Powell and Zielinski 1994

General Ecology

The fisher (Martes pennanti) is a medium-size mammalian carnivore and the largest member of the genus Martes…(38)

Adult male fishers generally weigh between 3.5 and 5.5 kg… adult female fishers weigh between 2.0 and 2.5 kg… (38)

Only two intensive, radio-telemetry based habitat studies have been published on fishers [in the West], one in northwestern California (Buck et al. 1983) and the other in Idaho (Jones 1991)… Two additional studies have been completed in about the same locations in Montana (Heinemeyer 1993; Roy 1991) but both individuals studied fishers that were reintroduced from Wisconsin and Minnesota… Only two natal dens and one maternal den have been discovered and described in the West… Thus, the quantity of data on the ecology of fishers in the West is extremely low… (64)

Conservation Status and Threats

Between 1800 and 1940, fisher populations declined or were extirpated in in most of the United States and in much of Canada due to overtrapping and habitat destruction by logging… (39, 41) trapping and logging are not independent because logging increases access to forests by trappers (41)

At the same time that fishers were heavily trapped, their habitat was being destroyed… Either trapping or habitat destruction by itself could have dramatically reduced fisher populations; together, their effect was extreme. During the 1930's, remnant fisher populations in the United States could be found only on the Moosehead Plateau of Maine, in the White Mountains of New Hampshire, in the Adirondack Mountains in New York, in the "Big Bog" area of Minnesota, and in the Pacific States… (41)

Following the reduction in fisher populations, porcupine populations climbed to extremely high densities in much of the forested lands in the United States… (41)

…the range of the fisher in eastern North America has recovered much of the area lost during the first part of this century. The fisher is again living in areas from northern British Columbia to Idaho and Montana in the West, from northeastern Minnesota to Upper Michigan and northern Wisconsin in the Midwest, and in the Appalachian Mountains of New York and throughout most of the forested regions of the Northeast… (42)

Fishers may be on the verge of extinction in Washington… (42)

… the fisher population in the southern Sierra may be doing well, but it appears to be isolated from the population in northwestern California… (42)

It is our opinion that the precarious status of the fisher population in Washington and Oregon is related to he extensive cutting of late-successional forests and the fragmented nature of these forests that still remain. (64)

That the populations [of fishers in the Northwest] appear dangerously low should be sufficient to generate protection; discussions and research into the reasons should occur after protection. In our opinion, protection by the states of Washington, Oregon, and California has not been sufficient to improve population status. (66)

The status of fishers in the northern and central Sierra Nevada is unknown but the absence of recent observations suggests they are declining or barely holding steady… Fisher populations in the northern Rocky Mountains of the United States do not appear to be in as critical condition as those in the Pacific Northwest… If trapping seasons are regulated carefully in Montana to prevent overtrapping, fisher populations may slowly expand in Montana and Idaho… (66)

Reintroductions/Augmentations

Fisher reintroductions in the West: (Table 1, p. 40)
Idaho — 1962-3
Montana — 1959-60, 1988-91
Oregon — 1961 (failed)

Fishers have been released in eastern North America to reestablish populations… Releases have generally been unsuccessful in western North America… (42)

Healthy adult fishers appear not to be subject to predation, except fishers that have been translocated (44)

Reintroduction of fishers to the Cabinet Mountains of Montana was hindered by predation; of 32 fishers from Wisconsin released in the Cabinet Mountains, at least 9 were killed by other predators… (44)

Management Considerations (p. 61)
Where reintroductions are necessary, conduct them during the summer until additional research dictates otherwise.

Home Ranges/Minimum Area Requirements

Fisher population densities vary with habitat and prey, and density estimates in the northeastern United States have ranged from 1 fisher per 2.6 km2 to 1 fisher per 20 km2… (43)

The mean home range size for adult male fishers is 40 km2… nearly three times that for females… because the territories of male fishers are large, hundreds of square kilometers of suitable habitat may be necessary to maintain sufficient numbers of males to have viable populations… managed areas in the West may need to be at least 600 km2 in California… to 2000 km2 in the Rocky Mountains… of contiguous, or interconnected, suitable habitat. (57-58)

… home ranges overlap little between members of the same sex but overlap is extensive between members of opposite sexes… (59)

Management Considerations (p. 60)
Fishers, especially males, have extremely large home ranges and the largest ranges may occur in the poorest quality habitat. The management of areas large enough to include many contiguous home ranges will probably have the best chance of conserving fisher populations.

Trapping

Trapping has been one of the two most important factors influencing fisher populations… (44)

Fishers are also easily trapped in sets for other furbearers… Where fishers are scarce, the populations can be seriously affected by fox and bobcat trapping…(44)

Mathematical models for the fisher community in Michigan indicated that small increases in mortality due to trapping could lead to population extinction… (44)

During periods of high prey availability, juvenile fishers comprise a higher-than-average proportion of a trapped population; when prey populations are low and fisher populations decline, cohorts of old fishers comprise higher-than-average proportions of the population… (45)

… trapping may affect the abilities of fisher populations to respond to increasing prey populations… (45)

Sex ratios for natural fisher populations should be close to 50:50… [the] trapping bias toward males might skew harvested populations toward females… (45)

… trapping during January and February causes disproportionately high mortality of adult males, may decrease their numbers below that necessary to inseminate all females, and may even lead to population decline [Algonquin, Ontario example] (46)

Wherever fishers are trapped, populations must be monitored closely to prevent population decrease. (63)

Because fishers are easily trapped, where fisher populations are low they can be easily jeopardized by the trapping of coyote, fox, bobcat, and marten… Wisconsin designated fisher wildlife management areas… where lands sets for all furbearers were prohibited… During the two years that British Columbia closed the fisher season the incidental capture of fishers exceeded the legal capture the preceding year… The closure of all commercial marten trapping where their range overlaps that of the fisher in Washington and Oregon has been recommended by the [Forest Service]… Where commercial trapping of terrestrial carnivores occurs, the threat exists that fishers will be trapped and that their populations could be negatively affected… (63-64)

Management Considerations (p. 45)
Fishers are easily trapped and can frequently be caught in sets for bobcats, foxes, coyotes, and other furbearers. To protect fisher populations, trapping using land sets may need to be prohibited. Incidental trapping of fishers in sets for other predators may slow or negate population responses to habitat improvement.

Reproduction and Denning Habitat

Female fishers are usually sexually mature and breed for the first time at 1 year of age… (46)

Implantation is delayed approximately ten months, and, therefore, female fishers can produce their first litters at age two. Females breed again approximately a week following parturation… (46)

Why some females that have bred fail to produce litters is unknown, but nutritional deficiency related to stressful snow conditions is suspected because reproductive indices are higher in some areas of low snowfall… (46)

… the mean litter sizes for fishers from seven studies… ranged from 2.00 to 2.90… (46)

…female fishers in eastern North America and in the Rocky Mountains are highly selective of habitat for resting sites… they are probably highly selective of habitat for natal and maternal den sites as well (47)

Female fishers will use 1-3 dens per litter and are more likely to move litters if disturbed… Kits are often moved from natal to maternal dens at 8 to 10 weeks of age (47)

Management Considerations (p. 45)
The reproductive rates of fishers are low, relative to other mammals, and low density fisher populations will recover slowly.

Management Considerations (p. 48)
1. The recovery of fisher populations will be slow because fishers have small litters and do not produce their first litters until two year of age. Reproductive output of populations biased toward young fishers is limited by the inability of yearling males to breed effectively. Over-trapping may also bias the population toward young animals, further delaying recovery.
2. All natal and maternal dens in the West were found in large diameter logs or snags. These habitat elements may be reduced in stands that have been intensively managed for timber.

Diet

For fishers in the Cabinet Mountains of Montana, 50% of the prey remains found in 80 scats were from snowshoe hares… Mice and other small rodents constituted the next most common prey. Porcupines constituted 5-10% of the prey items eaten and deer eaten constituted less than 5% [% of carrion may have been underestimated] (48)

Snowshoe hares are the most common prey for fishers and have been reported as prey in virtually all diet studies… (50)

Management Considerations (p. 52)
1. Snowshoe hares area a major prey item almost everywhere fishers have been studied, including the Rocky Mountains. If this is confirmed from studies elsewhere in the West, managing for hare habitat might benefit fishers if it is not at the expense of denning and resting habitat.
2. In late-successional coniferous forests the presence of high densities of snowshoe hares or porcupines indicates the potential for a fisher population.

Habitat

Fishers occur most commonly in landscapes dominated by mature forest cover and they prefer late-seral forests over other habitats… In the Pacific states and in the Rocky Mountains, they appear to prefer late-successional coniferous forests… and use riparian areas disproportionately more than their occurrence… Everywhere they exhibit a strong preference for habitats with overhead tree cover …(52)

It is unlikely that early and mid-successional forests, especially those that have resulted from timber harvest, will provide the same prey resources, rest sites, and den sites as more mature forests. (52)

Buskirk and Powell (1994) hypothesized that physical structure of the forest and prey associated with forest structures are the critical features that explain fisher habitat use, not specific forest types. Structure includes vertical and horizontal complexity created by a diversity of tree sizes and shapes, light gaps, dead and downed wood, and layers of overhead cover. Forest structure should have three functions important for fishers: structure that leads to high diversity of dense prey populations, structure that leads to high vulnerability of prey to fishers, and structure that provides natal and maternal dens and resting sites. (53)

Fishers appear to be restricted to areas with relatively low snow accumulation… On the Olympic Peninsula and on the west slope of the Cascade Range… where snowfall is greatest at highest elevations, fisher sightings in the past 40 years have been confined to low elevations… Fishers in Idaho and Montana select flat areas and bottoms and avoid mid-slopes… (54)

The fishers in all three Rocky Mountain studies… selected riparian areas, which have relatively gentle slopes, dense canopy, and perhaps protection from snow… (55)

Fishers avoid nonforested areas… Fishers have avoided areas 25 m across and less in the Midwest… Large forest openings, open hardwood forests, recent clearcuts, grasslands, and areas above timberline are infrequently used in the West… (55)

Aversion to open areas has affected local distributions and can limit population expansion and colonization of unoccupied range… An area of farmland in Upper Peninsula Michigan delayed expansion of the population to the north by at least 15 years… and the Penobscot River delayed expansion of fishers to eastern Maine for over a decade… (55)

The canopies of, or cavities within, live trees are the most commonly used rest sites reported in eastern and western studies… In the published western studies, logs were of secondary importance, followed by snags… the average diameters of trees used as resting sites were 55.8 cm in Idaho… 114.3 cm in California… (56)

Resting sites reported in studies in the western United States tend to occur predominantly in closed canopy stands. Jones (1991) analyzed canopy closure at 172 resting sites in Idaho and found that fishers preferred to rest in stands that exceeded 61 percent canopy closure during summer and winter, and avoided stands with less than 40 percent closure. Canopy closure at 34 rest sites in northcentral California averaged 82%… (56)

Because the types of forests that normally contain resting and denning sites may be more limiting than foraging habitat within the fisher range in the West, they should receive special consideration when planning habitat management. (57)

The extensive, clearcut logging done during the 1800's and early 1900's, together with trapping, decimated fisher populations all over the continent. Because fishers are associated most frequently with relatively unfragmented, late-successional forests, recent clearcut logging continues to affect fisher populations today through its profound effects on forest landscapes. Large nonforested areas are avoided by fishers, especially during the winter, and the fact that extensive areas of the Pacific Northwest have been recently clearcut… may be the reason fisher populations have not recovered in some parts of this region… (64)

Provided there are large patches of late-successional conifer habitat nearby, fisher populations should be able to tolerate incidents of stand replacement disturbances. Small patch cuts interspersed with large, connected, uncut areas should not seriously affect fisher populations. In fact, these small-scale disturbances may increase the abundance and availability of some fisher prey. Large clearcuts and numerous, adjacent, small clearcuts of similar age should seriously limit resting and foraging habitat for fishers during the winter. This, in turn, may limit fisher population size… Forestry practices aimed at maximizing wood production and minimizing rotation times will probably have detrimental effects on fisher populations. (64)

Management Considerations (p.57)
1. In the western mountains, fisher prefer late-successional forests (especially for resting and denning) and occur most frequently where these forest include the fewest large nonforested openings. Avoidance of open areas may restrict the movements of fishers between patches of habitat and reduce colonization of unoccupied but suitable habitat. Further reduction of late-successional forests, especially fragmentation of contiguous areas through clearcutting, could be detrimental to fisher conservation.
2. Large physical structures (live trees, snags, and logs) are the most frequent fisher rest sites, and these structures occur most commonly in late-successional forests. Until it is understood how these structures are used and can be managed outside their natural ecological context, the maintenance of late-successional forests will be important for the conservation of fishers.

Fragmentation/Connectivity/Movements

… in areas where there has been extensive, recent logging that fragments forests extensively, fisher populations have not recovered, perhaps because fishers appear sensitive to forest fragmentation… (42)

If remnant populations in the Pacific Northwest and Rocky Mountains are reduced in number and sufficiently separated they may not be capable of recolonizing depopulated arears… (45)

No studies have investigated large-scale habitat preferences, as might be found across the pronounced elevational gradients in the western mountains, yet fishers may have critical preferences on this large scale… (53)

Individual fishers appear to use different scales in choosing where to perform different behaviors.. Where to establish a home range is decided on a landscape scale; where to hunt is decided on a scale of habitat patches; where to rest is decided on a scale of both habitat patches and habitat characteristics within patches… (53)

Fishers can travel long distances during short periods of time but travel, about 5-6 km per day on the average. (60)

In Idaho, two 1-year-old males established ranges after moving 26 and 42 km, respectively… (60)

Buck et al. (1983) thought that forested saddles between drainages were important linkages for fisher movements (has not been studied)… Large open areas retard population expansion… (61)

It is possible that forest fragmentation may affect predation on fishers by other predators. If fragmentation causes fishers to travel long distances through unfamiliar habitat (especially unpreferred habitat) in search of mates, the fishers might be subject to predation. (62)

Management Considerations (p. 43)
1. Isolated populations are of special concern and must be monitored.
2. Forest fragmentation may result in the isolation of populations.
3. Reintroductions would be most likely to succeed if translocated animals from similar habitats in the same ecoprovince.

Management Considerations (p. 45)
Population densities of fishers are low, relative to other mammals, and can undergo fluctuations that are related to their prey. These fluctuations make small or isolated populations particularly prone to extirpation.

Management Considerations (61)
Fishers are capable of moving long distances, but movements may be restricted in landscapes with large nonforested openings. The maintenance of contact between individuals and subpopulations and the recolonization of unoccupied habitat may be facilitated by reducing the size of openings.

Fishers vs. Martens and other carnivores

… in the West martens tend to occur at higher elevations than fishers… sympatric in the southern Sierra Nevada, northern Idaho… (61)

Fishers have been reestablished in areas inhabited by foxes, coyotes, bobcats, and lynx, which suggests that competition with these other predators is not limiting to fisher populations. (62)