NATIONAL GLACIERS PARK, Mont. – In civilization, the invaders change the language, food and customs of the places they conquer. Invading fish do not get on tanks or chariots, but their disturbance leaves almost warlike marks on the ecology.
This competition currently takes place between the native Bull Trout of Montana and the Invasive Lake Trout in the Flathead River Basin. New research indicates that even though the lakers ran like Genghis Khan, the bulls could latch on if they get help.
A new study from the University of Montana’s Flathead Lake Biological Station shows just how big and long-lasting invasive species are on regional waterways.
âOnce we get to a tipping point, things turn quickly for Bull Trout,â said Shawn Devlin, aquatic ecologist at the Flathead Biological Station and co-author of the study.
âThis work has shown that if you give the bulls a chance before this tipping point – before they are in a spiral that they will never come back from – they can be managed for conservation,â he added. âAnd the good news is that these lakes were invaded for a lot longer than expected, but it took longer than expected for the effects to be felt. It was a great discovery. It gives managers hope that there is more time below this tipping point than we thought. “
Bull trout in lakes play the same ecological role as grizzly bears on land – the # 1 predator in their natural habitat. They eat other fish, grow and reproduce slowly.
Lake trout occupy a similar niche in their native waters of the Great Lakes and rivers of the Midwest. But they have a crucial spawning advantage.
Bull Trout go through a life cycle similar to that of salmon hatching in small streams before reaching maturity in large rivers and lakes and then returning to spawn in the same stream where they were born. It also makes them vulnerable to many other predators when they are young. as human threats such as river dams, irrigation systems and sedimentation from logging or road construction.
Lake trout spawn on rocky outcrops in deep water. While grizzly bears and eagles can harass Bull Trout in their shallow spawning streams, few competitors reach lake trout egg deposits. And when they grow up, lakers eat the same bull trout target fish.
Since being artificially introduced to Flathead Lake in the early 20th century, lake trout have become a popular game fish due to their ability to grow to the size of a lunker. Second, a separate effort to improve Swan Lake’s artificial Kokanee salmon population by adding mysis shrimp had an unintended consequence. The tiny shrimp made their way down the Swan River to Flathead Lake, where they became a new food source for lake trout. The lake’s population rapidly expanded, sending ripples into the ecology of all other fish species in the system.
Such transformations are called “trophic cascades”. In the case of Flathead Lake, the young lake trout supplanted the Kokanee for zooplankton and other tiny organisms, while the mature lakers destroyed the Kokanee schools. They also preyed on native cutthroat trout and bull trout, depleting both their populations and their food reserves.
And then lake trout began to spread through the Flathead River system, invading bull trout strongholds on the west and south sides of Glacier National Park. The Hungry Horse Dam kept them from going far into the Bob Marshall Wilderness Resort to the south. But Lakes McDonald, Logging, Quartz, Bowman, and Kintla have all seen their Bull Trout populations plummet.
Lead author of the study, Charles Wainright of the US Geological Survey, spent 49 days prowling 10 remote lakes in Montana. This included the battlefields of Glacier Park like Quartz and Logging and Arrow lakes, as well as sites in the Bob Marshall Nature Complex that still support Bull Trout habitat, like Big Salmon Lake.
The park’s water bodies represent almost a third of all Bull Trout habitat in the lower 48 states. Bull trout have âthreatenedâ status under the federal Endangered Species Act.
âIf we lose Bull Trout in these lakes, the system will never go back to what it looked like,â Devlin said. What the study found is that one species doesn’t just overeat the other. Everything around them is affected.
âThe whole lake is important, not just the traditional food route from the little things to the big things,â Devlin said. âBull trout are not good at finding other foods. When they cannot get the big fish they were eating in the middle of the lake, they are forced to go to the shallows and littoral areas with sub-optimal food. Then their rate of growth is stifled. Meanwhile, lake trout grow like gangbusters.
This change also affects everything around the two species of trout: phytoplankton, insects, frogs, spiders, and anything that feeds on or falls into the lake. As Bull Trout shift from eating other fish to consuming insects, this affects populations of insects as well as other trout such as cutthroat and rainbow trout which hunt insects. The entire food web becomes exhausted and can unravel.
Which brings us to the other important finding of the study: the time factor.
By examining both the ratios of invading fish to native fish and what everything was eating, the study assessed how – how permanent an invasion had become. And it turned out that the process is taking longer than most researchers expected.
This gives wildlife managers more options. Late-stage interventions could be as complicated as Glacier Park’s efforts to create bull trout sanctuaries while using gillnets in infested lakes. An early invasion could be a matter of simple fishing regulations, such as an unrestricted catch of lake trout in protected waters. Flathead Lake is past this point.