North Shore tributaries to Lake Superior have been too cold this month to coax the annual steelhead run, but fisheries managers this week saw a sign that the spawning push is nigh.

On Monday at Knife River, 41 steelhead were swimming in the Department of Natural Resources fish trap 18 miles north of Duluth. Nick Peterson, the agency’s migratory fish specialist, said the finding marked the first meaningful stream activity of the season.

“Males are usually the first to arrive and that’s what was in there,’’ he said. “We had a small push over the weekend.’’

For the trend to continue, flows must remain strong and water temperatures need to climb a few digits to around 40 degrees. Every time this spring that stream water temps started to warm, they crashed from onsets of snow and cold rain, said Cory Goldsworthy, area fisheries supervisor for DNR. He will be watching this week to see if plunging overnight air temperatures will continue to have a chilling effect.

“Normally by now we’d be hot and heavy into the run,’’ Goldsworthy said.

The two biologists said there is still room for a normal season. A year ago at this time, you could cross-country ski over some North Shore streams. But despite last winter’s late thaw, steelheaders recorded an estimated spring catch of 3,200 fish — more than the historic shorewide average of 2,800.

“The fish still showed up, but they showed up at a different time,’’ Peterson said. “That’s kind of what we hope for this year.’’

The widely anticipated annual season for the exceptional rainbow trout is dictated by river conditions as the fish seek a return to upstream spawning grounds. The migration requires strong flows of warming water — a combination that can be short-lived depending on the North Shore’s snowpack and other factors.

Peterson and Goldsworthy said there’s a couple of subplots playing out this year.

The majority of the steelhead run in Minnesota each year is made up of 4-, 5- and 6-year-old trout. In 2019, the returning group will include classes of fish that didn’t show up very well in DNR counts of young trout as they departed the Knife River for Lake Superior as smolts.

“The number of smolts was really low in those years, so we are watching that,’’ Peterson said. “If the steelhead catch is low this year, it wouldn’t be surprising.’’

But Peterson was quick to add that catch rates this year may not suffer. That’s because so many variables factor into steelhead survival while they mature in Lake Superior. “You can’t put a lot of weight on one thing or another,’’ he said.

Indeed, catch rates have boomed in some years when the corresponding counts of the fish as smolts were disappointing.

The recent trend in catch rates has been upward. Minnesota steelheaders fish 18 North Shore streams from Grand Portage, Minn., to Duluth. In 2018, the catch rate of steelhead was 8.7 angler hours per fish caught. The historic average is 11.4 angler hours per fish caught. The peak since spring surveys of angler success started in 1992 came in the late ice-out year of 2015, when the rate was 5.8 hours angling per fish caught.

Minnesota’s wild steelhead population is naturally reproducing, but assisted by a hatchery program that has changed in the past couple of years. DNR’s overly expensive French River Hatchery was decommissioned and the annual put-and-take of stocked Kamloops rainbow trout was halted to stop crossbreeding between the “loopers’’ and steelhead.

As North Shore anglers catch and keep the dwindling Kamloops population, DNR is stocking two Lake Superior tributaries each year with genetically pure, pond-raised steelhead hatched in the DNR’s Spire Valley facility in Remer, Minn.

On Monday, Goldsworthy’s staff stocked the French River with 60,000 yearling steelhead. Another 60,000 each year will be released into the Lester River. Those fish, recognizable by a clipped adipose fin, can be kept as table fare as replacements for the Kamloops. For all unclipped, wild steelhead, the rule continues to be catch-and-release.

The new, clipped steelhead program will allow anglers a daily bag limit of three fish at least 16 inches in length. (No harvest upstream of posted boundaries to Lake Superior.) Fish provided to the Lester and French this year should be available for harvest when they return in three to five years. Their survivability in Lake Superior improves if they hold in the rivers for two years before migrating.

Goldsworthy said the DNR’s backup plan for sustaining the wild steelhead population is a broodstock program in Altura, Minn., at DNR’s cold-water Crystal Springs Hatchery. When those hatchery fish mature in a few more years, they will replace the steelhead broodstock previously maintained at French River.

“We’re still building the broodstock so it’ll be there to fall back on,’’ Goldsworthy said.

Dave Zentner of Duluth, an avid trout angler and conservationist, said more habitat improvement projects are in the works for the North Shore. There’s a push now to improve the assessment and selection process, he said.

On Monday, Zentner heard that 41 steelhead were counted in the DNR’s Knife River trap.

“It’s been duck hunting weather,’’ he said. “I’ve got to pull my stuff together. I’m going to finally fish.’’