Flathead Lake, in northwest Montana, is the largest natural lake by surface area in the western contiguous United States. Known for its clear, clean waters, it is considered one of the cleanest lakes globally. This lake, approximately 28 miles long, 15 miles wide, and 370 feet deep, is important for both the local ecosystem and recreation. Its southern portion lies within the Confederated Salish and Kootenai Tribes’ Flathead Indian Reservation, highlighting its cultural and environmental significance.
Fish Native to Flathead Lake
Historically, Flathead Lake supported about 10 native fish species. These include the Bull Trout (Salvelinus confluentus), once the lake’s top native predator. Bull trout migrate, living in the lake and returning to cold, clear gravel beds in tributary streams of the Flathead River’s North and Middle Forks to spawn. This species is sensitive to habitat changes and is listed as threatened under the Endangered Species Act.
The Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi), Montana’s state fish, is another native species. While some populations live in the lake, these trout are more common in cooler tributaries. The Pygmy Whitefish (Prosopium coulterii) is also native. Other native species include the Mountain Whitefish, Largescale Sucker, and Northern Pikeminnow.
Fish Introduced to Flathead Lake
Flathead Lake’s fish composition changed significantly with the introduction of non-native species, starting in the 1890s. Fisheries managers introduced over 19 non-native species, often to enhance sport fishing. Lake Trout (Salvelinus namaycush), brought from the Great Lakes region around 1905, were one such introduction. Their populations remained low for decades.
Mysis Shrimp (Mysis diluviana), a small crustacean, were released into lakes upstream of Flathead Lake by Montana Fish, Wildlife & Parks between 1968 and 1976. This was intended to provide food for Kokanee Salmon (Oncorhynchus nerka). These landlocked sockeye salmon, introduced in 1916, were popular sport fish until the mid-1980s. Lake Whitefish (Coregonus clupeaformis), introduced around 1890, also saw population growth after the Mysis introduction.
Ecological Dynamics: The Impact of Introduced Species
Mysis shrimp introduction significantly altered Flathead Lake’s food web, leading to unforeseen ecological consequences. Instead of feeding kokanee salmon, Mysis shrimp competed for zooplankton. Their nocturnal migration pattern also made them unavailable to sight-feeding kokanee. However, Mysis shrimp provided an abundant food source for young, deep-dwelling lake trout.
This led to a rapid increase in the lake trout population, making them the lake’s dominant predator. The growing lake trout population then decimated the kokanee salmon fishery through predation, causing its collapse. This ecological shift also impacted native fish like bull trout and westslope cutthroat trout, making them vulnerable to increased lake trout predation. The trophic cascade even affected bald eagle populations that historically relied on kokanee salmon.
Conservation Efforts for Flathead Lake’s Fish
Flathead Lake’s fisheries are cooperatively managed by Montana Fish, Wildlife & Parks and the Confederated Salish and Kootenai Tribes. A main goal is to reduce introduced species, especially lake trout, to protect and recover native fish. Targeted fishing regulations, like the semi-annual “Mack Days” contests, encourage anglers to harvest lake trout; hundreds of thousands have been removed since 2002.
The Confederated Salish and Kootenai Tribes also use gillnetting to remove lake trout and lake whitefish. Harvested fish are processed and sold by Native Fish Keepers, a non-profit, with proceeds supporting native fish conservation. Beyond population management, conservation includes habitat restoration projects to improve wetlands and riparian areas along the Flathead River and its tributaries. These efforts provide the cold, clean, and connected aquatic environments needed for native trout to thrive.