Invasive species are major drivers of ecosystem change
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.
Zebra mussel impacts on walleye
Understanding impacts in a whole ecosystem context
Map of Minnesota's large walleye lakes included in this study, colored by presence of invasive zebra mussels (yellow), spiny water fleas (purple), or both (red).
Phase 1: Effects of zebra mussels and spiny water fleas in Minnesota's large lakes
2017-2019
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. From 2017-2019, we investigated 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 used stable isotope analysis to characterize the food web of each of the large lakes, and historical data collected by the Minnesota DNR to assess potential changes in first year growth of walleye and yellow perch associated with invasion.
Objectives:
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 focused 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 provided critical information on the impacts of two strongly interacting AIS in the context of economically important fisheries.
Key results:
- Age-0 walleye were >10% smaller at the end of summer following invasion by either AIS. Age-0 yellow perch growth decreased following zebra mussel invasion, although this effect was not statistically significant.
- Smaller length at the end of the growing season was associated with decreased survival to later life stages for walleye in 7 of the 9 study lakes.
-In general, juvenile walleye relied on offshore prey resources in invaded lakes. Combined with reduced growth rates, these results suggest that as zooplankton food resources decline following invasion, young walleye are not sufficiently accessing alternative prey resources to maintain pre-invasion growth rates.
These results demonstrate that zebra mussels and spiny water flea influence the growth rates of age-0 walleye and that a wide range of food resources and habitats support walleye in these lakes. Declines in growth rates of young walleye are an early signal of potential negative effects on walleye. This information can guide managers on the most effective and sustainable walleye harvest and stocking strategies in invaded lakes.
Hansen, G.J.A., Ahrenstorff, T.D., Bethke, B.J. et al. (2020). Walleye growth declines following zebra mussel and Bythotrephes invasion. Biological Invasions. https://doi.org/10.1007/s10530-020-02198-5
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.
Phase 2: Zebra mussel impacts on walleye populations and mercury concentrations
2019-2021
Zebra mussels profoundly impact lake ecosystems, but their impacts on walleye are not well-known. Following Phase 1, this second phase seeks to investigate the effects of zebra mussel specifically on changes in growth, recruitment, and mercury concentrations of walleye, without the confounding effects of spiny water fleas. We are also interested in how these dynamics play out in smaller, more typical Minnesota lakes, versus the highly unique large lakes that were the focus of Phase 1.
These effects have important implications for harvest, stocking, and consumption of walleye in Minnesota lakes. Quantifying the effects of zebra mussels on walleye recruitment will inform proactive management and allow for realistic goal setting and data-driven public communication following species invasions before a crisis hits. Quantifying food web structures in uninvaded vs. invaded lakes with indicate the mechanisms for walleye population changes or resilience. Understanding sources of mercury in walleye and how they are influenced by zebra mussels is critical for fish consumption advisories in Minnesota lakes.
Objectives
1. Assess the effects of zebra mussel invasion on walleye recruitment in Minnesota lakes. This is being investigated by graduate student Holly Kundel, read more here.
2. Quantify the relative reliance of age-0 and adult walleye on littoral vs pelagic resources in invaded and uninvaded lakes.
3. Quantify the impacts of zebra mussels on mercury concentrations in and pathways to walleye.
Master’s student Naomi Blinick will characterize the food webs of 14 Minnesota lakes using stable isotope analysis of carbon and nitrogen to quantify the reliance on littoral resources and trophic position of walleye, and how these are influenced by invasion status. Biological samples of fish, macroinvertebrates, and zooplankton will be collected over two summer field seasons in partnership with numerous MN DNR partners. In collaboration with researchers from USGS, she will also quantify mercury concentrations in walleye tissue and characterize the mercury stable isotope composition in our study lakes to identify pathways of mercury bioaccumulation and how it is influenced by zebra mussel-induced shifts in food web configuration.
You can watch her research update at the MAISRC 2020 Research and Management Showcase on YouTube.
This research is conducted in collaboration with the MN DNR (link) and USGS, and is funded by the Minnesota Aquatic Invasive Species Research Center with funds from the Environment and Natural Resources Trust Fund.
Key results
Objective 1
Results coming soon!
Phase 3: Quantifying Heterogeneous Impacts of Invasive Zebra Mussels on Walleye Habitat, Food Webs, and Mercury Concentrations
2021-2023
In phase 3, we look to build upon previous research to quantify food web changes in walleye and yellow perch in lakes invaded with zebra mussels. Stable isotopes and various modeling techniques will calculate the impact zebra mussels have on walleye and yellow perch. Explore the different aspects of the project below!
Field and lab work:
Throughout the summer of 2021 and 2022, Denver and his crew intensively sampled 14 lakes across Minnesota. During these field visits, the crew collected fish, zooplankton, and benthic macroinvertebrates using seines, small-mesh gill nets, zooplankton tows, and kick nets. Additional adult fish were collected in collaboration with the Minnesota Department of Natural Resources gill net sampling. Over 2,400 stable isotope samples were collected and processed in the lab. Typical stable isotope procedure includes drying, grinding, and precision weighing of each sample.
Modeling:
The robust stable isotope data set build between phase 2 and 3 allows for cross-lake statistical exploration of zebra mussel impact on food webs. Walleye typically rely on offshore lake resources. Zebra mussels filter important offshore energy resources, creating new importance of nearshore zones in the walleye food web. In this project, we use Bayesian mixing models to calculate the difference in walleye offshore and nearshore resource reliance in lakes invaded with zebra mussels. The stable isotopes of carbon and nitrogen enable the exploration of the differences.
The timing of the project presented a unique opportunity to explore food web trends. Leech Lake was sampled as a reference lake in phase 1. During the time since phase 1 sampling, zebra mussels have been introduced and expanded across Leech Lake. We collected stable isotopic data throughout the summer for pre/post invasion analysis of the lake. This analysis can build upon our cross-lake study.
Early results suggest that walleye rely more heavily on nearshore energy pathways in lakes invaded with zebra mussels. Coincidentally, mercury assimilates into the food web at higher rates in the nearshore regions of lakes. To explore potential public health concerns related to food web shifts, we collaborate with the United States Geological Survey to analyze samples for mercury. Using Bayesian modeling, we examine related trends of zebra mussel invasion with higher concentration of mercury in fish.
Expanding the work:
During the project, zooplankton samples were preserved in ethanol for further processing in the lab. Thus, we needed to address the impacts of ethanol preservation on zooplankton stable isotopic values. Undergraduate researcher Tristan explored the affects during the summer of 2022 and looks forward to sharing results soon. This work is important in drawing accurate calculations of nearshore reliance in walleye stable isotope modeling.
The Minnesota Pollution Control Agency and Minnesota Department of Natural Resources maintain a large database of fish contaminated data. To explore the public health risk of mercury with the invasion of zebra mussels, we intend to explore historic trends of mercury in lakes with varying invasion status. These results will help inform our overall conclusions of zebra mussels impact on walleye mercury concentrations.