Flathead Lake, situated in Western Montana, holds the distinction of being the largest natural freshwater lake west of the Mississippi River in the contiguous United States. This vast body of water is roughly 28 miles long and 15 miles wide, covering over 120,000 acres, with depths exceeding 370 feet. The lake’s cold, deep, and clear waters support a complex fishery ecosystem. The management of this diverse fish population is a shared responsibility between Montana Fish, Wildlife and Parks and the Confederated Salish and Kootenai Tribes.
The Dominant Introduced Game Fish
The Lake Trout, often referred to as Mackinaw, defines the Flathead Lake fishery. Introduced in the early 1900s, this deep-dwelling char became the apex predator and the primary target for sport fishing. Lake Trout thrive in the cold, well-oxygenated environment, allowing them to reach impressive sizes.
Anglers typically catch Lake Trout ranging from three to eight pounds, but the species has trophy potential, with many fish exceeding twenty pounds. The population expanded significantly following the unintentional introduction of Mysis shrimp into the watershed in the late 1960s. This provided a dense, high-calorie food source, allowing Lake Trout populations to boom and dominate the ecosystem.
The Lake Whitefish is another introduced species contributing substantially to the sport and commercial fishery. Lake Whitefish are abundant and commonly weigh between three and four pounds, though some individuals top ten pounds. The harvest of both Lake Trout and Lake Whitefish is actively encouraged through liberal regulations and fishing events to help reduce their population pressure on native species.
The Native Treasures: Bull Trout and Cutthroat
The lake is home to several native species, primarily the Bull Trout and the Westslope Cutthroat Trout. The Bull Trout is an adfluvial char, meaning it spends its adult life in the deep lake waters before migrating into tributary rivers to spawn. This migration can cover over a hundred miles, as adults seek cold, clean headwater streams with low sediment levels for reproduction.
Bull Trout are a federally protected species, listed as threatened, which imposes strict conservation measures. It is illegal to intentionally fish for or retain a Bull Trout; any caught accidentally must be immediately released. These native predators can grow quite large in Flathead Lake, with some documented individuals reaching weights of up to twenty-five pounds.
The Westslope Cutthroat Trout is the only native strain of Cutthroat Trout in the Flathead River system and is Montana’s state fish. Known for the distinctive red or orange slash marks beneath their lower jaw, these fish face pressure from non-native species through competition and hybridization with introduced Rainbow Trout. Conservation efforts focus on preserving the genetic purity and recovering the numbers of this native trout throughout the watershed.
Other Common Species Found in Flathead Lake
Flathead Lake hosts a variety of other fish, including several introduced species popular with anglers. Yellow Perch are non-native panfish, typically six to ten inches long, providing popular fishing, especially during the winter ice fishing season. Smallmouth Bass and Largemouth Bass have also established populations, primarily inhabiting the shallower, warmer bays and sloughs connected to the lake.
Northern Pike, an introduced predator, are present, particularly in the northern sections and tributary mouths. These fish are known for their long, slender bodies and sharp teeth, often targeting other fish species. The harvest of Northern Pike is encouraged to prevent their further spread and impact on the ecosystem.
The lake’s diversity includes numerous smaller, non-game, and forage fish that form the base of the food web. Native species like Mountain Whitefish, Longnose Sucker, and Slimy Sculpin are present throughout the ecosystem. Other species, such as the Peamouth Chub and Redside Shiner, play an important role in supporting the larger fish populations.