Glacier Park lake trout netting shows promise

Scientists working in Glacier National Park and at the University of Montana may have found a solution to the decades-long lake trout invasion. Over the past 30-plus years, non-native lake trout from Flathead Lake have migrated upstream into the backcountry lakes west of the Continental Divide in Glacier National Park. Glacier’s native bull trout have steadily declined in abundance, because the larger and deeper-dwelling lake trout outcompete the locals for food. In addition to consuming more prey, lake trout are less likely than bull trout to be hunted by eagles, otters, and loons.

In 2005, biologists Bill Michels and Wade Fredenberg made a disappointing discovery at the park’s 900-acre Quartz Lake. Lake trout had established a self-sustaining, multi-generational population. The population included multiple size classes, and some individuals were already over 5 years old. Too late were the efforts to construct a fish barrier against invasion, begun the previous fall in Quartz Creek. The future looked bleak for Quartz Lake and other large, isolated lakes in western Glacier National Park.

Now, more than 10 years later, a viable suppression strategy has surfaced.

A new study indicates that despite the constraints of working in the backcountry, more than six miles from the nearest road, lake trout populations can be suppressed with gill netting techniques. Nets with specifically-sized mesh holes selectively catch lake trout by their gills, while allowing scientists to minimize accidental bull trout by-catch. From 2009 to 2013, gill netting removed 352 mature lake trout and 1,457 juvenile lake trout, during which time the bull trout population remained stable. Because bull trout were allowed to flourish while the lake trout’s population and growth rates were reduced, suppression via netting could also prove beneficial to other invaded lakes in Glacier Park.

Clint Muhlfeld, a research aquatic ecologist with the U.S. Geological Survey, who worked on the suppression study, said the gill netting technique definitely has positive implications for managing invasive species in other water bodies. Of 12 Glacier lakes, nine were colonized by lake trout, and in eight of those, bull trout were fundamentally extinct before the netting of lake trout began. Gill netting has provided a much-needed opportunity for bull trout populations to be restored.

“There’s hope,” Muhlfeld said in a recent interview. “If we can overfish the oceans and the Great Lakes, why can’t we overfish these small, glacially-carved lakes?”

If exploitation — heavy fishing — rates of lake trout can continue to exceed recruitment — the introduction of new individuals through reproduction — invasive trout populations will continue to decline. Bull trout populations have benefited greatly from dedication, proactive effort, and hard work, Muhlfeld noted.

In the Park, bull trout populations are managed by fisheries biologist Chris Downs and his staff. They’ve implemented suppression strategies, constructed fish passage barriers, and monitored the lakes on a five-year rotation. This five-year period has proven effective because lake trout live twice as long as bull trout, in addition to being larger.

“Lake trout pose a big threat and a big challenge,” Downs said, echoing the now-familiar sentiment. On a more hopeful note, Downs confirmed that biologists estimate there are now fewer than 100 adult lake trout in Quartz Lake.

Additionally, he noted that Glacier Park is developing a fisheries management plan for long-term conservation and restoration of lake ecosystems and native species.

The management plan is expected to be finalized within the next couple of years.