Early in the 20th century, biologists believed only 10 species of native fish lived in Flathead, the West's largest natural freshwater lake. But anglers and game officials had already introduced other species. "It was a Johnny Appleseed approach," Vashro says. "Throw in lots of stuff, see what makes it."
In 1920, a shipment of kokanee arrived from Bonneville, Ore. By 1940, kokanee, which are landlocked sockeye salmon, had replaced cutthroat trout as the most-caught fish, and they remained dominant until the 1980s.
Meanwhile, the lake trout, introduced from the Great Lakes in 1905, bided its time. "There are many cases in which a species arrives and it just sits around," says Daniel Simberloff, an authority on invasive species at the University of Tennessee. Then, he says, something changes to trigger a rapid expansion. In Flathead Lake, it was Mysis shrimp. Starting in 1968, state fisheries managers released the shrimp in several lakes of the Flathead watershed to enhance kokanee stocks; the shrimp moved downstream and reached Flathead Lake in 1981.
Instead of being a boon, though, Mysis competed with kokanee for their preferred food, zooplankton. The kokanee population collapsed. More importantly, younger lake trout feasted on shrimp, and the population soared. They ate nearly all of the remaining kokanee, as well as any other fish they could get their mouths around. Today, an estimated 1.6 million lake trout live in Flathead; more have since migrated and colonized most of the watershed.
Many of the remaining native fishes in Flathead Lake have dramatically declined, including bull trout, the top native predator.
