An Elusive Carnivore Confronts New Challenges
In 2010, my dog treed a young fisher in the woods bordering my neighbor’s Christmas tree farm. I was surprised to spot the animal in this portion of southeastern New Hampshire, within 5 miles of the University of New Hampshire campus. It seemed that fishers, those elusive inhabitants of deep boreal forests, had moved into the suburbs.
Like the return of black bears, wild turkeys, and bald eagles, the restoration of fishers (Pekania pennanti) is a conservation success. Less than 100 years ago, fishers were so rare in most of the United States that extirpation seemed likely, even as more sustainable populations remained in Canada. By the mid-1900s, however, the species’ prospects had improved dramatically, as thousands of acres of land in New England and the upper Midwest returned to forests. Closure of state trapping seasons and live trapping-and-transfer of fishers into vacant habitats further reversed the decline. By the 1990s, fishers had returned to a large portion of their historic range.
Yet the fisher’s return is a complicated story. Some well-established fisher populations have declined, even as animals have moved into surprising new habitats. These population shifts suggest the influence of new environmental conditions, but what are they? And what insights do they offer, beyond this species?
A Change in Survival Rates and Competing Predators
As fisher populations recovered in the late 20th century, many states reintroduced limited trapping seasons, along with requirements that trappers document their harvests. These records aren’t comparable to a census, but they can help state biologists gauge population trends, especially if large changes occur.
Since restoration, trapping numbers have gone up and down, often correlating to the commercial value of fisher pelts or with typical variation in predator/prey abundances. But in the past two decades, there has been a clear downward trend in some regions that were once considered fisher strongholds. In New Hampshire, trappers bagged nearly 1,200 fishers in 1998 – a record take for the state. But subsequent harvests dropped dramatically, despite a relatively steady number of trappers. State biologists incrementally reduced trapper bag limits, and those actions seemed to slow the decline but did not stop it. By 2022, the harvest was a mere 19 fishers. Such a decline in harvests does not mean the population dropped by more than 90 percent, but it does indicate there are substantially fewer animals. Likewise, in Minnesota, state biologists estimated that fisher populations dropped 50 percent between 2000 and 2015, and in 2022, the annual harvest dipped to the lowest level since trapping seasons were re-established in 1977.
Responding to the large decrease in Minnesota’s fisher population, John Erb and his colleagues at the state’s Department of Natural Resources monitored the survival of 114 radio-collared fishers. From that sample, they identified the cause of death for 57 individuals, including 26 from predation (46 percent), 18 taken by trappers (32 percent), 7 from natural causes (12 percent), and 3 from vehicle collisions (5 percent).
Field evidence and forensic DNA analysis indicated that bobcats were the most frequent predator of fishers. Of particular concern, 21 of the predator-killed fishers were females and 15 of those were killed while they still had dependent young, meaning their deaths inevitably resulted in the death of their kits. Such a pattern of loss could not be sustained over many generations, and Erb suspected that mortalities among fishers had increased during the past 10 to 15 years. He speculated that habitat changes were either directly or indirectly affecting fisher survival rates.
Fishers rely on cavity dens in large trees for protection from weather extremes and to escape from predators. Cavity trees are especially important to females raising their young. With that in mind, Michael Joyce, a researcher at the University of Minnesota Natural Resources Institute, reasoned that a scarcity of suitable cavity trees might explain why female fishers in particular were vulnerable to bobcat predation. In the Northeast, trees require 80 to 100 years to grow large enough to support fisher-sized cavities. Timber harvests in the state have decreased forest age, potentially creating a scarcity of suitable den sites.
To evaluate this possibility, Joyce built numerous “denning boxes” and placed them in logged forests occupied by fishers. Similar boxes had been used by nesting fishers in British Columbia, and denning boxes have also benefitted pine martens – close relatives of fishers with similar denning habits – in actively managed forests in Scotland. Joyce built the boxes specifically for female fishers by making the entrance holes too small for larger male fishers or other predators.
Although females brought their kits to the boxes in late summer, they never used the boxes while caring for young kits. The results of Joyce’s research strongly suggested that a shortage of den sites wasn’t the source of the fishers’ rapid population decline. So, what was the cause? Another possibility was competition with bobcats. Both animals rely on similar prey, and there was evidence of bobcats hunting adult fishers, and not just vulnerable young. Ecologists consider such predation by a dominant predator (such as bobcats) on a subordinate predator (such as fishers) to be an extreme form of competition.
In New Hampshire, bobcat abundance steadily declined from the late 1960s through the 1980s, a consequence of high pelt prices and declining prey abundance. By 1989, harvests ebbed to a historic low and bobcats were reclassified as a protected species in the state, a designation that continues today. With protection and an expanding prey base (restored wild turkeys), bobcats have rebounded (for more details, see “The Resilient Bobcat” in the Spring 2017 issue of Northern Woodlands).
In Minnesota, bobcat populations have also increased, doubling since the early 2000s. The patterns of bobcat increase and fisher decrease in both states are intriguing, as is the extent of predation on fishers by bobcats in Minnesota. Bobcats and fishers have coexisted for millennia and, until recently, there have been few documented instances of cats preying on adult fishers. But there is precedent for the reintroduction of a dominant predator directly limiting the population of a subordinate one. Consider, for example, the restoration of gray wolves to Yellowstone National Park, and its impact on the park’s coyote population.
In 1995 and 1996, conservationists captured 31 wolves in Canada and released them in Yellowstone. From 1996 to 1998, wolf attacks on Yellowstone coyotes reduced coyote density by 50 percent across the park, and up to 90 percent in the wolves’ core areas. In the Lamar Valley of Yellowstone, for example, the coyote population declined from 80 animals in 1995 to 36 in 1998, and the average family group of coyotes dropped from 6 members to fewer than 4.
A similar pattern may be occurring in the Northeast, where fishers thrived in a period of bobcat scarcity from the 1970s through the 1990s. If this is the case, current population numbers for fishers in Minnesota and New Hampshire may be more representative of a partially restored predator community. (“Partially,” because bobcats themselves are subordinate to wolves and catamounts, which inhabited the Northeast until the 19th century.) In other words, the recent decline of fishers in some areas may be a sign of environmental recovery.
Moving Into Town
Another surprising development in fisher population trends is the animals’ movement into suburban landscapes. Until recently, much of our understanding of fishers was based on a time when the species had already retreated from portions of its original habitat. As a result, species profiles of fishers strongly associated them with remote boreal forests. Likewise, experts considered fishers incompatible with more populated regions to the south. But as fisher populations have increased in the region, the animals have demonstrated adaptability to diverse habitats.
In 2009, the year before my dog treed a fisher in the neighborhood Christmas tree farm, biologist Roland Kays discovered a group of fishers living in and around Albany Pine Bush Preserve. This New York preserve comprises some 3,200 acres and is the remaining portion of an extensive pine barrens. Although the preserve is mostly undeveloped, a network of roads and a variety of commercial and residential developments surround it. With the aid of graduate student Scott LaPoint, Kays captured and equipped a group of fishers with radio transmitters to study how they were making a living in this environment.
The transmitters on the fishers recorded their location at 15-minute intervals. Drawing on this detailed information, Kays and LaPoint determined that fishers were living a surprisingly extroverted lifestyle. Rather than avoiding contact with humans, they regularly ventured into small patches of forests in neighborhoods of single-family homes, stores, eateries, and other businesses. Kays and LaPoint speculated that the suburban habitat had more potential prey than did the preserve. Indeed, tree squirrels, mice, cottontail rabbits, some songbirds, and other human-tolerant prey can reach greater densities close to town than in continuous forests.
Researchers have since documented fishers in other suburban and human-dominated areas. Remington Moll and his graduate student Andrew Butler, at University of New Hampshire, have been evaluating the use of trail cameras as a method to estimate fisher densities. As part of that study, they are placing cameras throughout the southern portion of the state and have documented fishers in Nashua and Manchester, New Hampshire’s largest cities. Erb and Joyce also have reports of Minnesota fishers moving into Minneapolis and St. Paul, as well as formerly unoccupied habitats in eastern North Dakota and northwestern Iowa. Similar to the coyotes in Chicago, fishers are demonstrating a remarkable ability to adapt to areas dominated by humans.
The Rodenticide Threat
Living closer to people comes with new population risks, however. Canine distemper is one hazard, with the potential to spread through many animals. Another, greater threat is widespread exposure to rat poison (rodenticide).
For the past several years, Jacqueline Frair and her students at State University of New York College of Environmental Science and Forestry in Syracuse (SUNY-ESF) have been investigating factors that affect the distribution of fishers. As part of that effort, graduate student Stephanie Cunningham captured and radio collared more than 170 fishers in the Adirondacks and Tug Hill regions of northern New York. During the handling process or shortly after release, three of the captured fishers died from what were later determined to be rodenticides.
These deaths were surprising because of the rural nature of the study areas. Most likely, those fishers had killed or scavenged mice or other rodents that had eaten poisoned bait, resulting in secondary poisoning. A decade earlier, research indicated that rodenticides were the cause of death for 10 percent of radio-collared fishers in the Pacific Northwest. Researchers traced many of those poisonings to illegal marijuana farms. That wasn’t the situation in New York.
Rodenticides are widely used on farms, in restaurants, and in some households to kill rodent pests. These poisons are designed to prevent blood clotting (anticoagulant) and usually result in a slow death by internal bleeding. Once a mouse or rat consumes the poison, it can take up to 10 days to die.
During that time, the rodent continues to move through its environment and can become an easy target for predators or be scavenged after its death. Because anticoagulant poisons do not break down quickly, they move up the food chain. Birds of prey, including owls, hawks, and eagles, often die after consuming rodenticide-poisoned rodents. More often, birds and mammals consume sub-lethal doses that have long-term effects, including decreased resistance to parasites and diseases. Affected wildlife may also exhibit altered behaviors that make them less able to survive. For example, in one study, researchers documented that house sparrows exposed to a single, sub-lethal dose of an organophosphate pesticide were 12 times more likely to be captured by a predator than were birds in the same flock that had no exposure to the pesticide.
To gain a better understanding of the extent of the impact rodenticides have on fishers, Frair’s student, Georgianna Silveira, conducted a regional survey using tissue samples taken from fishers harvested in Maine, New Hampshire, Vermont, New York, and Pennsylvania. Exposure rates to at least one rodenticide ranged from 53 percent in Maine to more than 90 percent in New Hampshire and Vermont, an alarming level considering the rural character of these states.
Silveira’s analysis further revealed several patterns of concern. She found that within the densely settled “hot spot” that spanned southern New Hampshire and Vermont, fishers showed exposure to more than one type of rodenticide, which would increase their vulnerability to adverse outcomes. Yet, rural areas with seasonal camps and low-density communities were not free of exposure. Finally, when Silveira reviewed tissue samples collected from the same locations over time, she discovered that exposure rates had increased by 15 to 17 percent during a 3-to 4-year period. With these findings, Silveira concluded that fisher populations may be able to persist and even grow with relatively high levels of rodenticide exposure, but widespread exposure may make the animals more vulnerable to other environmental stressors.
The Future of Fishers
This summer, New Hampshire Fish and Game and researchers at UNH will begin a three-year collaborative study to examine mortality patterns of fishers and to gain additional understanding of their environmental requirements. The research team will capture at least 100 fishers within three study areas and fit the animals with radio collars. In this large a study group, the researchers assume that some of the marked fishers will die; the radio collars are designed to alert the researchers if an animal stops moving long enough to indicate that it has died. This design will allow the team to quickly recover dead animals. To determine cause of death, the team will use field evidence, as well as detailed laboratory examination of carcasses. They will test tissue samples for exposure to diseases and chemical toxins. The findings from this study should reveal the impact of different factors on fisher populations, including predation, exposure to disease and rodenticides, and trapping.
The story of fishers’ re-establishment in much of their historical range shows the complexity of what it means to “restore” a species in a rapidly changing and frequently human-dominated environment. In many ways, we can take heart from the surprising adaptability of a species that, until recently, was considered a boreal-forest specialist. We can recognize, too, that not all species declines should be cause for alarm, if they’re connected to other conservation successes, including the return of competing species.
Yet the fishers’ example, and uncertain future, also shows wildlife’s continued vulnerability to human activities, especially in areas such as the Northeast, where people are present in even the most remote areas. Wildlife exposure to rodenticides is especially concerning, and is becoming problematic worldwide, with the potential to contaminate entire food webs. As we seek to conserve wildlife, we may need to look beyond a species’ immediate numbers and habitat needs, and consider the impacts of our activities in our own neighborhoods, including the products we put in our basements and cupboards.
No doubt, there will be additional, yet-to-be-identified factors that fishers will confront. With our help, this elusive animal will continue to thrive in our forests.
Fisher Facts
By the 1920s, fishers only inhabited the most remote northern forests. Commercial demand for their fur was exceptionally high at that time and a prime pelt could bring a trapper $300 or more. Converted to present-day dollars, that’s equivalent to $4,600. According to Robert Brander and David Books in their Natural History article, “Return of the Fisher,” “To trappers and loggers eking out a meager living in the woods, a single fisher could have meant more than a month’s wages.” So, spending a week tracking a single fisher for such a payday was not unheard of. It wasn’t until the 1930s and 1940s, when fishers were nearly extirpated from much of their range, that state wildlife agencies protected them and recovery began. However, populations in the Pacific Northwest continue to remain small and fragmented, even after decades of protection.
Fisher Arrives in the Big Apple
As described in a New York Times article in 2014, NYPD officer Derek Lenart was on patrol near Bronx Community College when he saw an animal dart in front of him and run beneath parked cars. Before it disappeared up a driveway and into a backyard, Officer Lenart snapped a photo. Roland Kays (now at North Carolina State University) later identified the photographed animal as a fisher. As Kays later noted on his blog, “Judging from the picture, this [is] a male fisher, likely a dispersing animal looking for a female and a new place to settle down. If he can find a place to sleep and something to eat, he might stick around. Bronx squirrels would make good fisher prey, but things could get really interesting if fishers start hunting rats in New York.”
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