A ‘Story of Invasions’

By Beacon Staff

To understand northwestern Montana’s lake trout debate, which has far-ranging implications for Flathead Lake and has caused entrenched gridlock between management agencies, it would help to understand some background on the fish in question and their waters. Amidst the back-and-forth rhetoric and statistics and environmental studies, it’s easy for the average non-angler, non-biologist observer to get lost. A little context always helps.

Flathead Lake is the largest natural freshwater lake in the contiguous western United States, just slightly bigger than Lake Tahoe. The lake has 10 known native fish species, according to a 2011 University of Montana scientific report by a team of researchers who are among the definitive experts on the Flathead’s aquatic ecosystems. The report, published in the Proceedings of the National Academy of Sciences, asserts that 14 nonnative fish species were introduced into the lake between 1890 and 1920, including lake trout, also called mackinaws or “macks.” The paper’s authors include lead author Bonnie Ellis and her husband Jack Stanford of the Flathead Lake Biological Station, as well as Barry Hansen of the Confederated Salish and Kootenai Tribes and Mark Deleray of Montana Fish, Wildlife and Parks, whose agencies are currently at odds in the lake trout debate.

Pre-1920 is referred to as the “Native Period,” in which native westslope cutthroat trout were the dominant catch of anglers. The following years, between 1920 and 1984, are called the “Kokanee Period,” indicating the rise of nonnative kokanee salmon in Flathead Lake. By 1940, kokanees – landlocked sockeye salmon – replaced cutthroat as anglers’ dominant haul. This was an important transition because it marked the kokanees’ establishment as a beloved game fish, and prompted subsequent management efforts to boost the species’ populations. Those efforts would backfire and lead to the explosion of lake trout, setting the stage for the ongoing dispute.

At their peak, kokanees thrived on multiple species of zooplankton, an abundant food source at the time. During this period, the biologists note that “native cutthroat trout remained at low but measurable densities, nonnative lake whitefish continued to expand, and nonnative lake trout remained at low densities.” Native bull trout, the report states, were the lake’s “dominant predator.” But by the late 1960s, drastic ecological changes were underway elsewhere in the Flathead drainage and making their way to the lake.

Those changes stemmed from a fateful management decision to introduce a tiny opossum shrimp called Mysis diluviana into five upstream lakes from 1968 to 1976. State fisheries managers transferred the shrimp from Alberta’s Waterton Lake, believing the small crustacean would “promote kokanee populations by increasing forage, an action based on erroneous interpretations of the results of such introductions elsewhere.” In a companion paper written for the academic journal “Montana Professor,” Ellis cites British Columbia’s Kootenay Lake as an example.

There, mysids were introduced for rainbow trout, “but the kokanee population proliferated instead.” For fisheries managers, it seemed logical that mysids would also spur kokanee proliferation in the Flathead.
But rather than provide more food for kokanees, the shrimp – which had drifted down into Flathead Lake by the early 1980s – actually fed on the salmon’s favorite food source, zooplankton. And they did it at night when the kokanees weren’t feeding, then retreated to the lake bottom when kokanees were sight-feeding during the day. Conversely, in Kootenay Lake, Ellis writes that the lake bottom’s unique contours resulted in the shrimp “being carried from deeper waters into a shallow bay where they become easy targets for salmon during the day.” The result in Flathead Lake: mysis shrimp populations exploded as zooplankton declined precipitously.

Furthermore, mysids also provided deep-water forage for lake trout, whereas there had previously been little food for young mackinaws at those depths. Eighty years after being introduced into Flathead Lake and living for decades in relatively small densities, lake trout suddenly boomed, surpassing bull trout as the lake’s top predator and triggering the collapse of the kokanee sports fishery. Like bull trout, westslope cutthroat also markedly declined, while peamouth and northern pikeminnow populations grew. Even bald eagles were affected; as their favorite prey, kokanee salmon, disappeared, the eagles dispersed. An “unusually large vernal bloom of blue-green algae” also appeared in 1988.

Freshwater Mysis shrimp | Courtesy photo

While early hypotheses attributed the kokanees’ decline to direct food competition from mysis shrimp, Ellis writes that “our recent bioenergetic research showed that the collapse of the kokanee population was best explained by the dramatic increase in the nonnative lake trout population and their subsequent predation on kokanee.” In other words, lake trout – fueled by mysis shrimp – basically ate kokanee salmon right off the food chain.

Ellis, a University of Montana professor with a Ph.D. in organismal biology and ecology, explains the impact of these changes succinctly: “the ‘mysid explosion’ period from 1985 to 1988 was certainly the most dramatic era in the ever changing food web of Flathead Lake.” And today’s great Flathead Lake management debate is rooted in those unintended consequences of mysis shrimp introduction, as are management dilemmas for other bodies of water throughout the watershed.

“This voracious piscivore now dominates the lake fishery,” the report says of lake trout. “Formerly abundant kokanee were extirpated, and native bull and westslope cutthroat trout are imperiled.”

The story of Flathead Lake, Ellis says, “is a story of invasions.” Altogether, 19 different nonnative fish species were introduced into the lake from the 1890s to the 1960s. It’s up to fisheries managers to analyze and respond to the changes wrought by those introductions – in some cases self-inflicted – and then implement sound policy, taking into account both scientific research and public desire. The lake trout dilemma is particularly tricky because of Flathead Lake’s distinct social and management dynamics: with the waters cutting through a reservation, the Confederated Salish and Kootenai Tribes and Montana Fish, Wildlife and Parks have co-management responsibilities, while federal agencies are involved as well. The U.S. Fish and Wildlife Service oversees species protected under the Endangered Species Act, and bull trout are listed as “threatened.”

A lake trout caught from Swan Lake. | Courtesy photo

In 2000, the tribes and FWP entered into a 10-year Flathead Lake co-management agreement. The plan called for using recreational angling and fishing tournaments as the primary methods for suppressing lake trout populations, with the goal of spurring bull and cutthroat trout populations. The biannual tournaments – called “Mack Days” – have grown into major fishing derbies, attracting hundreds of anglers and removing tens of thousands of lake trout each year. But since the co-management agreement expired in 2010, the tribes and FWP have failed to find middle ground on a path forward.

The tribes launched a study under the National Environmental Protection Act to explore the potential impacts of expanded lake trout suppression efforts in boosting native fish recovery, with the intent of forming a new science-based management plan. Last year, FWP publicly removed itself from the process, signaling a deep impasse between the tribes and state wildlife agency. Biologists and fisheries managers from FWP maintain that the lake currently has a balance of stable bull trout populations with a strong lake trout recreational fishery – a fishery utilized by both amateur anglers and professional charter services.

But tribal fisheries managers say bull trout need more help in their recovery. In June, the tribes, along with the Bureau of Indian Affairs, released a proposal that calls for reducing lake trout populations by up to 75 percent, though it provides options for smaller reductions as well. The new plan proposes continuing the old suppression methods of recreational angling and fishing tournaments, but also allows for trap-netting, gill-netting and bounties. The U.S. Fish and Wildlife Service backs the plan, as does the U.S. Geological Survey, putting FWP squarely at odds with the tribes and three federal agencies.

As of early September, the agency stalemate was firmly intact, with one more entity joining the fray. The Flathead Reservation Fish and Wildlife Board, consisting of seven members appointed by the tribes and state to help guide co-management efforts, announced at its Aug. 21 meeting that it favors additional lake trout suppression steps in order to encourage bull trout recovery. Officials from FWP have called for a third-party mediator to help resolve differences. The CSKT tribal council then agreed to seek aggressive reduction at a meeting recently, setting the stage for action.

Decades after a tiny shrimp wreaked havoc in the Flathead watershed, fisheries managers are miles apart on the best way to deal with the continued fallout. The UM research paper ends with a section asking a question: “Lake Trout Period – A Persistent State?” It’s an open question. But the report does make clear that under the current ecological alignment, extermination of native bull trout and westslope cutthroat “in the near future seems possible.”
“Recovery of bull and cutthroat trout will be difficult given strong food web control by the expansive lake trout population,” the report concludes. “On the other hand, water quality remains near the pristine or native state relative to most lakes with significant numbers of people in the catchment.”

All of which is to say there are consequences and challenges to whatever fisheries managers decide, as has always been the case. When you throw direct human intervention into a complex aquatic environment, along with other external factors like climate change, even the lake’s most knowledgeable scientists don’t know how it will all play out. But like everybody else with an interest in Flathead Lake’s future, they’re curious to see what happens next.