Sustaining walleye populations: assessing impacts of AIS
Phase II: AIS impacts on walleye populations and mercury concentrations
The overall goal of this project is to assess the impacts of invasive zebra mussels on walleye in Minnesota lakes. Zebra mussels profoundly impact lake ecosystems, but their impacts on walleye are not well-known, and likely vary among lakes. We will investigate the impacts of zebra mussels on walleye and identify lakes that are most vulnerable to negative impacts by focusing on three specific objectives related to recruitment, food web dynamics, and mercury concentrations.
First, we will quantify the effects of zebra mussel invasion on walleye recruitment using statistical analysis of data collected by the Minnesota DNR from hundreds of lakes since the 1980s. We will assess the influence of zebra mussels as well as other lake characteristics on recruitment success. Next, we will characterize the food webs of 15 study lakes using stable isotope analysis of carbon and nitrogen to determine which habitats and food resources support walleye in invaded and uninvaded lakes. Finally, we will quantify mercury concentrations in walleye tissue and characterize the mercury stable isotope composition in our 15 study lakes to identify pathways of mercury bioaccumulation and how it is influenced by zebra mussel-induced shifts in food web configuration.
Zebra mussel-induced changes in walleye recruitment, mercury concentrations, and food webs have important implications for harvest, stocking, and consumption of walleye in Minnesota lakes. The impacts of zebra mussels on walleye likely depend upon their ability to switch to alternative food sources if and when invaders cause zooplankton prey to become scarce. This ability to switch food sources likely depends on lake characteristics including size, depth, productivity, and fish community composition. Determining how zebra mussels affect walleye, and identifying characteristics of walleye populations that can withstand these invasions with minimal effect will allow managers to set realistic goals for future walleye production and harvest. Quantifying effects 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. Understanding sources of mercury in walleye and how they are influenced by zebra mussels is critical for fish consumption advisories in Minnesota lakes
This phase of this project will be relevant to a broader group of stakeholders by expanding to additional lakes and by including analysis of mercury concentrations. As in Phase I, we continue to leverage ongoing monitoring by the Minnesota DNR valued at hundreds of thousands of dollars, which allows us to sample many lakes at low cost. Additionally, the mercury measurements for this project will be provided in-kind, which presents an unprecedented opportunity to understand how AIS affect contaminant cycling and human health. Through these partnerships, we are able to maximize the relevance of our results at relatively low cost.
This project will quantify the impacts of invasive zebra mussels and spiny waterfleas on food webs and growth rates of young fish in Minnesota’s nine largest walleye-supporting lakes. The lakes – Cass, Red, Kabetogama, Rainy, Vermilion, Lake of the Woods, Leech, Winnibigoshish, and Mille Lacs – are at varying stages and combinations of invasion from spiny waterflea and/or zebra mussels. Red Lake was not infested with either species during field work.
Zebra mussels and spiny waterflea impact the food web by reducing native zooplankton and filtering algae from the water column. The ability of walleye to sustain high populations following invasion is likely based on their ability to adjust to alternative food sources as energy is shunted out of the open water zone and down to the bottom of the lake. Understanding what makes a walleye population more successful following an invasion will allow managers to more precisely target early intervention tactics, more accurately predict walleye production levels following invasion, and better understand impacts.
Researchers will collect samples of zooplankton, other invertebrates, and fish from both the nearshore zone and the open water zone of the lakes. Stable isotope analysis will be used to determine what the fish have been eating, what habitat in the lake is supporting their production, and at what trophic level they’re eating. This will tell us to what degree walleye rely on zooplankton as a food source, and how their diet changes when an invasive species is present. Researchers will also assess the effects of reduced zooplankton on the growth rates of walleye and yellow perch in their first year of life. It’s possible that young fish may be less impacted by invasive species if they can learn to pivot their diet as zooplankton abundance declines.
Understanding how these invasive species disrupt walleye food sources will allow managers to better project realistic levels of walleye production and harvest.
As of January 2019, samples of fish, benthic macroinvertebrates, and zooplankton have been collected from all nine lakes and have been sent for stable isotope analysis. This will allow researchers to characterize the food webs of these lakes with a high degree of accuracy. Data on age-0 walleye and yellow perch have also been collected from all of the lakes.
Initial results are showing slower growth of walleye in their first year of life in lakes invaded by zebra mussels and spiny waterflea. Yellow perch rates were somewhat slower in lakes invaded by zebra mussels, but these differences were not statistically significant. No changes in yellow perch growth were detected in lakes with spiny waterflea. Age-0 fish are most likely to be directly affected by reduced zooplankton populations. To analyze historical growth data, researchers used 50,012 individual walleye lengths and 176,983 individual yellow perch lengths that have been collected yearly since 1983.
Researchers compared age-0 walleye and yellow perch growth over 35 years, including pre- and post-invasion. 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. Using stable isotope analysis, researchers documented a high degree of variability in the resources supporting all life stages of walleye. 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. Variability in walleye diets among lakes may reflect differences in lake productivity or morphology, not necessarily the presence of AIS.
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.