Shotguns blazed as great flocks of bluebills knifed over Minnesota lakes.

In one of the greatest duck hunting seasons in state history, waterfowlers shot a remarkable 2.3 million ducks in 1951, including more than a half-million bluebills, also called scaup.

The highly coveted diving ducks routinely filled Minnesota's autumn sky in an annual fall spectacle.

Now, 60 years later, bluebills are an afterthought for most Minnesota duck hunters, an oddity that occasionally shows up over duck blinds. Last fall, most hunters could count on one hand the bluebills they saw or shot.

Total harvest was 7,358 -- lowest in history.

"You can't shoot something that isn't here, and they're not here,'' said Tom Tubbs, 74, of Rosemount, a longtime duck hunter and co-founder of the Minnesota Waterfowl Association.

Tubbs recalls shooting bluebills on Bear Lake in southern Minnesota, at Lake of the Woods, and at Shoal Lake in Manitoba, where "there were clouds and clouds of bluebills,'' he said.

What happened?

"They're just not here, I wish I knew why,'' said Steve Cordts, waterfowl specialist for the Minnesota Department of Natural Resources. "And when they are here, people don't target them.''

The continental bluebill population fell from a high of 7.9 million in 1972 to a low of 3.2 million in 2006 -- sparking bag limit cuts and research to figure out the cause. Waterfowl biologists still can't explain the decline.

"There is no smoking gun,'' said Al Afton, a waterfowl research professor at Louisiana State University and a former Minnesotan who has studied scaup extensively.

Concerned about the trend, federal officials lowered the bluebill bag limit from six in 1998 to three in 1999, then two in 2005. The low-point was in 2008, when the daily limit was one, except for part of the season, when two were allowed.

DNR officials feared waterfowlers would leave their sport as a result of the tight restrictions. And they perhaps did: Minnesota counted more than 140,000 duck hunters in the 1950s and 1960s and 122,000 as recently as 1998. Since then, duck hunter numbers have declined, hitting a low of 73,000 in 2010 before rebounding last year to 83,000.

Fewer duck hunters afield also contributed to the bluebill harvest decline.

"Historically, people hunted on larger lakes to target scaup and shot lots of them,'' Cordts said. "These days, diver-duck hunters are a pretty rare breed.''

The action at northwestern Minnesota's Thief Lake -- a traditional diver hot spot -- underscores the change. In the 1970s, 40 percent of the bag was scaup. Today, just 10 percent are bluebills.

But starting in 2006, something strange happened: Bluebill numbers gradually started to rebound. This spring -- less than a year after the low point in Minnesota bluebill harvest history -- the continental scaup population climbed 21 percent to 5.2 million, according to the U.S. Fish and Wildlife Service spring survey.

It's the first time in a decade they were above 5 million.

As a result, the federal agency OK'd doubling the scaup bag limit -- from two last year to four in Minnesota this fall.

Researchers can't explain the decline or the rebound.

"I don't think anyone has a real good explanation for why the numbers have gone up,'' Cordts said.

"Everybody's scratching their heads,'' said Afton.

Have bluebill numbers increased because of the recent harvest restrictions?

"That's possible, but I'm skeptical,'' Afton said.

Theories for the bluebill decline, meanwhile, include overharvest, decreased quality and quantity of food during winter and spring migration, accumulation of contaminants, and climate or habitat changes in key boreal forest breeding areas.

Research has shown contaminants probably can't be blamed.

"It's probably not one factor, but several different things going on,'' Afton said.

Clearly, habitat, including food, has deteriorated in Minnesota over the years.

Bluebills love arthropods -- fresh-water shrimp -- and they are tough to come by nowadays in most Minnesota waterways.

Afton has put satellite transmitters on scaup to track their movements, finding variances between spring and fall bluebill migrations through Minnesota.

"More birds go through Minnesota in the spring, but in the fall few come back,'' Afton said.

"They migrate back through the Dakotas in a circular migration. The food is better there than in Minnesota.

"Ducks aren't stupid.''

Doug Smith • dsmith@startribune.com