by Gustave Axelson
It was March, that in-between time when the bitter cold of winter lingers with the damp air of a new spring. And a wicked ice storm bombarded the north woods. Furious winds howled at over 50 miles per hour. An icy enamel encrusted every tree branch.
Huddled inside a hole in an aspen tree, just south of Chippewa National Forest, a mother fisher wrapped her long, slender body and bushy tail around her kits. Her luxuriant fur provided warmth and comfort for the newborns, still blind and mostly hairless. They nestled in against their mother and mewed like kittens. One moment, the family was cozy in their den. Then with a sharp and sudden crack, the wind ripped the roof off their hideaway, exposing them to the blowing cold and pelting ice.
A camera mounted on a telescoping pole captures fantastic footage of life inside a fisher den.
Fishers are nature's most proficient porcupine hunters—circling the porcupine, striking repeatedly with quick bites to the face, then retreating to stay clear of quills. After repeated facial bites, the porcupine is either dead or bewildered, at which point the fisher flips the porcupine on its back to feast on its belly. With this specialized technique for preying on porcupines, fishers have access to a proprietary food source with little competition from other predators.
Pregnant all Year
Female fishers often make dens in hollows in aspen trees about 20 to 30 feet high. In March they give birth to two to four kits. Then just 10 days later, the female fisher mates again—resulting in one of the longest reproductive cycles of any mammal (about 350 days). After initial development of the fertilized egg, implantation and further growth is delayed for 10 months until the following February, when active pregnancy begins.
Frantically, the mother fisher scurried down what remained of the aspen's trunk and into the night, searching for another hollow tree where she could move her kits. She didn't have much time. They wouldn't survive long in this squall of freezing rain.
Days later, Department of Natural Resources wildlife biologists Barry Sampson and Pam Coy tromped into the woods looking for the mother fisher. She was one of 48 fishers that they had radio-collared and were tracking in a research study of fisher reproduction and survival in Minnesota.
They found the toppled tree and started waving their radio antennas to see if they could relocate the mother fisher. From the look of the smashed den, it seemed her kits might join a growing list of fisher mortalities in the study.
While fishers are nowhere near threatened or endangered in Minnesota, Martes pennanti is a fur-bearer species with a population trending downward, according to surveys and trapping harvest records. The ongoing study, which began in 2007, has uncovered some possible reasons for their decline, including a question about habitat: How easy is it for fishers to find mature, hollowed-out aspen trees for dens?
It's a question that mother fisher faced on her frenzied search for a new den tree as she tried to rescue her newborns on that horrible stormy night.
20th-Century Comeback. Fishers are the larger, darker cousin of the pine marten. A medium-sized member of the weasel family, they measure about 2 to 3 feet long, with lush brownish-blackish fur. Fishers were once abundant and occupied all of wooded Minnesota, all the way down to the southeastern oak forests, but unregulated trapping for their prized pelts made them rare by the year 1900. Unregulated logging and slash-fed forest fires ate up their forest habitat in the early 20th century, and fishers were near extirpation in Minnesota by 1930. Fisher harvest was prohibited from 1929 to 1977; and the forests grew back, which allowed them to re-colonize the north woods, roughly the northern third of the state. Since 1977 a regulated harvest of fishers has been allowed.
Harvests were mostly between 1,000 and 2,500 fishers in the 1980s and '90s. The mid-2000s saw a run of harvests at the top of that range, culminating in a record-high take of 3,251 fishers in 2007.
"We cut the fisher season length in half after that high harvest year, and later reduced the [trapping limit] from five to two," says DNR furbearer biologist John Erb. "The 2006-07 harvest might have exceeded the sustainable level."
However, winter track surveys—when DNR biologists count wildlife tracks in the snow as a means of measuring furbearer populations—indicated the fisher population might have begun declining in 2001, prior to the run of high harvests. Erb suspected other factors could have been impacting fishers.
Minnesota still has about 6,000 fishers today, one of the highest fisher populations in the United States—but about 40 percent fewer than population estimates in Minnesota 15 years ago. And not much is known about their survival, reproductive ecology, or sensitivity to forest management in Minnesota, says Erb, who is responsible for monitoring the fisher population and setting sustainable trapping limits. So he launched the radio-collared fisher study five years ago.
Collaring and Tracking. Each year, the study begins in winter when DNR researchers put out cage traps baited with deer or beaver meat to lure fishers. A trapped fisher is sedated with tranquilizers, outfitted with a radio collar, and released. Then the DNR team—Erb, Sampson, and Coy—set out with radio antennas to track it and monitor its behavior.
Local volunteer Richard Nelles, who lives in a cabin in the study area, helps to trap and track fishers seven days a week. "It's always fun to get up and check the traps first thing in the morning," Nelles says. "I'm out there all alone, but I pump my fist and do a little dance if I get a fisher in my trap."
Female fishers find den trees around February. The team has noted that fishers mostly choose to den in aspens with trunks averaging almost 2 feet in diameter. Seventy-five percent of fisher dens in the study were in aspens. "Before this study, we didn't realize how much fishers rely on big, hollow aspens," says Sampson.
Fisher kits are born in early to mid-March. After the kits are a few weeks old, the mother fishers often move their kits to new den sites every few weeks—perhaps seeking new hunting grounds, avoiding den detection by predators, or because the old dens become contaminated with parasites, food scraps, and scat. To keep track of the fisher family, Erb's team mounts trail cameras near den trees to monitor relocations. If they find photos of a mother moving her kits one by one, hauling each one out by the scruff of the neck, the researchers know it's time to scan with their radio antennas to find the collared female.
Mother fishers also go out hunting a lot to feed their hungry family. Despite their name, fishers don't prey on fish. They scavenge on carrion and forage for fungi, berries, and nuts; but they mostly hunt a variety of forest animals—snowshoe hares, red squirrels, mice, and grouse. Fishers also eat porcupines. (The fisher is one of the few predators that has mastered the art of feasting on prickly porkies.)
Because they spend so much time looking for new dens, moving kits, and hunting in early spring, when there isn't much green leafy cover for hiding, mother fishers are often vulnerable to predators. And according to the study results, mother fishers in northern Minnesota are dying more often than Erb anticipated.
Identifying a Predator. Of 48 fishers with operational radio collars over the past five years, 27 died: two hit by cars, two trapped illegally, four trapped legally, four from natural causes, and 15 killed by predators. Nearly all of the predation deaths occurred in spring.
"We didn't expect such high predation on fishers," says Erb. Five male fishers were killed by predators: four by bald eagles and one by a mammal. Conversely, of the 10 female fishers killed by predators, nine were killed by mammals and only one by a raptor (likely a great horned owl). Of greatest concern, seven of the 10 females killed by predators were nursing mothers, which resulted in the subsequent deaths of 18 dependent kits.
Erb points out that one-third of all radio-collared female fishers in the study were killed by predators. "The level of observed adult female mortality [in the study] is not sustainable," he says.
To determine the mammalian predators of killed fishers, Erb's team swabbed wounds and clipped hair samples from the fisher carcasses and sent them to a lab to see if any remnant saliva from the bite wound could reveal a genetic ID. So far, two IDs have been made. Both were bobcats. While forensic results are still forthcoming on other carcasses, bobcats are suspected in several of the other fisher kills.
"Bobcats can put pressure on a fisher population in two ways," says Erb. "They compete with fishers for shared prey like grouse, hares, squirrels, and rodents. And bobcats are predators of fishers, particularly female fishers."
Erb says it's not a coincidence that bobcats are killing fishers in his study. Winter track surveys show the bobcat population has roughly doubled since 2000. During that same time, the fisher population has declined by 40 percent.
Trapping is another indicator of population trends. Just a decade ago, trappers took 10 fishers for every bobcat taken in Minnesota. Last year, for the first time in recent history, trappers harvested more bobcats (1,012) than fishers (903).
Erb believes that warmer winters with less snow (on average) over more than a decade partly explain why bobcats are now doing so well. The milder winters might allow bobcats, a more southerly species, to be more successful at hunting, overwintering, and reproducing, at the expense of fishers, he says.
Preserving Den Trees. Timber harvesting also plays a critical role, Erb says. Fragmented forests, clear-cuts, and brushlands might benefit bobcats, but fishers are northern forest specialists that depend on mature trees for dens and rest sites. And the more that fishers have to be out searching for suitable habitat, the more likely they are to be killed.
Erb notes that on the southern and western periphery of Minnesota's fisher range, pockets of their population seem to be increasing.
"Even though there's less overall forest [there], a higher percent of what's there is private forest that's not likely getting logged as intensively," says Erb. "That forest may provide better habitat—more big, old cavity trees and standing dead trees—which may explain why fishers are able to re-colonize those areas and do well there."
Trapping statistics correspond with Erb's theory. Last year, the highest rates of fisher harvest per 100 square miles were in Wadena, Crow Wing, and Otter Tail counties, at the southwestern fringe of the fisher range in Minnesota. In Otter Tail County, where no fishers were registered by trappers in 2000, the fisher harvest was 100 pelts last year.
In the northern core of Minnesota's fisher range (St. Louis, Aitkin, Cass, Itasca, and Koochiching counties), big, mature aspens with denning cavities have become less common as harvests of young aspen have increased on public land.
With Erb's research in mind as it revises timber harvest guidelines, the Minnesota Forest Resources Council is considering the need to leave suitable den trees, such as big aspens, in clumps or scattered around a harvest site.
The DNR has already implemented a program to emphasize leaving den trees on timber harvests on state lands. DNR foresters work with local wildlife managers to design timber sales with consideration for habitat.
"Because large-diameter aspen have economic value, they haven't always been left behind," says Tim Quincer, DNR forest wildlife coordinator. "John's research has been very helpful as discussions on leave-tree guidelines have occurred.
"Knowing the need fisher have for large-diameter aspen helps reinforce the larger message that we need to take care to leave some examples of all tree species and sizes during timber-harvest operations if wildlife habitat needs are to be met."
Cause for Optimism. Despite the downward trend of fishers in recent years, Erb sees cause for optimism.
"They're not going extinct anytime soon. We still have perhaps twice as many fishers as bobcats or wolves," Erb says. "I am optimistic that conservative [fisher] harvest management will allow us to move the population back upward in the short term, even if harvest was not the problem [that caused the decline]."
Indeed, after four years of conservative fisher harvests, winter track surveys of fishers ticked upward last year for the first time in seven years.
Safe in a New Home. On a bright spring day last May, DNR researchers Sampson and Coy finally caught up to the radio-collared mother fisher that had been forced out of her den, the one they affectionately called "snowstorm girl." She was holed up in a tree cavity in an old white pine.
They kept visiting the pine until they happened on a time when the fisher was out hunting, and then they snaked a long cord with a tiny video camera into the hole in the tree. The camera provided a live look into the den via a laptop computer. The researchers wanted to see if she had saved any of her kits.
After a few tense moments of lowering the cord and camera through the darkness inside the tree, the researchers looked at the laptop screen and saw two pairs of big, shiny eyes.
Two kits, probably hungry, anxiously awaited their mother's return with dinner.