MINNEAPOLIS (WCCO) — It has perplexed anglers and fisheries biologists for years now: What’s happening to the walleye in Lake Mille Lacs?

Now, after crunching 30 years of lake data, a University of Minnesota research team found some answers.

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Few have fished Mille Lacs more than Jerry Riege, of Capra’s Sporting Goods. And what he sees in the water is troubling.

“The problem with Mille Lacs is that it is only 38 feet at the deepest,” Riege said. “Back in the 60s and 70s, you could fish midday in four feet of water with a slip bobber and catch them, and couldn’t see the bottom.”

His observation’s confirmed by U of M research. CFANS Researcher Gretchen Hansen and her team looked at the lake’s water clarity and temperature over time.

“We know the walleye populations in Mille Lacs have declined over the past decades,” Hansen said.

(credit: CBS)

Walleye are light sensitive and prefer darker and cooler water.

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“So we found that the area of the lake that has suitable walleye habitat in terms of temperature and light conditions has gotten smaller over time,” Hansen said.

What was visible in eight feet of water can now be seen at 12. This is likely due to less runoff from septic systems, fertilized lawns and the infestation of zebra mussels.

“Zebra mussels invaded in 2005 and we saw some indications of additional clarity after that but the biggest change happened prior to the zebra mussels,” Hansen said.

To Riege, it’s a troubling trend.

“The whole dynamics of the lake have changed,” Riege said.

The U’s findings are not only helpful in understanding walleye habitat loss on Mille Lacs, but in hundreds of other Minnesota lakes. That as zebra mussels and environmental changes continue to grow.

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Researchers say the clearer water poses a dilemma for natural resource managers. They will be challenged to match safe yearly harvests with shrinking fish habitat.