Gretchen Hansen's research on invasive species in lakes
Invasive species alter aquatic food webs and represent a major management challenge. New invasive species are discovered in Midwestern lakes each year, and developing appropriate strategies requires an understanding of their impacts, potential for spread, and management alternatives. Current research is ongoing to examine the impacts of zebra mussels and spiny water fleas on sport fish populations and food webs in Minnesota's large lakes. Past research has examined landscape-level distributions of a large number of aquatic invasive species, as well as conducted large scale experimental removal of a single invasive species (rusty crayfish) to better understand their role in aquatic food webs and to identify options for control.
effects of Zebra Mussels and Spiny water fleas in Minnesota's large lakes
Zebra mussels (ZM) and spiny water fleas (SWF) are aquatic invasive species with profound effects on lake ecosystems, although their impacts on fish are not well understood. We are in year two of a two year study examining the food web and population-level impacts of ZM and SWF on walleye in Minnesota’s Large Lakes. The Large Lakes are at various stages of invasion by ZM and/or SWF and provide a unique opportunity to study their ecosystem impacts. We are using stable isotope analysis to characterize the food web of each of the large lakes, and using historical data collected by the Minnesota DNR to assess potential changes in first year growth of walleye and yellow perch associated with invasion.
1. Quantify the contribution of pelagic and littoral production supporting walleye and their trophic position in each lake to assess ongoing or potential food web impacts.
2. Assess the direct effects of reduced zooplankton biomass due to ZM and/or SWF invasion on the somatic growth rates and fitness of age-0 walleye and yellow perch.
By studying direct impacts to walleye growth within the context of the whole food web, we will be better positioned to interpret our results. For example, if we observe strong reliance of walleye on littoral resources in invaded lakes but no changes in age-0 growth, this would suggest that walleye are successfully able to adapt to changing conditions in that system. Conversely, reduced growth rates accompanied by a strong reliance on pelagic pathways in invaded lakes would suggest that declining zooplankton resources are negatively affecting walleye and that they are currently not compensating for these changes by shifting their resource base.
Our study focuses on Minnesota’s nine largest walleye lakes (>6,000 ha) because of their social and economic importance, rich sampling history, and invasion status. With a combined surface area of more than 640,000 hectares, these lakes account for 33% of Minnesota’s walleye harvest and contribute significantly to the $2.4 billion spent each year on fishing in Minnesota. The Large Lakes are at various stages of invasion by ZM and/or SWF and provide a unique opportunity to study the ecosystem impacts of these notorious invasive species. This study will provide critical information on the impacts of two strongly interacting AIS in the context of economically important fisheries.
This research is collaborative with the Natural Resources Research Institute at the University of Minnesota-Duluth and is funded by the Minnesota Aquatic Invasive Species Research Center with funds from the Environment and Natural Resources Trust Fund.
whole-lake removal of invasive rusty crayfish
Once an invasive species establishes a self-sustaining population, few options exist for mitigating its negative effects. Eradication or control of invasive species can be successful under certain conditions, but most eradication attempts have occurred on small islands. Eradication efforts can produce unexpected results in terms of community dynamics and ecosystem properties, particularly when an invader has been established for long time periods, exhibits strong interactions with other species, or alters physical properties of an ecosystem.
Rusty crayfish are native to the Ohio River Valley, and established populations in Wisconsin Lakes in the 1970s. Rusty crayfish eliminate aquatic plant beds and negatively impact panfish and invertebrate populations, including snails and native crayfish. Rusty crayfish were experimentally removed from Sparkling Lake, a 64 hectare lake in Vilas County, Wisconsin, from 2001-2008 via intensive trapping and changes in sport fish regulations in an attempt to restore the lake to pre-invaded conditions. The removal was successful in reducing rusty crayfish catch rates by over 99%, and the native virile crayfish increased in abundance 100-fold over this time period. As of 2015, rusty crayfish in Sparkling lake remain at low densities, and the aquatic ecosystem has exhibited substantial changes as a result. Monitoring of Sparkling Lake pre- and post-removal was conducted by the North Temperate Lakes LTER.
Hansen, G. J. A., C. L. Hein, B. M. Roth, M. J. Vander Zanden, J. W. Gaeta, A. W. Latzka, S. R. Carpenter. 2013. Food web consequences of long-term invasive crayfish control. Canadian Journal of Fisheries and Aquatic Sciences. 70: 1109-1122
Abundance distributions of invasive species
Invasive species are recognized as a leading driver of environmental change. Their impacts are often linked to their population size, but surprisingly little is known about how frequently they achieve high abundances. A nearly universal pattern in ecology is that species are rare in most locations and abundant in a few, generating right-skewed abundance distributions. Together with many collaborators, we collated abundance data from over 24,000 populations of 17 invasive and 104 native aquatic species to test whether invasive species differ from native counterparts in statistical patterns of abundance across multiple sites. Invasive species on average reached significantly higher densities than native species and exhibited significantly higher variance. However, invasive and native species did not differ in terms of CV, skewness, or kurtosis . Abundance distributions of all species were highly right skewed, meaning both invasive and native species occurred at low densities in most locations where they were present (6% and 2% of the maximum densities observed within a taxa, respectively). The biological significance of the differences between invasive and native species depends on species-specific relationships between abundance and impact. Recognition of cross-site heterogeneity in population densities brings a new dimension to invasive species management, and may help to refine optimal prevention, containment, control, and eradication strategies.
Hansen, G. J. A., M. J. Vander Zanden, M. J. Blum, M. K. Clayton, E. K. Hain, J. Hauxwell, M. Izzo, M. S. Kornis, P. B. McIntyre, A. Mikulyuk, E. Nilsson, J. D. Olden, M. Papes, S. Sharma. 2013. Commonly rare and rarely common: comparing population abundance of invasive and native aquatic species. PLoS ONE 8(10): e77415. doi:10.1371/journal.pone.0077415.
sea lamprey control in the great lakes
Sea lamprey are an invasive, parasitic fish species in the Great Lakes that kill large numbers of lake trout and other salmonids. A bi-national, multi-million dollar control program exists to suppress sea lamprey populations and allow higher numbers of lake trout to exist in each of the Great Lakes than would be possible in the absence of such a program.
For my master’s thesis, I conducted an adaptive management experiment across the entire Great Lakes basin to evaluate the performance of an alternative method of assessing sea lamprey populations and ranking streams for treatment with lampricides. I compared the differences in the total number of sea lampreys that would be killed as a result of each assessment method using population estimates generated from the assessments and independent population estimates from mark-recapture studies on a subset of streams. I found that the alternative assessment method outperformed the status quo method, and the international body charged with coordinating sea lamprey management in the Great Lakes subsequently adopted the alternative assessment method in 2008.
Following the completion of the empirical study, I collaborated with an economist to analyze thissystem using an economic modeling approach and to determine the conditions which would favor one assessment method over another. I also worked in collaboration with several researchers to develop a management strategy evaluation (MSE) style model of sea lamprey management. MSE models include components of the entire managed system, and incorporate uncertainty in everything from demographic processes to the enforcement of management actions. We used this model to test the efficacy of various management actions and to direct future research by identifying key uncertainties.
Another component of my master’s work incorporated historical sea lamprey survey data collected from over 300 Great Lakes streams over 50 years. I used generalized linear mixed models to analyze patterns in larval sea lamprey growth and recruitment from over 30,000 surveys. I found that consistent patterns exist in sea lamprey recruitment that could be used to more effectively assess streams for treatment.
Hansen, G. J. A and M. L. Jones. 2008. Evaluating an alternative larval assessment method for Great Lakes sea lampreys: A case study in adaptive management. Canadian Journal of Fisheries and Aquatic Sciences 65: 2471-2484.
Hansen, G. J. A. and M. L. Jones. 2009. Variation in larval sea lamprey demographics among Great Lakes tributaries: A mixed-effects model analysis of historical survey data. Journal of Great Lakes Research 35: 591-602.
Jones, M. L., B. Irwin, G. J. A. Hansen, H. A. Dawson, A. J. Treble, W. Liu, W. Dai, and J. R. Bence. 2009. An operating model for the integrated pest management of Great Lakes sea lampreys. The Open Fish Science Journal 2: 59-73.
Fenichel, E. P., and G. J. A. Hansen. 2010. The opportunity cost of information: An economic framework for understanding the balance between assessment and control in sea lamprey management. Canadian Journal of Fisheries and Aquatic Sciences 67: 209-216.