ISLAND PARK — Idaho Fish and Game, working alongside the Henrys Lake Foundation, has begun a new research effort to determine whether releasing larger, older hatchery-raised hybrid trout into Henrys Lake can improve fish survival rates and strengthen one of Idaho’s most prized fisheries.
Henrys Lake, located in the Island Park area of eastern Idaho near Yellowstone, is among the most popular angling destinations in the state. The lake depends almost entirely on hatchery supplementation to sustain its trout populations. In a normal year, hatchery fish account for an estimated 85 to 90 percent of the lake’s Yellowstone Cutthroat Trout population, while all hybrid trout — a Rainbow Trout and Yellowstone Cutthroat Trout cross — are hatchery-produced. Brook Trout are somewhat less dependent on stocking, with roughly 25 to 50 percent coming from hatchery releases.
To keep up with angler demand, the lake typically receives between one million and 1.5 million fingerling trout annually.
Declining Survival Rates Prompt New Approach
Over recent years, Fish and Game biologists noted a troubling trend: fingerling trout stocked into Henrys Lake were surviving at lower rates, leading to reduced catch numbers and diminished overall fishery quality. Water quality issues and predation by larger fish are thought to be the primary factors driving mortality, especially during a young fish’s first year of life when it is most vulnerable.
To investigate potential solutions, Fish and Game and the Henrys Lake Foundation launched a size-at-release evaluation beginning in 2025. The premise is straightforward — fish that are larger and more developed when they enter the lake may be better equipped to avoid predators and survive the challenges of their first season.
Because state hatchery facilities were already operating at full capacity, the Henrys Lake Foundation arranged rearing space at a private hatchery to raise the additional fish needed for the study.
Two Groups, Two Release Dates, One Goal
Biologists separated the study fish into two distinct groups, using pelvic fin clips as identifying markers. Roughly 17,000 fish with a left pelvic fin clip were released in fall 2025, coinciding with the traditional fingerling stocking of 1.5 million fish. The remaining group, marked with a right pelvic fin clip, stayed at the private facility through the winter months to continue growing.
On May 4 of this year, approximately 19,000 of these larger, more mature juveniles were released into Henrys Lake. The theory is that the added size and conditioning these fish gained over the winter will translate into better survival odds once they enter the wild.
Over the next several years, Fish and Game biologists will track the survival rates of both marked groups to evaluate whether the delayed, larger-size release method produces measurable benefits. That data will inform future stocking decisions for the lake’s long-term management.
The project involved contributions from the Henrys Lake Foundation, along with volunteers and students who helped with the fish-marking process.
Anglers Asked to Help with Research
Fish and Game is encouraging anyone fishing Henrys Lake in the coming seasons to play an active role in the research. When catching hybrid trout, anglers are asked to check the pelvic fins for clips. Identifying which group a fish belongs to gives biologists valuable real-world data on how each stocking strategy is performing.
The collaborative structure of the project — combining state resources, private hatchery capacity, and volunteer labor — reflects a growing model for managing high-demand public fisheries under budget and infrastructure constraints.
What Comes Next
Fish and Game biologists will continue monitoring fish populations at Henrys Lake over multiple seasons to gather statistically meaningful survival comparisons between the two stocking groups. Results from the study are expected to shape hybrid trout stocking practices not just at Henrys Lake, but potentially at other heavily managed Idaho fisheries as well. Anglers, conservation groups, and fishing organizations interested in supporting the effort can connect with the Henrys Lake Foundation for volunteer opportunities.
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