Henrys Lake Quarterly Newsletter
April 2021



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 this season. 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.

2021 Spawn

This spawn season was certainly one for the books. We operated as a small crew who remained on call for the month of February and March. We operated the trap from January 22 through March 3, 2021 and we were able to trap 7,221 unique fish.

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Jenn Vincent, IDFG

Yellowstone Cutthroat Trout
This year we collected an estimated 2,080,890 fertilized Yellowstone Cutthroat Trout eggs over 3 days. These eggs were incubated on site at the Henrys Lake facility for approximately 28 days before they reached their "eyed up" stage. At this stage you can look at the egg and see two little black eyes of the fish inside. If you look long enough you can even see them moving inside the egg! Check out the picture below, can you see the eyes?


Once they have reached this stage, staff sort through the eggs removing any impurities or eggs that were not fertilized. Next the eggs are counted and transported to the Mackay Hatchery where they will finish incubation, hatch, and grow to fingerling trout before being stocked back in to the lake this September.

Check out the video below to see us live in action spawning Yellowstone Cutthroat Trout in 2020.


Henrys Lake Yellowstone Cutthroat Trout spawn 2020

Hybrid Trout (Yellowstone Cutthroat Trout x Rainbow Trout)

Creating a sterile Hybrid Trout is a collaborative effort between multiple hatcheries and involves a time-sensitive "recipe" that staff need to follow down to the second.

On a hybrid spawn day, staff at the Hayspur hatchery start their day in the wee hours of the morning collecting milt from male Rainbow Trout broodstock on site at the Hayspur hatchery. Milt from each individual male is bagged, put on ice and driven over 3.5 hours to staff at Henrys Lake. At Henrys Lake, staff will collect eggs from female Yellowstone Cutthroat Trout and fertilize the eggs with the Hayspur Rainbow Trout milt. Next the eggs are washed, and disinfected at certain time points following fertilization to clean off the excess milt and limit fungal infections. Next these eggs undergo a pressure-shock treatment for 5 minutes at 9500 psi. This high pressure interrupts cell division in the egg and creates what is referred to as a triploid fish which are unable to reproduce. By stocking the lake with sterile Hybrid Trout we can provide anglers with the opportunity to catch some trophy trout and multiple trout species in the lake while protecting our native Yellowstone Cutthroat Trout genetics and spawning habitat.

This year we collected and fertilized an estimated 926,810 Hybrid Trout eggs over 2 days. These eggs will then go through the same process as described above for the Yellowstone Cutthroat Trout before travelling to the American Falls hatchery to finish incubation, hatch, and grow to fingerling trout to be stocked back into the lake in June 2021.

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Jenn Vincent, IDFG


Graduate Study Update
By: Darcy McCarrick

Over the past two years, historical environmental and biological data have been compiled and processed. These data include air temperature, reservoir volume, stocking rates, catch rates of each species, and hard structures from Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. Hard structures, such as scales and otoliths, form rings that mark annual growth similar to tree rings. This allows biologists to estimate age by counting the total number of rings and evaluate annual growth by measuring the distance between rings (increments). Check out Figure 1 below, and try your hand at counting the rings.

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Darcy McCarrick

Growth of the hard structure is proportional to the growth of the fish and the increments can be used to estimate lengths at previous ages. Back-calculated lengths at age of Yellowstone Cutthroat Trout were compared across decades (Figure 2). Two interesting patterns were revealed by this comparison. First, we noticed increased variation in size at age through time. The second pattern is that Yellowstone Cutthroat Trout in the most recent two decades are slightly larger on average than Yellowstone Cutthroat Trout in the past.


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Darcy McCarrick

Growth was further evaluated with mixed effects modeling. This technique produces growth coefficients that represent growth of all Yellowstone Cutthroat Trout sampled each year. Due to concerns with estimates from scales, growth coefficients were only calculated for years we had otolith data (1994 – 2019). Growth patterns were cyclic and have improved in recent years (Figure 3).

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Darcy McCarrick


Linear models were used to identify relationships between Yellowstone Cutthroat Trout growth and temperature, water level, stocking rates, and relative abundance of Utah Chub, Brook Trout, Yellowstone Cutthroat Trout, and hybrid trout in Henrys Lake. The top model explained variability in growth by a negative relationship with Brook Trout and Utah Chub catch rates and a positive relationship with Yellowstone Cutthroat Trout stocking levels. Yellowstone Cutthroat Trout growth was higher when Utah Chub and Brook Trout abundances were low compared to when Utah Chub and Brook Trout abundances were high. Growth improved when the number of Yellowstone Cutthroat Trout stocked increased. Multiple mechanisms could be driving these patterns.

Growth modeling suggests Utah Chub have a negative effect on Yellowstone Cutthroat Trout growth. Although the specific mechanism is not known in Henrys Lake, diet overlap and competition between Utah Chub and salmonids have been documented in other systems. Utah Chub and trout feed on plankton and macroinvertebrates, so high abundances of Utah Chub may reduce food availability for Yellowstone Cutthroat Trout in Henrys Lake. Another potential mechanism is density dependence. Henrys Lake has finite resources that can support a finite number of fish. High abundances of fish might approach resource limits and reduce growth of Yellowstone Cutthroat Trout. A negative relationship with trout catch rates further supports this hypothesis.

A negative relationship with Brook Trout catch rates was also in the top model. Brook Trout have been associated with reduced growth and recruitment failure of Yellowstone Cutthroat Trout in multiple systems. However, most research conducted on Brook Trout and Yellowstone Cutthroat Trout focused on streams and rivers where competition was occurring at early life stages. Limited information is available about the effects of Brook Trout on adult Yellowstone Cutthroat Trout in lake systems. Our hypothesis is that Brook Trout might outcompete Yellowstone Cutthroat for oxygen-rich cold water. Brook Trout are more thermally sensitive than Yellowstone Cutthroat Trout and Brook Trout may be congregating on springs and other cold-water sources in periods of high temperatures (i.e., summer), limiting access for the other species. The relationships with Utah Chub and Brook Trout warrant further investigation to identify the mechanisms causing the observed trends.


April to June 2021

  1. Gillnetting: Our annual gillnet survey takes place just after ice-off (April/May). Information gathered from this survey allows us to monitor trends in the abundance, size, age structure, and survival rates of trout and chubs in Henrys Lake.
  2. Henrys Lake Opener: If you are like me you are eagerly awaiting the Henrys Lake opener! This year the lake will open to fishing on Saturday May 28th, 2021. Our staff will be around the lake on opening day weekend conducting a creel survey. We will interview anglers to better understand the current state of the fishery and how fishing is on the lake. Keep an eye out for staff, we would love to talk with you all.
  3. Electric fences: Each June, through a cooperation with multiple landowners and ranchers we install miles of electric fencing along Duck, Howard, Targhee, and Kelly Creeks and along the lake. These fences help to keep livestock out of the riparian areas of Henrys Lake tributaries and off the shoreline of Henrys Lake. By limiting access to these important tributaries by livestock we can protect vital in stream habitat for Yellowstone Cutthroat Trout.
  4. Fry Traps: This year we plan on reinstalling fry traps in the main tributaries of the lake. Our goal is to capture fry that have emerged upstream and are travelling to the lake. Through the collection of DNA via a fin clip we will be able to identify if these fry are from spawning wild or hatchery Yellowstone Cutthroat Trout. The number and identity (wild vs. hatchery) of these fry will provide us with valuable insight into the current wild contribution to the lake and amount of successful spawning occurring at Henrys Lake.

2022 - 2024 Fishing Regulations

We are currently scoping for public comments on our fishing regulations throughout the region. Are there any fishing season changes you would like to see for 2022-2024? Help shape fishing seasons in the Upper Snake Region by submitting your ideas by April 11.

Click on the following link for a recording of our virtual open house where we answered questions from the public on April 1st. You can also submit your thoughts through a web form at this link:


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: How does harvest impact fish populations?

A:     There are multiple drivers that impact populations of fish. If you picture a scale, on one side we have gains to the fishery and on the other losses. If one side "tips" more than the other it can trigger a response to the population. The population may gain fish to the population through natural reproduction or hatchery supplementation.

While the main loss to the population is mortality, which can occur through natural or fishing mortality. Natural mortality is the mortality that occurs in the population without additional loss and can change from year to year. Natural mortality is influenced by habitat quality which can include the amount of space available, and cover types; abiotic or environmental factors which can include temperature, dissolved oxygen, and nutrient levels; and mortality due to age or disease. Fishing mortality is the mortality caused by anglers. The amount or type of fishing mortality is what is regulated by Idaho Fish and Game fishing regulations. When fishing mortality increases to the point that is negatively affecting the population, Idaho Fish and Game can respond in one of two ways. Managers can increase the number of hatchery fish added to the population or they can implement new fishing regulations or adjust current fishing regulations on the waterbody which will adjust angler behavior geared to subsequently reduce fishing mortality, and increase abundance in the fish population.

Our next quarterly newsletter will be sent out in July 2021 so stay tuned!
If you have any questions, thoughts or concerns please contact Regional Fisheries Biologist Jenn Vincent at

If you, or someone you know would like to be added to our distribution list, please send an email to idfgreply@idfg.idaho.gov, request to be added to our emailing list and choose the subscription option “Upper Snake Region”.


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Jenn Vincent, IDFG