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Flathead Lake study points to shrimp as cause of decline in biological diversity

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It hasn't been the same since 1986, and Flathead Lake may never be again.

For years, scientists have been concerned about water quality and increasing nutrient levels in the largest freshwater lake in the western United States.

Meanwhile, the kokanee salmon, bull trout and cutthroat trout populations in Flathead Lake have struggled to survive or died off completely as introduced lake trout took dominion.

Though scientists at the Flathead Lake Biological Station suspected those two phenomenons were somehow interrelated, they couldn't prove that hypothesis. A lingering hypothesis nags at scientists, of course, so a team from across the Pacific Northwest set out to analyze more than 100 years of data regarding food-web structure and the lake's dynamics.

Four-and-a-half years later, they now know their hypothesis was correct. Water quality in Flathead Lake and the rise and fall of different fish populations all point to the same culprit: introduction of the mysis shrimp in the 1980s.

The significant ecological breakthrough shows how the introduction of invasive species can affect a large lake's biological diversity. The groundbreaking study was published in one of the world's most prestigious scientific journals, the Proceedings of the National Academy of Sciences, and was highlighted in last week's issue of Nature magazine.

"I'm getting calls from all over the world on this," said Jack Stanford, director of the University of Montana's Flathead Lake Biological Station at Yellow Bay.

In the 1980s, proposed mining in the North Fork of the Flathead River threatened Flathead Lake water quality, which was already under assault from development in the upper basin and along the lake's shores.

Then came a ban on phosphate detergents in towns around Flathead Lake as increasing nutrient levels became a concern to valley residents.

Since 1977 - when the biological station first began gathering year-round data - algae growth (a sign of decreased water quality) spiked 30 percent in Flathead Lake. A majority of that growth - 21 percent - occurred in one year alone, the same year the mysis shrimp population peaked in the lake.

Mysis shrimp were introduced into Swan, Ashley and Whitefish lakes by the state in the early 1980s to enhance growing Kokanee salmon populations. Mysis eventually invaded Flathead Lake, where they turned out to be anything but beneficial.

The data gathered at the station, which span 120 years, show that Flathead Lake has several distinguishable fish periods.

Prior to 1920, native bull trout and cutthroat dominated the lake. Though nonnative lake trout existed, they remained at low densities because of the lack of deep-water food sources.

Kokanee salmon dominated the lake from 1920 to 1984. They replaced cutthroat trout as the dominant catch for anglers. Both fish competed for the same food source, zooplankton, and cutthroat lost out.

The outlet of Lake McDonald in Glacier National Park was a popular spawning location for kokanee. The fish attracted many eagles, which fed on the salmon and drew large crowds of park visitors.

Toward the end of this period, the kokanee population was estimated to have reached 2.3 million.

Then the mysis shrimp made their way into Flathead Lake and the food web changed drastically.

The mysis population swelled from only a few in 1982 to 70 billion or

80 billion in 1986. Suddenly lake trout, bottom-floor feeders, had something to eat and their numbers soared. The non-native fish thrived while the natives suffered.

"During that period, the population of kokanee in the lake fell and never recovered, the bull trout declined and the lake trout came to the dominant top predator," said Bonnie Ellis, a UM research assistant professor at the biological station who worked full time on the study. "At the same time as the kokanee declined, bald eagle numbers dropped with the collapse of their primary prey."

Efforts are under way to preserve the bull trout, now a federally endangered species. However, the link between water quality and fish populations shows that recovery of the bull trout may present a conundrum of sorts. Reducing lake trout in order to allow bull trout to thrive may accelerate mysis population growth, which could in turn lead to increased algae growth and decreased water quality.

Which is bad for bull trout.

Whatever the answer, the study speaks to the importance of looking at Flathead Lake's ecosystem in terms of its food web - a more holistic approach.

One of the most important and unexpected findings in the study was the peak mysis number and its correlation with the rapid algae growth. The role mysis played in the changes of the Flathead Lake food web in 1986 is what scientists refer to as a "trophic cascade."

"The mysid invasion changed the entire food web and in that way altered the water quality in the Flathead Lake by increasing algae growth," Ellis said.

Not only are the study's findings significant, but so, too, is the scope of the study, Stanford said. A database of more than 100 years of research data for a big lake is unusual.

With the study published in a prestigious journal, the next step is to build a computer database to model the future, Stanford said. While trout lake and mysis numbers have stabilized, it's only a matter of time before something disturbs the food web.

That something might be climate change, Stanford predicts. Since 1977, the winter water temperature in the lake has risen 2 degrees Celsius.

"That's a major change in a lake that big," he said.

And many of the lake's inhabitants, including mysis, thrive in cold water.

Reporter Chelsi Moy can be reached at 523-5260 or at chelsi.moy@missoulian.com.

 

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