A new research initiative at Henrys Lake near Island Park is examining whether releasing larger hatchery trout will improve their chances of surviving to catchable size, according to Idaho Fish and Game officials.
The evaluation, conducted jointly by state wildlife managers and the Henrys Lake Foundation, responds to declining survival rates among stocked fingerling trout observed in recent seasons. Biologists suspect water quality issues and predation from larger fish are killing significant numbers of young trout during their vulnerable first year.
Heavy Reliance on Hatchery Production
Henrys Lake ranks among Idaho’s most heavily fished waters and depends almost entirely on annual stocking to sustain trout numbers. Wildlife officials estimate that hatchery-raised fish account for 85 to 90 percent of the lake’s Yellowstone cutthroat trout population each year. Every hybrid trout in the lake—crosses between rainbow trout and Yellowstone cutthroats—originates from hatcheries.
Brook trout show less dependence on supplemental stocking, with roughly 25 to 50 percent coming from hatchery releases. To maintain fishing quality, managers typically stock between one million and 1.5 million fingerlings annually.
Despite this substantial effort, catch rates have fallen and overall fishery performance has weakened as fewer stocked fish reach adult size.
Two Groups Marked for Comparison
The study began in 2025 when researchers produced an extra batch of hybrid trout specifically for the evaluation. With state hatchery facilities operating at capacity, the Henrys Lake Foundation arranged for rearing space at a private facility to raise the additional fish.
Biologists separated the study fish into two distinct groups using pelvic fin clips for identification. Approximately 17,000 trout received a left pelvic fin clip and were released in fall 2025 alongside the standard fingerling stocking of 1.5 million fish.
The remaining fish, marked with right pelvic fin clips, stayed at the private hatchery through winter to gain size and strength. On May 4, roughly 19,000 of these older, larger juveniles entered Henrys Lake. Managers expect the increased size at release will give these fish better odds against predators and environmental stress.
What Anglers Can Do to Help
Over the next several years, Fish and Game will track the marked groups to measure relative survival rates and determine whether delayed, larger-size stocking should become part of long-term management at Henrys Lake.
The project received support from numerous volunteers and students who assisted with marking fish. Wildlife officials are asking anglers fishing Henrys Lake in coming seasons to check the pelvic fins of any hybrid trout they catch. Reporting clipped fish will help biologists evaluate whether the new stocking approach improves survival and fishing success.
What Comes Next
Results from the multi-year study will guide future stocking decisions for Henrys Lake and potentially inform management strategies at other high-demand fisheries across eastern Idaho. If larger fish prove significantly more likely to survive, managers may adjust hatchery production schedules and release timing to prioritize quality over quantity in annual stockings.