Sustaining walleye populations: assessing impacts of AIS
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.
Having a better understanding of the impacts of spiny waterflea and zebra mussels on food webs is critical for walleye management. 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.