Fishing in Island Park
Henrys Lake is a popular destination for sport fishing. The most common species in the lake is Yellowstone cutthroat. They make up 55 percent of the trout population. The average size in 2015 was 15 inches but biologists found some fish over 20 inches. The second most prevalent species is rainbow-cutthroat hybrids, which make up 24 percent of the population. The average size is 19 inches, although Idaho Fish and Game found hybrids longer than 25 inches. Brook trout make up 21 percent of the population. The average length is 16 inches but biologists found several Brook trout that exceeded 20 inches in length. There is a fish hatchery on the north side of the lake that helps to replenish the lake.
Henrys Lake is home to a variety of migrating birds. The western and eastern edges of the lake provide wetland/marsh habitats that serve as breeding areas for many birds in spring. Notable at Henrys Lake are White Pelicans, Widgeon, Lesser Scaup, Cormorants, Red-necked and western Grebes, coots, mallards, bufflehead, Ring-necked Ducks, Canada Geese, Blue-winged Teal, eared Grebe, Killdeer, common Merganser, common tern, Cinnamon Teal, Trumpeter Swans, Great Blue Heron, California Seagulls, Bald Eagles, Swainson's Hawks, Red-tailed Hawks, Red-winged Blackbirds, cowbirds and more.
Island Park Reservoir
is operated by the U.S. Bureau of Reclamation in Fremont County, Idaho. The dam lies in Targhee National Forest near Island Park. The zoned earthfill dam was built between 1937 and 1939 as part of the Minidoka Project, which provides water to irrigate farmland in Idaho's Snake River Plain.
The dam provides only water storage, impounding 135,000 acre feet (0.167 km3), which is distributed by the Cross Cut Canal to farms in Fremont and Madison counties in Idaho, and Teton County in Wyoming. The Island Park and Grassy Lake reservoirs were built as an alternative to construction of a larger project that would have flooded the Falls River area of Yellowstone National Park.
The Island Park reservoir is also known to have rainbow, cutthroat, and brown trout.
Information from https://en.wikipedia.org/wiki/Island_Park_Dam
Henrys Fork river
The Henry’s Fork watershed encompasses two million acres and over 3,000 miles of rivers, streams and canals in eastern Idaho and western Wyoming. This rich expanse supports healthy populations of fish and wildlife, including several threatened and endangered species, as well as a wide range of high-quality recreational experiences. The Henry's Fork area offers a high-quality recreational experience, with some of the finest trout fishing in the entire United States.
Information from: https://henrysfork.org/fishing-river
Henrys Fork-Box Canyon is a 4.1 mile river that is heavily trafficked during the summer. Fishing on the box is strictly single barbless hooks and is only catch and release. So most of those who fish on the Box are fly fisherman.