Trolling shallow for walleyes over the deepest water in the lake was a method of fishing so unorthodox in Minnesota 30 years ago that it landed Bruce Carlson on the pages of a national fishing magazine.

Carlson, a retired University of Michigan professor whose interests include human embryology, fish biology, lake biology and gerontology, discovered his pattern and slowly perfected it before his mid-1980s debut in In-Fisherman.

He came to learn that walleyes in Ten Mile Lake near Hackensack would suspend themselves in the top 20 feet of water over 150-foot depths to forage on cold-water ciscoes that swam to the surface near dusk to feed on plankton.

He'd drag a Rapala near the top during the early walleye season, hooking small and large specimens at a consistent pace. As the surface water warmed in July and August and the ciscoes stopped swimming all the way to the top, he lowered his bait 15 to 20 feet. The two-hour period before and after sunset worked best — dictated by the temporary but reliable ascent of the forage fish from below the lake's thermocline.

Fast forward to July 2016. Carlson's fishing magic still works, and Minnesota is spending millions of dollars to protect the phenomena in a host of cold-water cisco refuge lakes threatened by pollution. July 1 will mark the second consecutive year of special funding for a DNR project that aims to curb development and stop deforestation around deep lakes containing ciscoes, also known as tullibees.

"A lot of anglers understand the connection, and it's important that they do,'' said Peter Jacobson, a limnologist with the Department of Natural Resources.

Jacobson has documented a serious, habitat-related decline in ciscoes, a high-calorie forage fish important to walleyes, muskies, northern pike and lake trout. The average statewide abundance of the species has declined by about 60 percent over the past 30 years.

Ciscoes need cold, oxygen-rich water. In the long run of planet warming, scientists believe their existence is doomed in shallower lakes like Mille Lacs. But in 176 lakes like Ten Mile that have deep canyons of cold water, their survival won't be threatened as long as the depths are well oxygenated.

It's a realization that Jacobson said has given urgency to stopping nutrient pollution — especially in the refuge lakes that lie in fast-developing counties like Hubbard, Cass, Itasca, Otter Tail, Aitkin and Crow Wing. Runoff from developed land is feeding algae growth. When the algae dies, it sinks and absorbs the limited amount of oxygen that's locked in for the summer on lakes that have a thermocline.

Heather Baird, a shoreline habitat specialist for the DNR, said the agency has been acquiring natural lakeshore property and obtaining permanent conservation easements on other undeveloped land around targeted lakes. In back-to-back years now, the DNR has obtained a total of $2 million in special funding for the initiatives from the Lessard-Sams Outdoor Heritage Council.

About $1.2 million will be paid to private landowners for permanent easements while $780,000 will go for the purchase of Aquatic Management Areas, land with lake frontage that can be used by the public for hunting, trapping and fishing.

Baird said forest and wetlands work as giant sponges to filter out nutrients. The large tracts allow clean water to feed lakes from underground springs. She said the best conservation results occur when at least 75 percent of surrounding land is kept out of development.

"We've received a lot of support,'' Baird said. "The lake associations in these areas know it's important.''

Some of the well-known refuge lakes receiving attention for shoreland preservation include Big Sand and Kabekona near Park Rapids, Ten Mile, Big Trout in upper Crow Wing County and Pelican, north of Brainerd. Baird and Jacobson said cisco refuge lakes that lie further north are less at risk right now because forest lands in those areas are intact.

Individual counties also are stepping up to protect land around the deep-water tullibee lakes, as are certain conservation districts under the Board of Soil and Water Resources.

Meanwhile, Jacobson said his tullibee research continues. DNR crews in north-central Minnesota soon will be deploying vertical gill nets to continue the annual surveying of the fish. The nets hang like curtains in the water, dropping to within a meter of the bottom.

Carlson said he's not sure if his cisco-centric walleye method is applicable to all the refuge lakes. That's because tullibees grow larger on many lakes than they do on Ten Mile. Still, he said, walleye, northern pike and muskies all grow faster in lakes that hold ciscoes.