ON FROZEN LAKE MILLE LACS - As the sun set Thursday over this seemingly endless sheet of snow and ice, Bill Marchel set the hook on what he thought was a walleye.

We were about 7 miles from shore, in a heated fish house, peering through cylinders of ice. Surrounding us, some nearer, others farther, lights shone through small windows of other fish houses, as anglers inhabiting them hoped, as Bill and I did, that Mille Lacs and its walleyes and perch would treat them generously.

But in winter, this big lake is more than a place for Minnesotans to fish. It's also a work site, particularly for those who -- like the ice road truckers of television lore -- keep the lake's winter roads open, and transport people and goods over them.

Terry Thurmer is one.

For 21 years, Thurmer has owned Terry's Boat Harbor on the north end of Mille Lacs. In winter, while others on the lake wonder whether walleyes will bite and when, or whether, they should move their fish house from one flat to the next, Thurmer worries about keeping his heavy-duty plow trucks running, about whether a crack will open in one of the many ice roads he maintains, and whether his rental houses are parked in the right spots, so his customers can catch fish.

"In this business, it's inevitable that you have truck breakdowns," Thurmer said. "It also seems inevitable that if it's going to snow, it'll snow on Fridays, just before everyone comes up for the weekend."

Thurmer plows and maintains about 40 miles of Mille Lacs ice road in winter, the main thoroughfares of which are about 150 feet wide.

"If ithey were narrower, and a strong wind came up, snow would fill them in," Thurmer said.

The Mille Lacs ice-road business has changed over the years. Time was, said Thurmer, that no one drove on the lake unless it was covered with at least 15 inches of ice.

"Now in December you see people driving when there are only 9 or 10 inches," he said. "But not me. And not on my roads."

Thurmer's minimum ice depth for travel is 12 inches, and in December, employing an auger and a four-wheeler, he'll drill holes 6 or 7 miles onto the lake to make sure a full foot of ice covers his portion of the lake.

"If you start driving big trucks and wheel [fishing] houses on ice that is much thinner than that, nothing good will happen," he said.

Even when a foot of good ice covers Mille Lacs -- and right now, in most parts of the lake, there's a couple of feet, or close to it -- pressure ridges can form and the lake can open up, exposing long, watery seams.

Usually these close again soon, perhaps overnight, as rising and falling temperatures cause the huge ice mass to expand and contract.

Thurmer, who deploys steel "ice bridges" to cover the fissures when they occur, takes these occasional schisms in stride.

"The first thing people need to know about driving on ice is that they need to drive slow," he said. "Just like they show on the 'Ice Road Truckers' TV show, if you drive fast, what you're doing is pushing a wave of water ahead of you under the ice, and when it reaches a weak spot, such as a crack, it can blow up."

Eighteen-wheelers that tote supplies to mines and resorts in winter across the Canadian north travel only about 8 miles an hour. On Mille Lacs, Thurmer says no one should drive faster than 15 miles an hour.

Winter anglers also take risks if they free-lance across Mille Lacs from one spot to another, off established ice roads. Not only can pressure ridges present problems, people can get stuck. Or lost.

"We get calls fairly regularly from guys on the lake who don't know where they are," Thurmer said.

Some weeks, Thurmer and his co-workers blow as much as $2,000 for gas to keep their plows going. To recoup expenses, he charges vehicles $10 a day in road fees, or $25 for a weekend, Friday to Sunday.

"We go out as far as 9 Mile Flat and also cover most of the other major flats," he said. "The previous owner didn't plow anywhere near that much. But I knew if people had roads to get to the flats in winter, they'd go."

The other evening, not far from Bill and me, Roger Kramer of Eagan was in his fish house, hunched over a couple of icy holes, accompanied by his German shorthaired pointer, Sadie. Like us, Kramer had answered winter's siren call and was looking for a few fish.

Also not far away, Roger Bialke of St. Francis jigged in his cozy fish house, a plump perch lying at his feet.

And the fish on the end of Bill's line when he set the hook?

The one he hoped was a walleye?

A perch, as it turned out.

Soon, Bill rebaited his jigging spoon with a minnow head, and dropped the rig again to Mille Lacs' cold, dark bottom.

We had more fishing to do before we followed our truck headlights over the long ice road leading to shore.

Dennis Anderson • danderson@startribune.com