The glory days of one of America’s greatest walleye lakes might be gone.

Mille Lacs Lake no longer might be able to support the bountiful walleye stocks that made the 207-square-mile lake legendary among fishermen, yielded annual hauls of a half-million pounds destined for frying pans, gave rise to an economy of resorts and guides and justified a truck-sized statue of the state fish along U.S. 169 in central Minnesota.

That’s the sobering suspicion — still a suspicion, but a growing one — of what has now become an international chorus of scientists studying the lake.

And it’s not the fault of anglers, American Indian netters or misguided management by the Department of Natural Resources.

“Blame,” if one needs to point fingers, probably should be leveled at the Clean Water Act of 1972, a warming climate and possibly invasive non-native species such as zebra mussel, spiny water flea and Eurasian water milfoil.

The lake’s increase in the number of northern pike appears to be a growing factor in the size of the walleye population, and the double-crested cormorant, a fish-eating bird whose numbers also are growing, might become a player. But neither is believed to be the driving factor for why the productivity of the walleye factory could be declining.

The bottom line for walleye anglers is this: Although the walleye population might rebound from its current level — the lowest known population since people have been measuring it — we should expect conservative limits for how many walleyes we can keep for years to come.

These are the ideas, conclusions and recommendations from a blue-ribbon panel of walleye experts that released its final report Friday at the DNR’s “Roundtable,” an annual gathering of fishing, hunting and conservation interests.

BLUE-RIBBON REPORT

The notion that Mille Lacs simply can’t produce the same numbers of walleyes it used to isn’t the headline of the report, which I’ve posted at blogs.TwinCities.com/outdoors — and which everyone who cares about the lake should read themselves.

The headlines are basically these:

— The DNR isn’t missing the boat. The fundamental problem with Mille Lacs walleye is, as the DNR has said, the population is falling because young walleye aren’t surviving to adulthood, even though plenty — more than enough — are hatching each spring. This problem dates to at least 2000, even though the resulting decline in the adult walleye population didn’t manifest itself until about 2005.

— Fishing and netting aren’t to blame, but because we can manage the killing of walleye by people, prudence demands conservative harvests for the foreseeable future.

— The young walleyes aren’t starving or dying from disease. They’re being eaten — and mostly by bigger walleyes, which remain, bar none, the most prolific predator in the lake. Last summer, northern pike, probably for the first time in recent history, ate more newly hatched walleye than walleye did.

— Don’t bother stocking walleye. That could make the cannibalism cycle worse. (Mille Lacs walleye population is entirely self-sustaining.)

— Increasing water clarity, invasive species and falling populations of ciscoes, or tullibees, are among the likely causes behind walleye cannibalizing themselves to the point of hurting the entire population.

Those are the headlines. But read deeper into the report and press its authors, as well as DNR researchers studying Mille Lacs, and the longer-term picture — the one in which fewer walleyes might be Mille Lacs’ new normal — is undisputed as a growing concern.

NEW NORMAL

“Lower productivity” is the term scientists use to describe the phenomenon. It helps to think of a lake as having a finite amount of energy that can be spent on growing life.

“There’s pretty strong evidence the lake is less productive, and that’s in the water clarity,” said Paul Venturelli. Venturelli is a faculty member of the University of Minnesota and point person for the five-member blue-ribbon panel, which the DNR convened in 2014 essentially to audit its Mille Lacs work and bring its own expertise to bear.

The panel is no sham. It includes the world’s leading researchers on sander vitreus, aka the walleye: Jim Bence and Travis Benden of Michigan State University, Nigel Lester of the Ontario Ministry of Natural Resources and the University of Toronto, and Lars Rudstam of Cornell University and Oneida Lake Field Station. Of relevance: Rudstam’s Oneida Lake, the crown jewel of New York’s walleye fishery, has found itself in its own new normal, where lower productivity has meant walleye populations that are about two-thirds what they were from the 1960s to the 1990s.

Federal clean water laws of the 1970s began a cascade of changes that resulted in, among other things, fewer and cleaner septic systems along the shores of the Mille Lacs. Think of it simply as stink if you like, but sewage also contains “nutrients,” — that’s energy — and there is less of it in the lake now. This is evident by less algae, or phytoplankton, which forms the base of the food chain on which walleyes ultimately depend.

Zooplankton eat phytoplankton, and today the mass of zooplankton is at the lowest level ever recorded. This is why the water is so much clearer, at least 1.5 times clearer than it was in the mid-1990s.

In addition to the likelihood that there is less energy in the lake as a whole, invasive species might be diverting energy that used to be enjoyed by walleyes. Zebra mussels, first discovered in 2005, filter algae and sequester minerals such as calcium in their shells, while invasive spiny water fleas, first detected in 2009, also use energy to live. Because of their namesake microscopic prickles, spiny water fleas aren’t the preferred diet of small fish, so that energy similarly is diverted away from the walleye food chain.

Tullibees, which can be an important source of food for walleyes, appear to be in a long-term decline in Mille Lacs, thanks to a warming climate and the increasing prevalence of summer heat waves, which can cause massive die-offs. The long-term prognosis for tullibees is poor. Although several state and local programs are seeking to shore up tullibee populations in lakes across northern Minnesota, scientists believe Mille Lacs is too shallow and too far south to support the fish in the long term, if climate trends don’t change.

So walleyes need to find energy elsewhere, and they have turned to their own young. Cannibalism is well-known among walleye populations on any lake, but it looks as if Mille Lacs walleye are eating too many of their kind to sustain their historical numbers.

For those who might conclude the cannibalism problem is the result of too many big walleye, that appears to be a red herring. The DNR’s predator study of what’s eating what in the lake has reaffirmed what biologists already knew: Pound for pound, bigger walleyes eat less than smaller ones. A single 10-pound walleye eats less than 10 1-pound walleyes. In other words, if the lake is flush with medium-sized walleyes — keepers that have been hard to come by for anglers — there would be even more cannibalism.

SPECULATION

What puzzles the panel is why yellow perch, a favorite walleye food, hasn’t filled the void more.

Yellow perch numbers are high on Mille Lacs, yet it doesn’t seem to be curbing walleye cannibalism enough. One guess is that the clearer water has created a “separation” between where walleye like to roam and where perch like to hide: Simply put, maybe the walleye have gone deeper than the perch — and the proliferation of non-native Eurasian water milfoil weedbeds might be helping perch hide.

To be clear, much of the panel’s final report falls somewhere on the spectrum between suspicion and speculation. But there are constellations of evidence suggesting the lake’s current state of affairs — and its current stringent regulations — aren’t a one-off blip that soon will rectify itself with a return to years such as 1992, when anglers could take a record 1.2 million pounds of walleyes.

For example, cannibalism among walleye populations often leads to boom-bust cycles. But when that happens, and the productivity (total energy) of the system is strong, the fewer fish that remain show growth spurts, gobbling up that extra energy no longer taken by their peers. That’s not happening on Mille Lacs, Venturelli said.

“If the population is going down but growth rates aren’t going up, that really suggests the productivity is either going somewhere else in the system or the lake is becoming less productive,” he said. He suspects both.

The possibility of a less-productive Mille Lacs has been discussed openly by DNR researchers for several years, but they’re cautious about any conclusions.

TIME WILL TELL

DNR fisheries chief Don Pereira notes that the class of walleyes hatched in 2013 appears to be surviving well, and those fish will be bending anglers’ rods in the coming seasons. No one is predicting Mille Lacs can’t be a good walleye lake in the future, but the concern is it won’t be the great one.

“Only time will tell if that’s true,” Pereira said. “We do know the harvest is going the have to be low for some time.”

“Low” like the roughly 180,000-pound safe-harvest level of 2013, which was half the year before? Or “low” like last year, when the number was slashed to 60,000 pounds, resulting in a two-walleye limit with a hard-to-hit harvest slot between 18 inches and 20 inches (or one over 28 inches)?

“Low like this past year,” Pereira said.

Sobering.

And, although the report recommends managing pike to protect walleye and monitoring cormorants to see some sharpshooters can help, there’s no apparent solution to larger trend.

“The solution is to limit our impact on walleye until we can figure out better what’s going on,” said Venturelli. “But if the lake can only support fewer walleye, then it can only support fewer walleye.”

Dave Orrick can be reached at 651-228-5512. Follow him at twitter.com/OutdoorsNow.