Species introductions to lake could be disaster

In 1968, mysis shrimp were transplanted by Montana Fish, Wildlife and Parks from Waterton Lake, Alberta, to several lakes in northwestern Montana. Whitefish Lake and McGregor Lakes received the first plant in June of that year and were also part of the second plant in September. During 1968, 1975 and 1976 mysis shrimp were transplanted to 12 lakes in our area, including; Ashley, Bull, Crystal, Dickey, Holland, Little Bitterroot, McGregor, Middle Thompson, Spar, Swan, Tally and Whitefish Lakes.

Six of the original 12 lakes were found to contain mysis following initial introductions; Ashley, Little Bitterroot, McGregor, Swan, and Whitefish. Mysis then drifted downstream from Swan and/or Whitefish Lake(s) to populate Flathead Lake where they were first collected in 1981.

The mysis introduction was expected to provide a food source for benthic-feeding fishes such as lake trout, and for pelagic planktivores such as kokanee salmon. However, the expected increased forage for kokanee salmon from mysis shrimp was based on premature and erroneous interpretations of the results of such introductions elsewhere.

It is estimated that the Whitefish Lake mysis population peaked and reached carrying capacity sometime from 1973-1976. Mysis reside near the lake bottom during the day and vertically migrate to near surface waters at night to forage, preferring the larger bodied and slower zooplankton species. These same zooplankton species were also the preferred prey of kokanee salmon, a daytime ocular feeder.

In 1976, the kokanee salmon population declined dramatically and later became extirpated in Whitefish Lake. The selective feeding by mysis on larger zooplankton species also marked a shift in the zooplankton and phytoplankton communities of the lake.

Conversely, non-native lake trout and lake whitefish populations have benefited from the mysis plant. The rise of lake trout populations also hurt the kokanee salmon population since adult lake trout feed extensively on other fish; and kokanee were on the dinner menu.

Ultimately, the food web chaos caused by this half-inch invertebrate has profoundly altered the food web and nutrient dynamics of Whitefish Lake. Other notable biological introductions to Whitefish Lake include; lake whitefish (1910), brook trout (1925), lake trout (1941), Northern pike ( about 1970). Any additional biological introductions such as aquatic invasive species can only spell disaster to a system that has been in biological flux for the past 100 years.

In September, WLI conducted a mysis shrimp survey on Whitefish Lake via a grant from the Whitefish Community Foundation to determine their current spatial distribution and population levels.

Mike Koopal is the executive director of the Whitefish Lake Institute.