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Founding population size of an aquatic invasive species

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Founding population size of an aquatic invasive speciesAbstractIntroductionStudy area and populationsMethodsResults and discussionAcknowledgmentsReferencesSHORT COMMUNICATIONFounding population size of an aquatic invasive speciesSteven T. Kalinowski•Clint C. Muhlfeld•Christopher S. Guy•Benjamin CoxReceived: 24 July 2009 / Accepted: 24 December 2009Ó Springer Science+Business Media B.V. 2010Abstract Non-native species of fish threaten native fishesthroughout North America, and in the Rocky Mountains,introduced populations of lake trout threaten native popu-lations of bull trout. Effective management of lake troutand other exotic species require understanding thedynamics of invasion in order to either suppress non-nativepopulations or to prevent their spread. In this study, weused microsatellite genetic data to estimate the number oflake trout that invaded a population of bull trout in SwanLake, MT. Examination of genetic diversity and allelefrequencies within the Swan Lake populations showed thatmost of the genes in the lake trout population are des-cended from two founders. This emphasizes the importanceof preventing even a few lake trout from colonizing newterritory.Keywords Exotic species  Lake trout  Invasion BottleneckIntroductionInvasive species threaten the biodiversity of aquatic eco-systems worldwide (Vitousek et al. 1997), and are con-sidered the second greatest threat to biodiversity loss inNorth America (Mooney and Cleland 2001). Non-nativefishes have homogenized fish faunas throughout NorthAmerica, impacting many species and populations ofnative fishes through competition, predation, and intro-gressive hybridization (Rahel 2000). Invasions of intro-duced taxa often disrupt the structure and function ofecosystems, reduce biological diversity among nativespecies, and impose huge economic costs (Mack et al.2000). Therefore, understanding how exotic species invadeecosystems is critical for conserving many native aquaticspecies.Lake trout (Salvelinus namaycush) are large, long-lived,top-level predators native to deep, cold, oligotrophic lakesof Canada and northern parts of the United States,including the Great Lakes (Behnke 2002). During the late19th and early 20th century, lake trout were widely intro-duced into lakes and reservoirs outside their native range(Crossman 1995). More recently, the species is expandingits range in the western United States through dispersal andunauthorized translocations (Behnke 2002). While laketrout occupy an important ecological niche as a top-levelpredator in lakes where they are native, they have oftenbecome predators and competitors with native fishes inlakes where they have been introduced (Ruzycki et al.2003; Koel et al. 2005; Martinez et al. 2009).Lake trout recently colonized Swan Lake, MT and nowthreaten a native population of bull trout (S. confluen-tus).The objective of this study was to determine thenumber of lake trout that founded the Swan Lake popula-tion. Lake trout are unique relative to other aquaticS. T. Kalinowski (&)  B. CoxDepartment of Ecology, Montana State University,310 Lewis Hall, Bozeman, MT 59717, USAe-mail: [email protected] C. MuhlfeldU. S. Geological Survey, Northern Rocky Mountain ScienceCenter, Glacier National Park, West Glacier, MT 59936, USAC. S. GuyU. S. Geological Survey, Montana Cooperative Fishery ResearchUnit, Montana State University, 301 Lewis Hall, Bozeman,Montana 59717, USA123Conserv GenetDOI 10.1007/s10592-009-0041-8invasive species because they are long-lived and have ahigh age at first reproduction. Thus, understanding theeffects of founding population size on the rate of popula-tion expansion will provide useful information for ecolo-gists wanting to control the expansion of lake trout in lenticecosystems.Study area and populationsSwan Lake is 10.8 km2and located in the Seeley-SwanValley of western Montana. The lake morphometry ischaracteristic of glaciated lakes in the region. However,Swan Lake is relatively shallow compared to other lakesthat contain lake trout in the Intermountain West. Thenative population of bull trout in Swan Lake is one of themost robust extant populations in the United States (Fraleyand Shepard 1989; Rieman et al. 1997). Bull trout are listedas ‘‘threatened’’ with extinction under the US EndangeredSpecies Act, which provides a legal mandate to protect theSwan Lake population. Unfortunately, lake trout werediscovered in Swan Lake in 1998 (Vernon 1998). Bull troutand lake trout are both top-level predators (Donald andAlger 1993), and lake trout consistently displace native bulltrout in lake systems where they have become established(Donald and Alger 1993; Fredenberg 2002). The popula-tion of lake trout in Swan Lake appears to be growingrapidly. Approximately 7,000 lake trout [270 mm wereestimated to be in Swan Lake in 2008, and 94% of thesefish were between 3 and 5 years old (Cox and Guyunpublished).The source of the lake trout invasion in Swan Lake wasmost likely Flathead Lake. Flathead Lake is the largestfreshwater lake in the western coterminous United Statesand has had a large lake trout population since lake troutwere introduced in 1905. During the 20th century, lake troutspread throughout the Flathead Drainage Basin (Meeuwig2008), and were first caught in the Swan Lake basin in 1998.Swan Lake is connected to Flathead Lake by the SwanRiver which drains Swan Lake and enters Flathead Lake23 km downstream, and this is a plausible invasion route forlake trout. Fish movement between Flathead Lake andSwan Lake was impeded by Bigfork Dam, which was builtin 1902, but the dam was retrofitted with a fish ladder thatwas operational from 1959 to 1993. Lake trout could haveentered Swan Lake via the fish ladder or illegally intro-duced. If lake trout were introduced illegally, Flathead Lakeis the most plausible source, as it is only a 10 km drive fromFlathead Lake to Swan Lake. The second closest possiblesource is Whitefish Lake, which is 66 km by road fromSwan Lake. The exact mechanism of how lake trout enteredSwan Lake is unknown, but it is likely they have been in thesystem for several years because invasive species are oftenestablished before being sampled using standard samplingmethods (Munro et al. 2005).MethodsTissue samples of lake trout were collected from SwanLake (N = 389) in 2006 and 2007, and from Flathead Lake(N = 15) in 2007. Special care was taken to sample fishrandomly from Swan Lake. In 2006, lake trout in SwanLake were


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