A recently published research paper suggests that maintaining natural temperatures on Montana's Flathead River system may inhibit the movements of invasive lake trout to upstream waters.
The study was based on an analysis of telemetry data collected from 1996 through 1998 from radio-tagged lake trout for the purpose of learning more about their predatory tendencies, said Clint Muhlfeld, an aquatic ecology researcher with the U.S. Geological Survey who is based at Glacier National Park.
With more than 200 waters in the western United States occupied by lake trout, many of them connected to rivers and other lakes, there has been a growing interest in how the species disperses, Muhlfeld said.
“No one had really looked at or discussed the dispersal capabilities of lake trout in interconnected lake and river systems,” he said, adding that data collected for the study showed “enormous” invasive capabilities of lake trout that now occupy most of the lakes on the western front of Glacier National Park.
Mysis shrimp that were introduced in the Flathead Basin from 1968 to 1976 are widely regarded as the cause of a lake trout population boom in Flathead Lake that reached a point where the species started to radiate into upstream waters.
“The incidence of lake trout entering the Flathead River was rarely observed prior to 1989, yet angler creel data suggested that lake trout use of the Flathead River substantially increased in the 1990s,” states the study published in the Fisheries Management and Ecology Journal. “Changes in the thermal characteristics of the Flathead River also may have affected lake trout distribution in the system.”
Lake trout widely used the river system during summer months prior to the 1996 installation of a selective withdrawal system at Hungry Horse Dam that allowed river temperatures to be regulated by drawing warmer water from Hungry Horse Reservoir.
The new system basically restored natural temperatures below the dam in river reaches that had previously been artificially cooled during summer months.
An analysis of the movements of radio tagged fish found they used the river during summer considerably less after the selective withdrawal was installed.
“Lake trout were detected in the river primarily during autumn, winter and spring, when water temperatures were cool,” the study states. “By contrast, fewer were detected when temperatures were warmest during summer and during high spring flows.”
The study concludes that maintaining natural water temperatures may be effective in “reducing the spread of non-native lake trout to conserve native fish populations in the Flathead River system and elsewhere.”
Muhlfeld said one aspect of lake trout he considers important is “enormous” dispersal capabilities that have had major impacts upstream from Flathead Lake, all the way into Glacier Park lakes and Canadian waters.
A rapid invasion of Glacier’s Quartz Lake has prompted an effort to suppress lake trout over the last few years.
Gill netting in the spring and fall has removed 1,600 lake trout so far on the 900-acre lake, and this fall’s five-week effort resulting in nets capturing all six adult “Judas fish” that were radio tagged and tracked to two distinct spawning locations. Because those fish were removed, it is believed that there was a high rate of success in removing adults that are still in the lake.
A fish barrier downstream from the lake is expected to prevent future invasions, and Muhlfeld is optimistic that the project will be successful in suppressing the lake trout population and protecting native bull trout and westslope cutthroat trout that are still abundant in the lake.