No one knows how many hours Minnesotans spend ice fishing each year.

But it's millions. Many millions.

The state's three most popular ice fishing destinations — Lake of the Woods, Lake Mille Lacs and Upper Red Lake — account for more than 5.5 million hours alone, according to the Minnesota Department of Natural Resources.

Yet one thing is certain. Increasingly, anglers are wetting lines in the confines of a highly mobile shelter on wheels that offers the comforts of home. And they are doing it with ever-improving high-tech equipment. This makes fishing easier and more fun. It also makes fish management more challenging.

"The rise of the pull-behind wheel house has revolutionized ice fishing," said Matt Jensen, marketing director of Minnesota-based Rapala USA. "Wheel houses are the hottest thing above ice, and they are driving many of the products being designed and sold."

An example, said Jensen, is the 40-volt lithium ion auger that starts with the flip of a switch. No gas. No exhaust odor. No yanking a cord. It even spins in reverse to flush slush down the hole.

Another example is the addition of an HDMI video port on underwater camera units. "Flat screen TVs are common in wheel houses," Jensen said. "These days many anglers and families are a watching a lure and any fish around it in larger-than-life high definition. It's fun. Exciting."

Still another advance is all-seeing sonar technology that scans the water column forward, backward, sideways and down. It does so while transmitting images to a video screen in real time. This means an angler can drill a hole, drop a transducer into it, and quickly take a panoramic peek of what is or isn't around. If a school of suspended crappies is spotted this way or that, well, that's where the next hole gets drilled.

As angler interest in ice fishing has grown, so has business interest in ice fishing. Rapala USA has acquired Strikemaster, which makes augers, and MarCum, a sonar and underwater camera manufacturer. "Both have become very hot brands for us," said Jensen. "We sold out of 40-volt augers before Christmas. Tens years ago, ice fishing was a small part of our business. Today, it's more than 15 percent."

Matt Johnson, "Ice Team" manager for Clam Corp. in Osseo, said the company is dealing in more of everything.

"Our annual sales have doubled over the past seven or so years," Johnson said. "We used to publish a six-page catalog. Now it's 96 pages. We used to offer six fish shelter types. Now it's 35. I now oversee 400 ice fishing pros who work in Canada and every state that has ice. We keep thinking there's a plateau, but we've yet to hit it."

Even Mossy Oak, a Mississippi-based company that licenses camouflage patterns to other businesses, is jigging for Minnesota ice fishing business. The company bought a booth at the recent St. Paul Ice Fishing and Winter Sports Show because modern ice fishing — unlike its comic depiction in the 1993 movie "Grumpy Old Men" — is part of a youthful and popular lifestyle.

"Our mission is to help people live their best life outdoors," said Jake Meyer, Mossy Oak's public relations manager. "We've developed patterns for coats and bibs that aren't, obviously, intended to camouflage anglers from fish but rather express an outdoor lifestyle."

Brett Drexler, general manger of Ice Castle Corp. in Montevideo, Minn., has witnessed this lifestyle evolution.

"Many of my customers are guys in their 20s or 30s who now have a better way to get their wives and kids on the lake," he said. "Shower. Toilet. Stove. Sleeping space. Furnace. It's all there. It makes spending a weekend on the lake fun and easy."

Lakeshore owners are also common customers. "We hear about new family traditions starting because grandpa and grandma have a wheel house and the grandkids want to be in it," he added.

Drexler said new sales of wheel houses, which range from about $20,000 to $40,000, are strong. He said the firm has been making and selling about 3,000 units per year in recent years.

"We've come a long way since 1997," he said. "That's when we built our first basic fishing shack on wheels. Today, we are the largest manufacturer of fish houses on wheels in the world."

The story is much the same at Voyager Industries, Inc., the Brandon, Minn., manufacturer of Yetti wheel houses. Voyager began building wheel houses nine years ago as way to fill a seasonal void in its aluminum and plastic manufacturing operations.

"Today we are building Yettis year-round because interest in ice fishing has exploded," said Mike Draper, sales and marketing manager. "Sales took off more than we ever could have imagined. We've had growth every year and are feeling good about next year, too."

It's different below the water

While fishing products advance above the ice, it is different below.

"Fish are still fish," said Chris Kavanaugh, northeast regional fisheries manager for the Minnesota Department of Natural Resources (DNR). "Because finding them has never been easier, we are likely removing big sunfish and crappies faster than they can be replaced in most bodies of water."

Kavanaugh said seasons, bag limits and length limits all work to keep lakes from getting "fished out," but waters can be "fished down" because humans, unlike most top predators, prefer to prey on what's rarest rather than most abundant.

"Understandably, people prefer to catch and keep larger bluegills and crappies," Kavanaugh said. "Yet when the biggest ones are removed faster than they can be replaced, the outcome is even greater numbers of small fish. That's because the species, in the name of self preservation, starts to mature at an earlier age, producing even more fish that can result in stunting."

Kavanaugh said the situation — high numbers of early-maturing small fish — is common in perch and northern pike populations across much of Minnesota. Perch can reach sexual maturity at just a few inches in length. Many northern pike in central and north-central Minnesota are sexually mature at age 2 when less than 20 inches long.

Gary Barnard, the DNR fisheries supervisor who oversees Upper Red Lake in Beltrami County, said the walleye population on Red remains excellent, in part because of annual fish population and angler surveys. The data enables managers to dial-in bag and length limit regulations that produce a well-balanced walleye population.

Still, like Kavanaugh, Barnard said the wheel house explosion has created certain management challenges.

"A fishing trip isn't what it used to be," he said, noting staff who calculate ice fishing pressure have had to change their methodology. "An angler fishing trip used to be several hours in length. Now, a single trip on Red Lake can be nonstop fishing for 72 hours. People are watching football on TV, making meals and doing other things while their lines are in the water."

Barnard likes to see the popularity and sustained interest. He said advances in comfortable fishing, including pop-up and flip-over shelters, have increased statewide fishing pressure even though fishing license sales are fairly flat.

Jeremy Smith and Mike Hehner, employees of Lindner's Angling Edge in Baxter, Minn., are fishing industry advocates for conservation. Both have biological degrees. Both wonder what fishing will be in 20 and 30 years.

"I am in the business of helping people become better anglers by using today's technology, and today's technology is incredibly cool," Smith said. "However, as ice anglers ourselves, Mike and I see firsthand how effective today's innovative equipment is, and therefore the importance of self-restraint and effective regulations."

Hehner offered a similar sentiment. "Because everything used to catch fish is improving — maps, lines, rods, reels, awareness of hot bites on social media, you name it — I try to get people to think about the implications for the future. There will be a tipping point at some point. When that time comes will be decided by us."

C.B. Bylander is a freelance writer. He lives near Baxter, Minn.