Minnesota scientists dive deep to learn why walleye are stressed

Gretchen Hansen and Carl Pedersen pull in a net.
DNR fisheries research scientist Gretchen Hansen and large lake specialist Carl Pedersen pull in a net on July 11, 2017 while conducting a fish population survey on Leech Lake.
Dan Gunderson | MPR News

To understand what's happening to walleye populations, it turns out you need to do more than just count fish.

"Walleye in these large lakes are being affected by large scale stressors like invasive species, climate change, so to understand and manage the population we need to understand what they're eating, what's eating them and how that's changing," said Minnesota Department of Natural Resources research scientist Gretchen Hansen.

Nine of the state's largest lakes are part of a new study that aims to gain a deeper understanding of how life works under the waves.

For decades the prevailing idea has been that to manage walleye, you monitor walleye populations and populations of fish they eat. But scientists now wish they'd been collecting more data about the basic food web.

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"It's difficult, it's expensive and I think fisheries management traditionally focuses on the fish that they're managing," said Hansen. "This study came out of the idea we wish we had been doing this all along but it's better to start now than to not start at all and to anticipate the big changes that are going to keep coming."

A variety of small aquatic invertebrates are collected to be tested.
A variety of small aquatic invertebrates are collected on Leech Lake to be tested.
Dan Gunderson | MPR News

Understanding who's eating whom in the mostly invisible underwater world of fish could help predict future changes in fish populations.

On a gray, drizzly morning, Hansen wades near the shore of Leech Lake, leaning against the weight of a seine net. Large lake specialist Carl Pedersen is chest deep in the lake at the other end of the 100 foot net.

The net collects young fish that hatched this spring and are still in shallow water near shore. This sweep of the net finds a half dozen species including walleye, perch, shiner minnows and bass.

The DNR has collected fish here for years. It's part of the annual survey the DNR uses to help predict populations of gamefish.

The fish are counted, measured and weighed. The state has more than three decades of this data. But what they don't have is more detailed data about the food fish eat and the interaction among species.

Most of the young fish are returned to the lake, but Hansen puts a few in plastic bags in a cooler to send to a lab for stable isotope analysis. Scientists will analyze nitrogen and carbon atoms in the fish. As part of this new study, crews are also collecting every part of the food web from zooplankton to insects to adult fish for lab analysis.

That's important because when a lake changes in response to warming water or invasive species, fish might need to adapt to new food sources in order to survive.

Leech Lake is newly infested with zebra mussels and Hansen says the changes caused by a growing population of the invaders might force walleye to look for food near shore rather than in deeper water. They could face more competition from fish species already foraging near shore.

Warming water will make walleye grow faster, but only if they can find enough food to sustain the growth.

A recently published study Hansen worked on in Wisconsin found warming water reduced walleye population in some lakes but not others.

Scientist Gretchen Hansen measures fish as part of a population survey.
DNR research scientist Gretchen Hansen measures fish as part of a population survey July 11, 2017 on Leech Lake near Walker, Minn.
Dan Gunderson | MPR News

Bass thrive in warmer water and walleye didn't do as well in lakes with strong bass populations. But the study didn't answer the question of why that's happening.

This study of Minnesota lakes will begin trying to tease out some of the answers. As an example, Pedersen points to one of the many predator prey interactions that might change on Leech Lake. He says walleye eat a lot of perch and cisco, a fish that needs cold water to survive.

"If we have warmer temperatures and warmer climate up here that important prey species for those walleyes is going to be gone," said Pedersen, explaining the simple answer might be that walleye will just eat more perch if cisco disappear, but the reality is more complicated. "The ciscoes, they have a higher energy content than perch does so they can eat one cisco versus two perch and get the same energy content and if they have to work harder to find those food it might create greater stresses for them."

And stressed walleye are more susceptible to disease.

But this is just one tiny interaction in this complex food web. Throw in some zebra mussels that eat the zooplankton and the baby walleye the crew is netting today will lose a critical early food source. Will they adapt and find other food sources?

Hansen says not only is the lake food web complex, every lake is different. She expects to find some common factors among the nine lakes being studied, but water quality, competition for food among fish species and varied fish genetics might all play a role in the fate of walleye. And she knows answering one question will likely prompt a dozen more

For lake managers like Pedersen, any science that will help predict or even better understand changes is welcome.

"If things are going to change we might have to come up with new tools we haven't used before to manage the lake," said Pedersen who accepts the reality that for better or worse, Minnesota lakes will be different in the future.

"We can prepare the public and prepare the resort owners and the business owners that make their living off this lake," he said. "Until you get to that new equilibrium where it kind of all balances out and we get to the new normal."