Isolation and protection of native species are important in the Flathead Basin because many non-native fishes were introduced into the basin, mainly in Flathead Lake, and some established strong populations. Management agencies purposefully introduced sport fish decades ago to increase angling opportunities. More recent introductions were surreptitious, by anglers desiring more diverse fishing, or accidental, via bait buckets. Non-native species introductions are now illegal in Montana. Unfortunately, competition for limited food supplies in the oligotrophic waters of the Flathead, and interbreeding with non-natives, reduced the vitality of the natives, especially native salmonids.

The most pervasive change in the fish fauna was caused by the unfortunate but purposeful introduction in the late 1970s of Mysis relicta, the freshwater opossum shrimp, whose native range includes the glacial lakes of the Canadian Shield and the Laurentian Great Lakes. Mysids become abundant in lakes by substantially reducing the large zooplankton. They avoid visual feeders such as kokanee and cutthroat in the lighted layers of the lakes by residing in very cold water on the dark lake bottom in the daytime.

Mysis Relicta

They are, however, excellent forage for juvenile lake trout and lake whitefish, which also were introduced from the Canadian Shield lakes, where mysids are their natural food. This relationship allowed lake trout and whitefish to substantially increase in Flathead Lake, creating a voracious predator trap. The kokanee received a double whammy from the mysids: their food resources were substatially reduced and their predators substantially increased.

Because planktivorous fish occur in very low numbers in Flathead Lake, the subadult and adult lake trout have little to feed on other than mysids. They have begun to disperse upstream and downstream out of the lake. Lake trout are often now caught in the rivers and they have invaded the front country lakes where mysids were not introduced. This presents an interesting natural experimental design for long-term research of food web dynamics and the strong interactive effects for the mysids with and without lake trout.

In the coming years, we will begin to examine the food web dynamics in this natural array of lakes in the Flathead Basin.

Bull Trout	West-slope Cutthroat Trout

Related Publications:

Stanford, J. A. and B. K. Ellis. 2003. Natural and cultural influences on ecosystems processes in the Flathead River Basin (Montana, British Columbia). Pp 269-284. IN: Baron, J. S. (Editors). Rocky Mountain Futures: An Ecological Perspective. Island Press, Covelo, California, USA. 325 pp.

Spencer, C. N. and B. K. Ellis.  1998.  Role of nutrients and zooplankton in regulation of phytoplankton in Flathead Lake (Montana, USA), a large oligotrophic lake.  Freshwater Biology 39(4): 755-763.

Stanford, J. A., B. K. Ellis, J. A. Craft and G. C. Poole.  1997.  Water quality data and analyses to aid in the development of revised water quality targets for Flathead Lake, Montana.  Open File Report 142-97.  Flathead Lake Biological Station, The University of Montana, Polson.  Prepared for the Flathead Basin Commission, Kalispell and Helena, Montana.  154 pp. + appendices.

Stanford, J. A. and J. V. Ward.  1992.  Management of aquatic resources in large catchments:  Recognizing interactions between ecosystem connectivity and environmental disturbance, pp. 91-124. IN:  Naiman, R. J. (ed.), Watershed Management.  Springer-Verlag, New York.

Dodds, W. K., B. K. Ellis and J. C. Priscu. 1991. Zooplankton induced decrease in inorganic phosphorus uptake by plankton in an oligotrophic lake. Hydrobiologia 211: 253-259.

Dodds, W. K., J. C. Priscu and B. K. Ellis. 1991.  Seasonal uptake and regeneration of inorganic nitrogen and phosphorus in a large oligotrophic lake: size-fractionation and antibiotic treatment.  Journal of Plankton Research 13(6): 1339-1358.

Spencer, C. N. and B. K. Ellis.  1990.  Co-limitation by phosphorus and nitrogen, and effects of zooplankton mortality, on phytoplankton in Flathead Lake, Montana, U.S.A.  Verh. Internat. Verein. Limnol. 24: 206-209.

Ellis, B. K. and J. A. Stanford.  1982.  Comparative photoheterotrophy, chemoheterotrophy and photolithotrophy in a eutrophic reservoir and an oligotrophic lake.  Limnology and Oceanography 27(3): 440-454