UMD researchers build new dens for forest fishers

A shortage of big trees to nest in may be one factor in the big decline in Minnesota’s fisher population over the last decade.

Michael Joyce (left), a wildlife ecologist with the Natural Resources Research Institute, and Reid Siebers, a UMD grad student, hang a fisher nest box on a poplar tree in the Superior National Forest near Hoyt Lakes. (Clint Austin /

SUPERIOR NATIONAL FOREST — One plywood box at a time, Michael Joyce and his field assistant, Reid Siebers, are trying to solve a north woods enigma.

What’s happening to Minnesota's forest fishers?

So this month and next, they will be humping dozens of the 52-pound boxes that Joyce built deep into the woods across the region to see if and where female fishers might use them to den and raise their young.

“They are a bit heavy to work with,’’ Joyce said with a grin as he backpacked one of the boxes into the woods off a Forest Service logging road southeast of Hoyt Lakes.

Fewer big trees, fewer fishers


A male fisher on a fisher nest box in Carlton County. ( NRRI photo)

The good news is that fishers, the big weasel-family resident of Minnesota’s north woods, like to build their dens in aspen trees, now the most common tree in the forest.

The bad news is that they need big trees, 20 inches or more in diameter, that are starting to crack and develop crevices, to build their dens in. And if you’ve spent any time in the woods lately you'd know trees that large are in short supply. Studies show fewer than 2% of the trees now standing in the woods are 20 inches or bigger.

That shortage of big trees to nest in may be one factor in the big decline in Minnesota’s fisher population over the last decade. The Minnesota DNR estimates the fisher population at about 7,000 statewide, less than half the 17,000 estimated in the early 2000s.

“As we’ve seen the fisher population drop about 50%, fairly quickly, we looked at denning sites as a limiting factor. We’re trying to see if there are some ways to get around that,” said Michael Joyce, lead researcher on the project. “We’re really concerned about fisher habitat decline.”

Joyce, a wildlife ecologist with the Natural Resources Research Institute arm of Minnesota Duluth, will monitor the dens to see if female fishers use them to rear their kits in spring. Each den box has a temperature sensor to see if and when the warm-bodied critters are inside. And each has a trail camera pointed at the front door to see what’s stopping by to visit. A glue strip will pull some fisher hair out so DNA samples can be taken to identify specific animals.

Big trees take time

The interior of a fisher nest box being installed in the Superior National Forest near Hoyt Lakes. (Clint Austin /


It takes about 80 years for an aspen tree to get 20 inches in diameter. But most forest land managers cut aspen every 40-60 years to make paper or boards, meaning most don’t get old enough enough for fisher nests.

Clearly, longer rotation ages — letting aspen get older before they are cut — would help create more cavity trees. Joyce said his efforts aren’t aimed at any restrictions on logging, but more at providing options for landowners and wildlife managers as the woods are used by both people and animals. He noted the state’s timber industry has a vested interest in making sure fishers don’t dwindle to endangered status, a mark that probably would bring restrictions on where and when big trees could be cut in some areas.

“We are definitely not anti-logging. But we are trying to see if we can strike a balance between human needs for wood products and what the animals need to survive in the woods,’’ Joyce said. “Maybe we can find some options, some tools for foresters and land managers on what they can do to keep some of the big trees standing.”

Where that’s not possible, or where no old trees exist, Joyce’s experiment will test if human-made nest boxes will help. First, researchers need to know if female fishers use them. Eventually, they’ll want to know if the boxes can actually increase the population, as what occurred with human-made wood duck and bluebird nesting boxes, credited with bringing those species back from the brink.

“If they (fishers) use them only where they already have good habitat, then it really doesn't make any sense to add boxes,” Joyce said. “But if they use them it the more marginal, sub-prime habitat, where there may not be another option for them, then we might make a difference. We need to figure that out.”

Over the past two years, Joyce and cooperators with the Carlton County Land Department put up a dozen fisher nesting boxes. Trail cameras showed female fishers on the boxes, but none were used to nest in, at least not yet. The cameras also caught bear, fox, pine marten, male fishers, a barred owl, raccoons and other critters on and near the boxes.


“We even had red squirrels mating on one box,’’ Joyce said.

This fall, Joyce is going all-in, putting up more than 120 boxes with the help of the Leech Lake Band of Minnesota Chippewa, the 1854 Treaty Authority, Vermilion Community College and UPM-Blandin Paper Co. The project was funded through a $190,000 grant from the Minnesota Natural Resources Trust Fund, the state’s profits from lottery sales.

The boxes are modeled on den boxes wildlife researchers used in British Columbia with some success: Up to 30 % of those boxes were used by female fishers to den. The 3-foot-high boxes are insulated to keep baby fishers warm. And the hole is small enough so predators (including male fishers in some cases) can’t get in. Female fishers give birth in March and take care of the kits through about May.

Bobcat bonanza

Michael Joyce (left), a wildlife ecologist with the Natural Resources Research Institute, and Reid Siebers, a UMD grad student, prepare to install a fisher nest box in the Superior National Forest near Hoyt Lakes. (Clint Austin /

Predators may also be playing a role in the fisher decline. Bobcats are the largest predator on fishers and bobcats tend to like young aspen forests for hunting. The more aspen that’s cut, the more bobcats prevail, and right now, bobcat numbers are growing fast.

The makeup of the forest “is really a big deal in figuring this out,’’ Joyce said.

John Erb, the Minnesota Department of Natural Resources' furbearer research specialist in Grand Rapids, agreed. His study of radio-collared female fishers found nearly all of them nest in bigger, old trees.

“I believe that changes in the forest, mostly age, likely explain both declines up north,’’ Erb said of fishers and their smaller cousins, pine martens. The lack of old tree habitat has probably held back any big rebound in fisher numbers, even though the trapping season and limits have been dramatically reduced.

Erb also said forest management and the move to younger forests “also likely played a direct role in bobcat increase, which then had additive impact on fishers and martens.”

Erb said that fishers and martens appear to have bottomed out a few years ago and that their numbers have stabilized in recent years. He also noted that fishers, while their numbers are stagnant in the north, have been expanding their range into central and southeast Minnesota.

Trappers also erecting boxes

Michael Joyce, a wildlife ecologist with the Natural Resources Research Institute, scouts a site with Reid Siebers, a UMD grad student, to place a fisher nest box in the Superior National Forest near Hoyt Lakes. (Clint Austin /

Members of the Minnesota Trappers Association already have been erecting den boxes for several years — more than 1,000 to date — mostly aimed at pine martens, but some that fishers have used, too, said Shawn Johnson, a Duluth trapper and education coordinator for the association.

There’s no hard data for those boxes, but anecdotal evidence shows that “yes, fishers will utilize artificial nesting cavities,’’ Johnson said.

Johnson says many trappers suspect that stagnant fisher populations in the core areas of the north woods are due in part to direct competition between fishers and an exploding bobcat population — not just because bobcats will kill fishers, but because they both also eat the same small mammals. Johnson also noted that red-backed voles appear to have experienced a significant decline in population while snowshoe hare numbers also have been down.

Johnson agreed that a younger forest also has helped bobcats.

“No doubt a young, successional forest has worked to assist bobcats in their expansion and population increases,’’ he said. “Fishers generally got along just fine before cats became the major dominating presence they have become in so many areas.”

Roller-coaster population

Michael Joyce, a wildlife ecologist with the Natural Resources Research Institute, installs a branch on a fisher nest box in the Superior National Forest near Hoyt Lakes. (Clint Austin /

Fishers have had an up-and-down century or so in Minnesota. They were trapped and logged nearly out of existence by the early 1900s, considered extirpated from Minnesota forests. But by the late 1950s, they started to pop up again, and by the 1970s, Minnesota was allowing trappers to kill a limited number of fishers each year. Then that harvest got too high, topping 3,000 in 1979, leading the DNR to close the season entirely by 1980.

Fisher numbers built again, peaking in the early 2000s. Again, trapping increased, to a high of 3,251 killed in 2006 during a 16-day, five-fisher season, just as the population began to dwindle yet again. By 2018, the DNR had cut the fisher trapping season to just six days, with a two-fisher limit, and only 510 were killed.

Johnson said the short season, coupled with chronically low fur prices, has reduced trapping considerably in recent years. Minnesota’s fisher and pine marten trapping season will run Dec. 21-29 this year with a combined limit of two.

About fishers

The fisher is a member of the weasel family, resembling a very large mink. It weighs as much as a red fox, but has much shorter legs. Fishers are extremely agile and active predators. Excellent tree climbers, they can out-climb martens and red squirrels.

They prey upon red squirrels, snowshoe hares, mice and voles and are the only natural predator of porcupines. Fishers will also eat insects and berries. Despite its name, the fisher does not catch or eat fish.


General description: The fisher is a medium-sized long-shaped predator that belongs to the weasel family.

Length: 24-30 inches long, including a long, bushy tail.

Weight: Female adults weigh 6-8 pounds; males weigh up to 18 pounds.

Color: Fur is a grizzled dark brown, blackish on the rump and tail, with a white or cream-colored bib on their chest.


Like most members of the weasel family, female fishers have what is called "delayed implantation." Females get pregnant in spring, just 10 days after they have given birth. For the next several months, the young exist as tiny embryos. Then, two months before being born, the embryos develop into fetuses.

One to five young fishers are born in a hollow tree, log or rock cavity. Within days after giving birth, the female will seek out a new mate. Young fishers stay with their mothers for just a few months. The young leave the female in early fall to find their own home territory.


Bobcats are the only major predator for fishers.

Habitat and range

Fishers prefer large areas of continuous forest, particularly older timber stands. They are adaptable, but avoid open areas. They prefer the edges of conifer stands when these are adjacent to stands of deciduous trees. Hollow trees are their primary denning sites, but they occasionally den in rock crevices, abandoned beaver lodges in dry ponds and old porcupine dens.

Photos: Dens for forest fishers Photos by Clint Austin

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at
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