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Researchers Study How Lake Trout Removal In Flathead Lake Might Alter Complex Food Web
Posted on Friday, March 16, 2012 (PST)

How would Flathead Lake’s complex food web and ecology change if an aggressive netting project started removing 140,000 lake trout every year?


That is considered an important question that has yet to be answered, but it is a subject being addressed in a study being conducted for the Confederated Salish and Kootenai Tribes as part of an environmental review for a proposed lake trout netting project on the lake.


Montana Fish, Wildlife and Parks officials recently cited the pending study as one reason why the tribes’ environmental impact statement for the project is lacking so far. The department, which co-manages the lake with the tribes, has withdrawn support from the process until state officials can review a more thorough environmental impact study.


“It is very important,” said Mark Deleray, the state fisheries biologist for Flathead Lake, referring to the pending food web study. “Flathead Lake and its entire aquatic system is very complex. All the levels are interrelated and they interact.”


The work being led by Bonnie Ellis, a research assistant professor at the University of Montana’s Flathead Lake Biological Station, actually is a two-part model developed at the University of Western Australia. It has been well tested and applied to aquatic ecosystems around the world.


The first part, Ellis explained, is a three-dimensional hydrodynamic model that accounts for physical influences and characteristics of the lake, such as solar radiation, air temperatures, wind, evaporation, humidity, currents and more.


The second part is a model that defines the interactions of the food chain, from fish to mysis shrimp to zooplankton and the various species of phytoplankton.


“The hydrodynamic model basically drives the biological model,” Ellis said.


Mysis shrimp, for example, avoid warmer water at shallower depths so temperature differences throughout the water column are critical to the model.


“It’s really important to have a really good physical model,” Ellis said.


When the models are combined, they will predict food web changes that result from changes in mysis densities throughout the lake.


Lake trout and whitefish eat mysis, which consume zooplankton, which in turn feed on the various phytoplankton algae species.


“The model might tell us that zooplankton densities increase and therefore algae abundancies will decrease,” Ellis said.


Or, it might indicate that with fewer lake trout, there will be higher mysis densities, a decrease in the abundance of zooplankton, and more algae growth in the lake. In other words, a decline in water quality.


“Depending on which groups of zooplankton the mysis feed on, we might see very different outcomes in terms of the phytoplankton” or algae growth, Ellis said.


Ellis said the model will be updated with a new count of mysis densities from last year that isn’t complete yet, and it will factor in work being done by a University of Washington researcher to predict mysis densities based on various lake trout removal scenarios. One of the alternatives in the EIS calls for removing as many as 140,000 lake trout annually.


According to Deleray, it’s estimated that somewhere in the neighborhood of 70,000 lake trout now are being removed annually by anglers.


In 2009, Ellis said, a mysis sample count was the third-highest recorded for Flathead Lake. There was an average of 90 mysis for every square meter of lake surface.


The all-time peak count was 129 per square meter in 1986, followed by 125 per square meter counted in 1996.


The 2009 count indicates an obvious reduction in predation pressure on the mysis population, either by lake trout or whitefish, Ellis said.


The tiny mysis shrimp is the best example of how the lake’s food web can be dramatically altered by a seemingly small change.


Mysis were introduced in five lakes upstream from Flathead Lake in the late 1960s and early 1970s in an effort to increase the size and abundance of rainbow trout. Mysis eventually found their way into Flathead Lake, where they first were detected in 1983.


In the years that followed their numbers increased exponentially, and they became an abundant food source for non-native lake trout that had existed in the lake in relatively low numbers since the turn of the century.


“Mysis really provided a food source for young-of-the-year lake trout, which previously had very little food,” Ellis said.


The lake trout population exploded, with larger lake trout having devastating impacts on a kokanee salmon fishery that no longer exists.


Lake trout subsequently have proliferated throughout the Flathead River system and its connected lakes with heavy impacts on native bull trout, a threatened species since 1998, and on westslope cutthroat trout populations.


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