The herring season is over.

Commercial fisherman Steve Dahl’s 18-foot steel skiff, pulled from the Knife River Marina in early December, sits on a trailer in his yard. A thin veneer of ice still clings to its gunwales. He fished the season right to the end.

Fishing was good.

“This fall, I was almost overwhelmed,” said Dahl, 64, of Knife River. “You’re doing 600, 700, 800 pounds a day. It wears you out.”

But the previous two falls were not so good, Dahl said. Fisheries biologists say Lake Superior’s herring population is in decline. They are not sure what’s causing the population to decrease, but they believe that an increase in predators and a warming climate may be factors.

The short answer is that not enough little herring are growing to maturity. Herring recruitment - young herring that become eventual spawners - has become more sporadic in recent decades, biologists say.

“I think we still have a lot of herring, but the trajectory of the population is what’s giving us pause,” said Dan Yule, a fisheries scientist at the U.S. Geological Survey’s Great Lakes Science Center in Ashland. “A lot of surveys suggest we are in decline.”

Herring not only are the lifeblood of commercial fishermen on Minnesota’s North Shore and elsewhere in the lake, but at smaller sizes, herring are also important as prey species for predator fish, especially lake trout.

The last decent “year class,” or crop of young herring, was in 2009, Yule said, and the last one before that was in 2003.

“To me, it’s a matter of simple population dynamics,” Yule said. “There’s less recruitment back-filling the adult population, and the adult population is being lost to old age.”

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Cory Goldsworthy, Lake Superior area fisheries supervisor for the Minnesota Department of Natural Resources, says herring make up the greatest portion of the big lake’s prey fish biomass. And those stocks are down significantly.

“We’re at the lowest prey fish biomass that we’ve ever seen in Lake Superior,” Goldsworthy said. “We’re very concerned, and we will be until we get another good year class of herring.”

Figuring out what’s causing poor recruitment of herring is difficult, he said.

“That’s like the million-dollar question on Lake Superior right now,” Goldsworthy said.

Meanwhile, Minnesota’s 25 commercial fishermen, accustomed to boom and bust years, often must work hard to net enough fish to supply North Shore restaurants and the November roe fishery.

Tougher times

For years, there has been an uneasy tension between Minnesota’s commercial fishermen and the DNR, which sets quotas regulating the commercial harvest. The best herring fishing is in November, when herring are usually more concentrated. At that time of year, herring are sought for their roe, or eggs, which are processed in Grand Marais and shipped to Europe. The November roe fishery is the most lucrative time of year for Minnesota commercial fishermen.

After smelt were first discovered in Lake Superior in 1946 and their population exploded, herring stocks plummeted in Minnesota. In 1973, the DNR closed the November herring fishery, a severe hardship to commercial fishermen. That season did not reopen until 2006. Ontario and Wisconsin did not shut down their commercial herring fisheries.

Minnesota’s closure lasted “way too long,” Dahl said.

“It almost did us in,” he said. “We had to pull our nets because of the November closure, and they were just setting nets in Ontario and Wisconsin. Forty to 50 percent of my annual income is from four weeks - the third week of October to about Thanksgiving.”

Several Minnesota commercial fishermen began salmon fishing in Alaska during those years to make up for the lost income.

The smelt population began to decline in the 1980s. The herring population had three excellent year classes in the late 1980s and a couple more decent classes in the 1990s, Goldsworthy said.

But since 2006, herring catches have declined in two of Minnesota’s three fisheries management zones. Although commercial fishing quotas remain fairly low in Minnesota, many commercial fishermen are unable to reach their quotas, Goldsworthy said.

“We’re stretched on this last year class,” said Clint Maxwell, 63, a Beaver Bay commercial fisherman. “So we’re waiting on the next good run.”

Just last month, for the first time, Wisconsin set a herring quota for its 10 state commercial fishermen, although there is no quota for that state’s tribal commercial fishermen. The DNR and tribes are in the process of trying to work out a new commercial fishing agreement.

“We’ve heard a lot from different jurisdictions - Minnesota and Ontario - about unregulated (herring) harvest in Wisconsin waters,” said Terry Margenau, Lake Superior fisheries supervisor for the Wisconsin Department of Natural Resources in Bayfield.

Assessing herring numbers

Minnesota commercial herring fishermen are a resilient breed, many of them descendants of families with 100-year histories of North Shore herring fishing. Fishing with one or two people in small skiffs, they set suspended nets offshore and hope to intercept schools of moving herring. They generally credit the Minnesota DNR with doing a reasonably good job of managing the commercial fishery.

“They (DNR officials) put quotas on. They adjust it up and down. They do a pretty good job. On the North Shore, it’s managed better (than in other jurisdictions on the lake),” said Harley Toftey of Grand Marais, a commercial fisherman who also sells herring at his Dockside Fish Market in town.

The commercial take of herring represents a tiny fraction of overall herring mortality, the DNR’s Goldsworthy said. Many more are eaten by predators or die before they can enter the adult population.

The U.S. Geological Survey does both hydroacoustic and net trawl surveys to monitor the herring population. Through those annual lakewide assessments, they can determine year-class success when herring reach age 1. That allows the Minnesota DNR to offer commercial fishermen a heads-up about projected year-class strength and possible changes in quotas.

But some commercial fishermen question fisheries biologists’ data about the herring population.

“How do they really know how many fish are out there?” Dahl said. “Four or five years ago, the North Shore was dead. Bayfield had a whole bunch. I told Cory (Goldsworthy), you may be missing a whole bunch (in surveys). There’s so much we don’t know about herring.”

Toftey contends it’s difficult for scientific surveys to sample a lake as large as Superior.

“That’s just a snapshot,” Toftey said. “You gotta be in the right place.”

But Yule, at the USGS in Ashland, says his teams have sampled 82 sites around the perimeter of the lake since 1977, and the data from those surveys consistently show a decline in the herring population.

“All of our surveys indicate the same thing to us,” Yule said. “I feel pretty confident that we have a handle on the trajectory. If the data were telling us different stories, I’d be less confident.”

Importance of ice

When a fisheries population changes, Goldsworthy said, it’s usually not the result of a single factor. The herring population declined rapidly when the smelt population was booming in Lake Superior. But now, the smelt population has dropped significantly as the lake trout population has rebounded.

“And we’re still seeing issues with sporadic recruitment (of herring),” Goldsworthy said. “It’s never just one thing that will solve an issue.”

So, what factors might be causing the sporadic success of herring year classes in Lake Superior? “Part of it is environmental,” Goldsworthy said. “Ice cover is believed to have an impact on a successful recruitment event.”

Yule, at the USGS, says research data supports that contention.

“A lot of the larval herring, we think, are starving,” he said, “because they’re hatching and not finding enough available zooplankton.”

Zooplankton are typically microscopic organisms drifting in the water and are an important part of the marine food chain.

“We’ve seen a pattern where recruitment of herring is strongest in years when we have a high degree of ice cover,” Yule said. “And we’re seeing less ice cover in recent years. We think there’s a linkage there.”

Ice cover on Lake Superior declined by 79 percent from 1973 to 2010, according to a study by a team of Great Lakes scientists published in the Journal of the American Meteorological Society in 2011.

Scientists are not sure just why the presence of zooplankton correlates to years of more ice cover, Yule said. It might be that the presence of ice means less current affecting herring eggs, he said. Or it might be that ice allows less light to penetrate the water, making it more difficult for predators to find larval herring to feed upon, Yule said.

“Understanding the reasons for survival of larval fish is the Holy Grail,” Yule said. “It’s been tantalizingly difficult to solve.”

Setting more net

Meanwhile, Minnesota’s commercial fishermen must often set more net to catch the same number of herring they caught in better years. DNR research shows herring catches per 1,000 feet of net have declined since 2006 in two of three zones. The northeasternmost zone, from the Cascade River to the Canadian border, is faring the best, according to the DNR.

Fisheries biologists say the changing nature of the lake’s ecosystem, with lake trout rebounding strongly in Minnesota water, may be holding the herring population down. The lake trout population had collapsed due to sea lamprey infestation and overharvest in the 1950s and 1960s. Salmon, an introduced species, also are at the top of the food chain, are now self-sustaining in the lake.

More hungry mouths mean more pressure on the lake’s prey fish populations, including herring.

“It could be the rehabilitation of lake trout, and that the lake trout are eating more age 1 and age 2 herring before they can become adults and reproduce,” the DNR’s Goldsworthy said.

Growth rates of Minnesota’s lake trout are declining in part because the prey fish biomass has been declining for the past 10 to 15 years, he said.

Yule, with the USGS, agreed.

“(The decline of prey fish) has to do with having high numbers of predators in the lake at this time,” Yule said. “I think that’s why all the forage fish populations are down. When the herring do hatch out, they have a higher probability of being eaten by lake trout.”

Both Goldsworthy and Yule believe that as humans, we might have to change our expectations about what the Lake Superior fishery will look like in the future.

“As the lake trout have recovered,” Yule said, “what we expected from our forage fish population - maybe we have to think about lessening those expectations.”

Many lakes across Canada, he said, are dominated by lake trout and tend to have fewer forage fish.

“I think in many respects, Lake Superior is moving toward some of those lakes in Canada that are predator-heavy,” Yule said.

Goldsworthy believes that is what might be happening.

“This might be more what the Lake Superior ecosystem is going to look like,” he said, “rather than when we were battling sea lamprey and had low lake trout abundance. It’s our human perception of what things should look like compared to what naturally it might look like.”

In the meantime, Dahl and other Minnesota commercial fishermen are waiting for the next good herring year class to materialize. Yule said weather conditions in the winters of 2012-13 and 2013-14, when more ice was present on Lake Superior, were a positive sign.

“That was a good thing to have happen for herring,” Yule said.

Several fishermen, particularly nearer the Canadian border, saw smaller herring showing up in their nets this fall. They’re hoping that those fish, perhaps from the 2014 year class, offer promise for the future.