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Tiny animals may have a big impact on Minnesota fish

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Spiny waterflea
Tiny spiny waterfleas eat zooplankton, taking food that fish eat. A spiny waterflea infestation means about half of the tiny zooplankton disappear.
Courtesy Donn Branstrator, University of Minnesota Duluth

Aquatic invasive species live in hundreds of Minnesota lakes and streams. 

Zebra mussels are the most visible and get most of the attention. The spiny waterflea gets less attention, but experts say it might be a greater threat to Minnesota fishing. 

Both are established in Lake Mille Lacs. 

Cluster of mussels
A cluster of zebra mussels rest on a piece of driftwood pulled from Pelican Lake in July 2012.
Ann Arbor Miller for MPR | File 2012

Research shows invasives deplete the food supply in lakes, and scientists think that could affect the survival of young fish. People pay attention to the fish at the top of the food web, but fish depend on the mostly invisible life at the bottom of the food web, and that's where invasive species are changing Minnesota lakes. 

The simple version of lake life goes like this: Microscopic algae plants are eaten by tiny animals called zooplankton. Many fish, like walleye, eat zooplankton until they're big enough to eat other fish. Some fish eat zooplankton their entire life. 

Insects also play a role.

That's the Minnesota lake life cycle. Until you add invasive species. 

Zebra mussels filter algae from the water, taking food that zooplankton eat. Spiny waterfleas eat zooplankton, taking food that fish eat. 

Two fisherman floated on Lake Mille Lacs.
Two fisherman floated on Lake Mille Lacs in early August as the sun set.
Matthew Hintz for MPR News

DNR zooplankton expert Jodie Hirsch is monitoring zooplankton in 10 big walleye lakes. In four lakes infested with the spiny waterflea —  Mille Lacs, Lake of the Woods, Rainy Lake and Kabetogema — she sees a troubling trend. 

"We're seeing a 40 to 60 percent decrease in the native zooplankton biomass in these lakes," she said.

The DNR wouldn't allow Hirsch to talk about her Mille Lacs research, but data she provided show the effect is essentially the same in all the lakes. A spiny waterflea infestation means about half of the tiny zooplankton disappear. 

"They're so small they may be overlooked, but they're pretty important in the whole food web," she said. Newly hatched fish rely on them for food, but they face competition from spiny waterfleas.  

A spiny waterflea is less than a half-inch long; one can sit comfortably on the tip of your finger. They're mostly tail, sharply barbed.   

That tail keeps then safe from young fish. Lab experiments show that when young fish try to eat the spiny waterflea but can't swallow it, they get frustrated and eventually give up. 

High water drowned out the wild rice crop.
DNR zooplankton expert Jodie Hirsch said she is concerned about the spiny waterflea population she's detected in Lake of the Woods and three other Minnesota lakes.
John Enger | MPR News

In some cases, the tail punctures the gut of larger fish, although certain species, like sunfish, appear to thrive on the spiny-tailed animal.

So how does that affect fish survival and growth in a lake? That's where the science starts to get a bit fuzzy. The work by Hirsch and other scientists leaves no doubt that spiny waterfleas change the food web in lakes. But what that means for fish is still mostly speculation. 

Ryan Maki is the aquatic ecologist at Voyageurs National Park, where spiny waterfleas are changing the food web in Rainy, Kabetogema and other lakes. He said losing half of the lower food web changes the energy balance in a lake.

"That decrease is so significant that there is general agreement among resource managers that there must be an effect, but we really need to study it to assess in more detail what it is," he said. 

It's at Voyageurs, in northern Minnesota, that scientists are getting the first peek at how the loss of zooplankton in a lake might affect fish. 

Researchers looked at growth rates of young yellow perch, a favorite food of walleye. What they found is that yellow perch are growing more slowly in lakes infested with the spiny waterflea. 

W. Charles Kerfoot, professor of biological sciences at Michigan Technological University, said the theory is that if zooplankton-eating fish like yellow perch grow more slowly, predators like walleye will have less food. 

"Going up like a wave in the food web, there is this diminished energy," Kerfoot said. "And so right at the top the fish are going to start having serious problems in the long run. And I want to emphasize 'long run,' because fish are not dumb. They will switch around to different food."

The adaptability of fish complicates efforts to predict what might happen in a large, diverse lake like Mille Lacs.  

Rick Bruesewitz, DNR fisheries manager for the Aitkin area, said there's no question the spiny waterflea and zebra mussel infestation are changing the energy flow in Mille Lacs. But how much of the lost zooplankton is needed to support life in the lake, and how much is surplus food? 

"It's by no means a simple question," Bruesewitz said. The change in the zooplankton supply might mean there's not enough energy to support all the fish, or it might mean the lake simply needs time to find a new energy equilibrium. 

Sunset over Rainy Lake
The sun sets over Rainy Lake, where DNR researcher Jodie Hirsch is concerned about the spiny waterflea infestation.
Derek Montgomery for MPR | File 2013

"Right now, with the way things are changing, we could be losing energy at one level but gaining it at another," he said. "That's probably the biggest giant question mark that I have."

Invasive species are just part of the food web puzzle in Mille Lacs.  The list of shape-shifting puzzle pieces includes water quality, fish harvest and climate change. 

"The worst lakes to do this in are the lakes where they're all going on simultaneously, right?  Like, Lake Mille Lacs is a tough one." 

A more aggressive research effort is needed to understand how the changing food web will affect fish survival, said Donn Branstrator,  associate professor at the University of Minnesota Duluth.  He studies the life cycle of the spiny waterflea.  

One thing seems clear, he said. The changes are long-term. 

"The ecosystems don't seem to reverberate or recover from a spiny waterflea invasion," he said. "There seems to be permanent changes in the lower food web."

Branstrator plans a study of DNR fish data to look for any trends in fish growth rates. Additional study of fish growth and survival is also planned on lakes infested with spiny waterfleas in northern Minnesota.

But as one fisheries manager admitted, researchers might not recognize the impact of invasive species on fish populations until they see it in the rear-view mirror.