Throughout their range fishers prefer closed canopy habitats and avoid open areas. In a mixed-wood forest in central Alberta, open fields and stands with <50% total canopy were used less than expected based on availability (P<0.05). In addition, average canopy cover at 72 locations of 4 fishers was significantly (P<0.001) greater than at 70 random sites adjacent to fishers' home ranges [17]. In coniferous forests of north-central Idaho, hunting (P≤0.0001) and resting (P≤0.001) fishers selected dense (≥81%) and avoided open (≤20%) canopies. In addition, summer fisher locations had significantly (P≤0.0001) greater canopy cover (69.5%) than random sites (59.4%) [66]. Average canopy closure at 599 resting locations of 36 fishers in 2 California study areas was 93.4%, which was significantly (P<0.0033) greater than the 88.8% average canopy closure at random sites [140]. In the southern Sierra Nevada of California, stands with canopy closure of 60% to100% occupied the highest proportion of area (66.3%) within fisher home ranges. Females' (n=8) home ranges included more dense-canopy area (71.7%) than males' (55.6%, n=4). [141]. In predominantly open, upland hardwoods of upper peninsular Michigan, fishers used open areas significantly (P<0.001) less than available in the study area and in the vicinity of tracks [97]. In 75-year-old, even-age hardwood, mixed, and coniferous forests of northern New Hampshire, there was a significant (P<0.01) difference between the use of various canopy classes and their availability. Fishers avoided stands with 30% to 50% (observed=4, expected=18.5) and 51% to 80% cover (observed=13, expected=26.8) and preferred stands with 80% to 100% cover (observed=445, expected=416.7) [67].
Despite their preference for closed-canopy habitats, there are cases where fishers use open areas. For instance, in south-central Maine, natal dens occurred in areas with <50% cover (21.9%) more often than random points (5.0%). Although availability of cavities was not measured, the lack of use of this relatively open habitat at other times, and the stand history, suggested that cavities were comparatively abundant in this area. In addition, most dens were near areas of relatively greater cover [90]. Although recent clearcuts and alpine zones were typically avoided, pole/sawtimber stands with 10% to 49% canopy closure were used in accordance with availability by translocated fishers in coniferous forests of northwestern Montana [110]. In addition, clearcuts in 75-year-old, even-age hardwood, mixed, and coniferous forests of northern New Hampshire were used (observed=24) in proportion to availability (expected=24.3) in summer [67].
Fishers prefer structurally complex habitats. Although canopy diversity was not significantly (P=0.138) different at locations of translocated fishers and random sites in a mixed-wood forest in central Alberta, canopy diversity was 1 of 4 variables in a significant (P<0.001) function for discriminating fisher locations and random sites [17]. The lack of fisher hunting (observed=21%) and resting (observed=3%) sites in coniferous forests of north-central Idaho with only 1 canopy level suggests that structurally complex forests were preferred [66]. Multiple analyses of structural features at 599 resting locations of 36 fishers in 2 California study areas, one predominantly comprised of Douglas-fir, white fir, Oregon white oak, and tanoak stands and the other primarily Sierran mixed conifer, ponderosa pine, red fir, and montane hardwoods habitats, implied a preference for a range of sizes and types of structural elements, including greater variation in bole size than random [140]. Selectivity for specific cover of various shrub layers and high volumes of coarse woody debris suggests the importance of structural complexity to fishers in coniferous forest of south-central British Columbia [132].
Fishers generally prefer coniferous stands and avoid hardwood stands. In northern hardwood-eastern white pine-eastern hemlock (Pinus strobus-Tsuga canadensis) forests of north-central Massachusetts and southwestern New Hampshire, maternal dens were located more often in conifer and less often in hardwood canopy (P<0.001) when compared with the relative availability of these overstory types [101]. In predominantly open, upland hardwoods of upper peninsular Michigan, resting fishers avoided (P<0.05) northern hardwoods and selected lowland-conifer and other habitats [97]. In a northwestern Connecticut forest comprised of hardwoods, softwoods, and mixed woods, fishers used hardwoods as rest sites (62%) less than expected (73.1%, P<0.05) in winter [69]. In south-central Maine, aerial locations (n=782) of 43 fishers occurred in coniferous forest significantly (P<0.05) more than expected based on availability in fall, winter, and spring and occurred in deciduous forest significantly (P<0.05) less than expected all year [10]. In a south-central British Columbia forest dominated by Douglas-fir, lodgepole pine, and hybrid white spruce, transient fishers (n=15) used coniferous habitat (56%) more than expected (P<0.05) based on availability (32%) [131]. Fishers reintroduced to coniferous forests of northwestern Montana avoided "hardwoods" (P<0.01), preferring western redcedar/western hemlock (Thuja plicata/Tsuga heterophylla) (P<0.001) and mixed-conifer stands (P<0.01) [110].
Despite an apparent preference for conifers in many areas, there are exceptions to this trend. For instance, in a mixed-wood forest of central Alberta, stands with ≥50% total cover comprised of >75% deciduous species were preferred [17]; and in northwestern Connecticut fishers showed no apparent preference for coniferous habitats dominated by eastern hemlock and eastern white pine [69]. Although fishers preferred (P<0.01) western redcedar/western hemlock and mixed-conifer stands in northwestern Montana, subalpine fir stands were avoided (P<0.001) [110]. In some areas, fishers selected habitats with a hardwood component. For example, fishers in south-central British Columbia selected (P<0.05) stands with 21% to 40% deciduous cover and avoided stands with no deciduous component in the summer [132]. In 75-year-old, even-aged forests of northern New Hampshire, fishers occurred in habitats comprised of 51% to 74% coniferous species significantly (P<0.05) more than expected based on availability [67], and in northern Michigan fishers preferred stands with intermediate (14-76%) amounts of deciduous canopy cover [120]. There are several possible explanations for differences in fisher selection of cover types, including certain habitat types being in better condition in some areas than in others [17], prey availability and efficiency of capturing prey [97], and variation in availability of habitats across the fisher's range [140].
Fishers generally prefer stands with large trees. In a predominantly deciduous and mixed-wood forest in central Alberta, tree height averaged 20.7 feet (6.3 m), and tree diameter at breast height (DBH) averaged 2.6 inches (65.5 mm) at 72 locations of 4 translocated fishers. These values were significantly (P=0.027 and 0.041, respectively) greater than at 70 random sites (tree height=14.4 feet (4.4 m), tree DBH=1.7 inches (44.0 mm)) adjacent to fishers' home ranges [17]. In Douglas-fir forest of northwestern California, fishers were positively associated with the density of large (>35 inches (>90 cm) DBH) Douglas-fir trees [105]. Fishers in sugar maple (Acer saccharum), red maple (A. rubrum), and northern red oak (Quercus rubra) forests of the Ottawa Forest in northern Michigan appeared to prefer habitats with trees larger than 10.6 inches (27 cm) DBH [120]. Although fishers occurred (observed=56) in low (0-20 feet (0-6.1 m)) canopy forests significantly (P<0.01) more than expected (expected = 40.2) based on availability in 75-year-old, even-age hardwood, mixed, and coniferous forests of northern New Hampshire, this was most likely due to their strong preference for the wetland habitat type that was comprised entirely of this canopy class [67].
Understory characteristics and composition may be important to fishers in some areas. In a predominantly deciduous and mixed-wood forest in central Alberta, the density of woody stems was significantly (P=0.001) greater at 72 locations of four translocated fishers (25.3 stems/m²) than at 70 random sites (12.5 stems/m²) adjacent to fishers' home ranges. In addition, woody stem density was 1 of 4 variables in a significant (P<0.001) function for distinguishing fisher locations from random sites [17]. In an area of south-central British Columbia dominated by Douglas-fir, lodgepole pine, and hybrid white spruce, specific structural features of the of the forest floor were selected, including shrub layer and canopy closure, and high volumes of coarse woody debris. It is suggested that the complexity provides habitat for fisher prey species, and that too much cover near the ground could reduce hunting success, which may explain the avoidance of stands with >80% canopy closure of the low shrub layer [132]. In predominantly grand fir and subalpine cover types of north-central Idaho, fisher winter (11.2%) and summer (8.8%) use sites had significantly (P≤0.0004) greater understory coverage of Pacific yew (Taxus brevifolia) than random sites (3.6%) [66]. In northern hardwood-eastern white pine-eastern hemlock forests of north-central Massachusetts and southwestern New Hampshire, coniferous understory was used more and hardwood understory was used less than would be expected based on availability in the subunit (P<0.001) [101].
Due to their importance as Denning/resting habitat, density and size of snags and coarse woody debris are important characteristics of fisher habitat. In predominantly grand fir and subalpine fir cover types of north-central Idaho, there were significantly (P≤0.0002) more large (9.5-inch (24.1-cm) DBH) snags on fisher summer use sites, and 3 of 4 snag size classes had significantly (P=0.0001) greater densities on winter use sites than random sites. In addition, fisher summer use sites had greater densities of 3 of 5 log size classes (P<0.02), and winter use sites had significantly (P=0.034) greater densities (23.3 m³/ha) of large (>21 inches (54.6 cm) in diameter at small end) logs than random sites (10.7 m³/ha) [66]. In an area of south-central British Columbia dominated by Douglas-fir, lodgepole pine, and hybrid white spruce, fishers avoided stands with no coarse woody debris in both summer and winter, selected stands with >200 m³ of coarse woody debris/ha in the summer, and selected stands with >50 m³ of coarse woody debris with diameters >7.9 inches (20 cm) in winter [132]. In a mixed-wood forest in central Alberta, the diameter of downed logs was significantly (P=0.01) greater at 72 locations of 4 translocated fishers (28.1 mm) than at 70 random sites (19.2 mm) adjacent to fishers' home ranges [17].
Fishers select different habitats throughout the year. As detailed in the Denning/resting habitat section, fishers are more likely to use burrows and coarse woody debris for resting in winter than in summer [10,66,69,130]. In addition, open habitats may be used more often in summer. In 75-year-old, even-age hardwood, mixed, and coniferous forests of northern New Hampshire, clearcuts (observed=3, expected=9) were avoided (P<0.01) in winter, but were used (observed=24) in proportion to availability (expected=24.3) in summer [67]. In predominantly hardwood forest of northwestern Connecticut, hardwoods were used as resting sites (62%) less (P<0.05) than expected (73.1%) in winter but in accordance with availability in summer [69]. Similarly, in an area of south-central British Columbia dominated by Douglas-fir, lodgepole pine, and hybrid white spruce, nonforested stands were avoided in winter, but used in accordance with availability at the stand scale in summer and autumn [132]. Lack of cover [67], decreased snow interception [132], and thermoregulation [69] have been suggested as reasons for decreased use of deciduous habitats in winter. Use of successional classes varied seasonally in coniferous forests of north-central Idaho. Young forests were used more in winter (P≤0.0001), while mature forests were preferred (P=0.028) in summer. This seasonal change in habitat selection may have been related to changes in prey availability [66].
Habitat selection can also vary between sex and age classes. Males used conifers in proportion with availability, while females avoided conifers in mixed-wood forests of central Alberta [17]. In hardwood, mixed and coniferous forests in northwestern Connecticut, females used significantly (P<0.05) smaller cavity trees (x DBH=21 inches (54.3 cm)) than males (x DBH=29 inches (73.0 cm)) [69]. Females used snags as resting sites (31.7%) significantly (P<0.001) more than males (18%) in 2 study areas of California [140]. In New Hampshire, females selected sites with significantly (P<0.01) higher elevations (1,824 feet (556 m)) than males (1,657 feet (505 m)) [67], while in the southern Sierra Nevada females may select lower-elevation sites compared to males [141]. Females had stronger habitat preferences than males in mostly Douglas-fir and white fir forests of northwestern California [30] and in Sierran mixed-conifer, ponderosa pine, red fir, and montane hardwoods habitats of southern Sierra Nevada [141]. However, in New Hampshire females were only selective of cover types in winter, while males were selective year-round [67]. Juveniles were less selective than adults in predominantly Douglas-fir and white fir forests of northwestern California [30]. Many of these differences are likely due to biology. For instance, the relatively small females (See sexual dimorphism) can occupy smaller trees, and wider-roaming males are more likely detected in relatively rare habitats [17]. Some differences may reflect the comparatively larger males excluding females and juveniles from preferred habitat rather than fishers of different age and sex classes preferring different habitats [30].
Foraging habitat: Prey availability or diversity is an often suggested cause of fisher selection or avoidance of a given habitat [17,66,67,69,97,132]. For instance, one possible reason for the avoidance of coniferous forests in central Alberta was low prey densities due to the high degree of browsing pressure in these areas [17]. In south-central British Columbia, selection of structurally complex coniferous forests may have been related to prey availability and hunting success in these habitats [132]. In Maine [10], northwest Connecticut [69], and central Alberta [17], the diversity and availability of prey in hardwood and mixed forests may explanation the lack of a stronger associations with conifers. In addition, in north-central Idaho the change in preference from mature and old-growth coniferous forests in summer to young coniferous forests in winter may reflect a seasonal switch in prey species [66].
Fishers do not appear as selective of hunting and traveling habitat as they are of resting and denning habitat. In primarily grand fir and subalpine fir cover types of north-central Idaho, fishers used pole-sapling forests (observed=13%) significantly (P≤0.001) more when hunting than when resting (observed=0%). Hunting fishers were less selective of canopy cover classes than resting fishers. Hunting fishers occurred in stands with 20-80% cover as expected based on availability, while only stands with 40-63% cover were used in accordance with availability by resting fishers. Hunting fishers also used forests with only one canopy level more often (observed=21%) than resting fishers (observed=3%) [66]. In Maine, fishers rested in coniferous forest more than expected in summer, while active fishers used comparatively diverse habitats [10]. In 75-year-old, even-age hardwood, mixed, and coniferous forests of northern New Hampshire stationary fishers occurred in the mixed coniferous-hardwoods and wetland associations more often than expected based on availability of these types. However, occurrence of traveling fishers in these habitat types was similar to their availability [67]. In predominantly open, upland hardwoods of upper peninsular Michigan, cover types used by resting fishers and traveling fishers were significantly (P<0.01) different [97]. The fisher's diverse diet may explain the greater variability of hunting habitats [10,67,99].
Denning/resting habitat:
Although fishers rest in many structures, they typically use downed logs, snags, or living trees.
Fisher resting sites in the snow, slash piles, and underground have been reported in Maine [40], Ontario [42], and the upper peninsula of Michigan [97]. In a coniferous forest of south-central British Columbia, 2 of 32 resting locations were in slash piles [132] and 4 of 86 were either under rocks or in underground burrows [130]. In predominantly hardwood forests of northwestern Connecticut, burrows were not used in summer, but comprised 19% of winter resting sites [69]. In south-central Maine, burrows were used most often from December to February [10], and in Wisconsin 2 of 8 winter rest sites were underground [55].
Fishers rest in downed logs and other woody debris [40,42,97]. Four of 14 resting and denning sites in upland mixed hardwoods and upland conifer habitats of Wisconsin were in downed woody debris [55]. In a coniferous forest of south-central British Columbia, 4 of 32 resting sites were in individual pieces of coarse woody debris. The pieces of coarse woody debris fishers used averaged 31.6 inches (80.3 cm) in diameter, which was significantly (P=0.001) larger than the average size (9.2 inches (23.4 cm)) of coarse woody debris available in the patch [132]. In predominantly grand fir and subalpine fir cover types of north-central Idaho, median diameter of the small end of logs used by fishers for denning was 21 inches (53.3 cm), and the median length of these logs was 47 feet (14.3 m). Logs were used as denning sites significantly (P=0.0005) more often in winter (27%) than in summer (8%) [66]. In coniferous forests of central British Columbia, the average temperature when fishers used coarse woody debris for resting was significantly (P<0.05) colder ( x= -10.7 °C) than when they used branches (x=2.4 °C) or cavities (x=1.3 °C) [130].
Snags are used as fisher resting sites. In predominantly grand fir and subalpine fir cover types of north-central Idaho, 8% of summer and 6.7% of winter resting sites were in snags. Snags used had a median DBH of 34 inches (86.4 cm) and a median height of 39 feet (11.9 m). Of the 13 snags used, 12 were grand fir and 1 was Douglas-fir. Three of the snags were young, 9 were soft and had sloughing bark, and 1 had no branches or bark [66]. In 2 study areas in California, female fishers used snags (31.7%) significantly (P<0.001) more than males (18.0%). The conifer snags used as resting sites had an average DBH of 47.2 inches (119.8 cm) [140].
Fishers rest on tree branches, in bird (Aves) and squirrel (Sciuridae) nests, on witches' brooms (Arceuthobium spp.), and in tree cavities. Fifty-seven percent of fishers' resting locations in coniferous forest of British Columbia were on branches [130]. In south-central Maine, open nests and tree branches were used in 70% of spring (March-May) and 94% of summer (June-August) resting sites [10]. In predominantly hardwood forests of northwestern Connecticut, fishers selected (P<0.05) nests as rest sites in summer (82%) and used them in accordance with availability in winter (30%). Twenty-one percent was crow (Corvus spp.) or raptor (Falconiformes or Strigiformes) nests, and the remainder was squirrel leaf-nests. Nests used for resting occurred at an average height of 35.4 feet (10.8 m) in trees with a mean DBH of 15.2 inches (38.7 cm) [69]. In coniferous forest of north-central Idaho, fishers rested on witches' brooms in 67.9% of live-tree resting sites. In addition, 91% of Engelmann spruce (Picea engelmannii,) and 56% of grand fir used for resting contained witches' brooms [66]. In coniferous forest of south-central British Columbia, hybrid white spruce used by fishers had an average of 3.2 witches' brooms, while hybrid spruce not used for resting had an average of 0.2 witches' brooms (P<0.001) [132]. In hardwood, mixed-woods, and coniferous forests of northwestern Connecticut, 18% of summer and 51% of winter rest sites were in tree cavities. The average height of cavities used by fishers was 17.4 feet (5.3 m) and the mean DBH of cavity trees was 23 inches (58.4 cm). Most (95%) of the cavity trees were hardwoods [69]. In south-central Maine, 34% of fall (September-November), 19% of winter (December-February), 19% of spring (March-May), and 6% of summer (June-August) resting sites were in cavities [10]. In coniferous forest of central British Columbia, 21% of resting sites were in cavities [130].
Fishers use relatively large individuals of several tree species as resting sites. In predominantly grand fir and subalpine fir cover types of north-central Idaho, 83.9% of temporary summer dens and 66.7% of temporary winter dens were in live trees, typically Engelmann spruce (63.4%) or grand fir (32.1%). Dens occurred at an average height of 54 feet (16.4 m) in trees with an average DBH of 22 inches (56.1 cm) [66]. California black oak (Quercus kelloggii) comprised 37.5% of rest sites in a southern Sierra Nevada study site, and Douglas-fir was used in 65.6% of resting locations in a northwestern California study area. The average maximum DBH of trees used by 21 resting fishers was 51.7 inches (131.2 cm), which was significantly (P<0.05) larger than the maximum DBH (43.7 inches (111.0 cm)) for random trees [140]. In south-central British Columbia the hybrid white spruce (x DBH=18.2 inches (46.3 cm)), black cottonwood (Populus balsamifera subsp. trichocarpa, x DBH=40.6 inches (103.2 cm)), and Douglas-fir (x DBH=43.7 inches (111.0 cm)) used by fishers for resting were significantly (P≤0.05) larger than the average size of these trees (x DBH=12.6-24.4 inches (32.1-62.1 cm)) in the vicinity of fisher locations [132]. In Maine, 2 fisher dens were in yellow birch (Betula alleghaniensis), and 1 was in a sugar maple [40].
Reuse of resting sites by fishers is uncommon. Percentages of reused resting sites were 13.8% in the southern Sierra Nevada and 3.5% in northwestern California [140]. In predominantly hardwood forests of northwestern Connecticut, 10% of summer sites and 24% of winter sites were reused [69]. Resting sites were rarely reused in south-central Maine [10].
Cavities are generally used for whelping litters and rearing young kits. These maternal dens occur in large live trees and snags of various species. Maternal dens in north-central Massachusetts and southwestern New Hampshire (n=56) [101], Maine (n=33) [90], and south-central British Columbia (n=5) [132] were all in cavities. However, a natal den in a hollow log was observed in northwestern Montana [110]. In Massachusetts and New Hampshire, 60% of maternal dens occurred in eastern white pine or eastern hemlock. Several hardwood species were used including northern red oak, red maple, sugar maple, and American beech (Fagus grandifolia). Species were used in accordance with availability, while snags were used (19%) significantly (P<0.01) more than would be expected based on availability (2%). The average DBH of maternal den trees (x=26 inches (66 cm)) was significantly (P<0.01) larger than the average DBH available (>90% were 13.4-23.2 inches (34-59 cm)) [101]. In south-central Maine, 95% of maternal dens were in hardwoods, primarily balsam poplar (P. balsamifera) and bigtooth aspen (P. grandidentata). Half of dens were in snags, 30% were in partially dead trees, and 20% were in living trees. The DBH of maternal den trees ranged from 9.8 to 36.2 inches (25-92 cm) [90]. In south-central British Columbia, the average DBH (40.6 inches (103.1 cm)) of the declining black cottonwood trees used as natal dens was significantly (P=0.025) larger than the average DBH (x=20.7 inches (52.5 cm)) of these trees in the vicinity of fisher locations [132]. Average height of cavities used for maternal dens was 20.6 feet (6.28 m) in north-central Massachusetts and southwestern New Hampshire [101] and ranged from 3 to 39 feet (0.9-12 m) in south-central Maine. In south-central Maine, cavities faced south (x=171° southeast) significantly (P=0.048) more than would be expected at random [90], while in north-central Massachusetts and southwestern New Hampshire fishers did not appear to select (P>0.05) maternal dens with openings of a particular aspect [101].
According to reviews, the fisher occurs from southern Yukon and southwestern Northwest Territories southeast through British Columbia and possibly extreme southeastern Alaska, Alberta, Saskatchewan, Manitoba, Ontario, southern Quebec, and New Brunswick to Nova Scotia. Its distribution extends south through several forested areas of the northeastern United States including Maine, New Hampshire, Vermont, northern New York, Pennsylvania, western Massachusetts, the upper peninsula of Michigan, and northern Wisconsin and Minnesota. There is also a population in West Virginia. In the western United States, fisher populations are known to occur in western Montana, the Idaho panhandle [96,98,99], the southern Sierra Nevada of California, the Klamath and Siskiyou mountains of northwestern California and extreme southwestern Oregon, and the southern Cascade Range of southwestern Oregon [14,139]. The fisher may be extirpated from Washington [14,121]. For more detailed summaries of the fisher's historic and current distribution, see these sources: [96,98,99]. For a map of the fisher's current and historic distribution, search the National Museum of Natural History's Mammal Species of the World website for fisher.
The following lists are speculative. They are based on distribution information reported no earlier than 1993 and the habitat characteristics and species composition of communities fishers are known to occupy. There is not conclusive evidence that fishers occur in all the habitat types listed, and some community types may have been omitted, especially in areas where fishers have not been recently documented. In addition, the following cover types provide habitat of varying quality for fishers. See Preferred Habitat for more detail.
Fishers are generalist predators. Their diets are comprised predominantly of snowshoe hares, North American porcupines (Erethizon dorsatum), small mammals, birds, carrion, and/or plants. Snowshoe hares are important prey throughout much of the fisher's range [40,110,133]. Snowshoe hares occurred in 39% of 215 fisher stomachs collected in British Columbia in winter [133] and 28% of 242 fisher digestive tracts collected in Maine from September to April [40]. North American porcupines are another often preyed-upon species [10,100,133]. North American porcupines occurred in 21% of 69 fisher scats collected Maine in winter [10] and 19.5% of 215 stomachs collected in British Columbia in winter [133]. Fishers prey on several small mammals including squirrels, shrews (Soricidae), voles, and mice (Muridae) [67,100,110,127,137]. Of 215 stomachs collected in British Columbia from November to February, 33.5% contained red squirrels (Tamiasciurus hudsonicus), 14.9% contained shrews, 23.3% contained southern red-backed voles (Clethrionomys gapperi), and 15.8% contained deer mice (Peromyscus maniculatus) [133]. Squirrel remains were found in 20.4% of 201 scats collected year-round in the southern Sierra Nevada and included western gray squirrels (Sciurus griseus), Douglas's squirrels (Tamiasciurus douglasii), and northern flying squirrels (Glaucomys sabrinus) [137]. Birds found in stomach contents or scats include Passeriformes [127], Galliformes [10,127,133], and Falconiformes [100]. Fishers also eat carrion, typically from ungulates such as deer (Odocoileus spp.) [10,100,127,133,137] and moose (Alces alces) [133]. American martens and fishers occur in fisher diets [66,133]. It not certain whether the fishers were preyed on or scavenged [133]. Common muskrats (Ondatra zibethicus) [10,127,133], northern raccoons (Procyon lotor) [10,100,127], reptiles (Reptilia), insects (Insecta), [127,137], and several genera of fungi [137] have been found in fisher scats or stomach contents. Fishers also eat plant material, including apples (Malus spp.) [10,127], common winterberries (Ilex verticillata) [10,100], manzanitas (Arctostaphylos spp.), currents (Ribes spp.) [137], black cherries (Prunus serotina), serviceberries (Amelanchier spp.), and blueberries (Vaccinium spp.) [100]. Willson [134] reviews fruits eaten by mammals including the fisher.
In some areas there are differences in diet between age classes and gender. For instance, in Massachusetts and New Hampshire juvenile fishers ate significantly more northern raccoons (P=0.002) than adult fishers, and females ate significantly (P=0.006) more eastern gray squirrels (Sciurus carolinensis) than males [100]. The stomachs of female fishers collected in British Columbia had significantly (P≤0.04) higher occurrence of small mammal remains than males [133], and in New Hampshire significantly (P<0.01) more males contained North American porcupine quills [67]. However, no significant differences in diet were observed between the sexes in Maine (P>0.1) [40] or Vermont (P=0.883). Adults and juveniles also had similar diets in Vermont (P=0.836) [127].
Fishers are likely capable of changing diets with changing availably of prey. For instance, in Massachusetts and New Hampshire fisher diets changed with the seasons. Fruits were common in summer and early autumn, while northern raccoons and eastern gray squirrels were common prey in winter. In addition, birds were not detected in the diets of fishers in the early autumn or winter [100]. There was a significant (P<0.1) shift in diet composition from autumn to spring in Maine, with snowshoe hare and white-tailed deer (Odocoileus virginianus) increasing and shrews decreasing in frequency as the winter progressed [40]. In Ontario, fishers displayed a delayed positive response to lagomorph (Lepus americanus and Sylvilagus floridanus) abundance, but appeared to use alternate prey during a period of low lagomorph abundance [27]. In northern Minnesota, predation of small mammals was negatively associated with predation of snowshoe hares for male and female fishers (P<0.02), suggesting the importance of small mammals during snowshoe hare population declines [74].
Reviews of fisher diet [83,99], hunting behavior [96,99], and ecological energetics [96] are available.
Little research is available on the effects of fire on fishers. The available information is mainly anecdotal and does not include varying temporal and spatial scales. Few data address the effects of various site and fire characteristics on fishers. As of 2007 few studies include estimates of fisher demographic parameters, and there are no comparisons between burned and unburned areas. Due to these limitations, the majority of the information that follows is speculation based on fishers' habitat and cover requirements.
Fishers are rarely observed in burned areas, and negative initial impacts of fire on fishers are widely reported. For instance, several reviews state that fishers are adversely affected by fire [64,75,109,112]. Other reviews note the lack of literature documenting fishers in recently (<25 years) burned areas [48] and the lack of habitat characteristics required by fishers in most early successional stands [68,81,109,129]. However, trappers occasionally observed fishers in burned areas of Ontario [42], and a review [99] states that "fisher populations should tolerate" severe disturbances such as stand-replacing fires as long as late-successional conifer habitat is nearby. Another review notes that American martens have been observed in high densities in postfire communities with complex structures, such as horizontal boles or dense herbaceous cover [102]. However, in a 21-year-old burn in the taiga of Mackenzie Valley, Northwest Territories, American martens had comparatively large home ranges, and occurred in unburned areas more and burned areas less than expected based on availability [76]. In addition, American marten demography in a burned stand of Alaskan taiga suggests the burn was a population sink [91].
Due to the relatively generalized nature of their foraging habitat (see Foraging habitat) and their varied diet (see Food Habits), fishers are likely more sensitive to effects of fire in resting habitat than in foraging areas. For instance, fire and/or mechanical harvesting at Blodgett Forest Research Station in the Sierra Nevada had significant impacts on the suitability of fisher resting habitat (P=0.024), but did not significantly (P=0.468) affect fisher foraging habitat suitability. However, early and late-season fires in Sequoia-Kings Canyon National Park, California, had marginally significant (0.06≤P>0.05) effects on both resting and foraging habitat [123]. Reviews and anecdotal reports suggest the open areas created by fire may provide improved fisher foraging habitat due to the increased vulnerability of fisher prey, as well as greater abundance of prey species and their forage in these areas [2,42,79,84,102,136]. The greater abundance of American marten in recent (<10 years old) burns compared to older stands of Alaskan taiga may have been due to the abundance and stability of small prey in the recently burned areas [91]. However, Pilliod and others [95] summarize cases where small mammal populations declined following thinning treatments, including prescribed fire. FEIS reviews of several fisher prey species, including snowshoe hare, red squirrel, northern raccoon, common muskrat, and deer mouse are available.
Fire consumes and alters snags and coarse woody debris used by fishers for denning and resting. In ponderosa pine forest of Arizona, volume of coarse woody debris was lower on recently (≤4 years) burned sites (2.99-4.85 Mg/ha) than on somewhat older (8-9 years) sites (5.85-10.03 Mg/ha) [92]. In a Sierran mixed-conifer forest, thin-and-burn and burn-only treatments resulted in a decline in the amount of fine and coarse debris, a reduction in the size of woody debris, and a shift to less decayed woody debris [65]. Almost half of ponderosa pine snags with DBH >6 inches (15 cm) were seriously damaged or destroyed after moderately severe surface fires in southeastern Arizona [63]. In an Illinois study where the majority of snags were oak (Quercus spp.) or elm (Ulmus spp.), burned areas had lower snag densities (P<0.2), and there was a negative relationship between density of snags and frequency and severity of prescribed fire. However, burned sites contained more snags rated as "excellent" (37%) than unburned sites (21%) [44]. Vulnerability of existing snags and coarse woody debris to fire depends on several factors including characteristics of the site, the fire, and of the snags and coarse woody debris. For instance, size and age/decay class can influence the vulnerability of a snag [44,63,77,111]. In addition, reviews note that charring of snags and coarse woody debris can alter their value to wildlife [28,95]. The suitability of burned snags and coarse woody debris as fisher resting and denning sites is not addressed in the literature.
Fire also creates new snags and coarse woody debris in the years following the fire. Amount of coarse woody debris was greatest in ponderosa pine forests of Arizona that burned by wildfire about 8 years previously [92], while studies of mostly pine (Pinus spp.) trees in the western United States found the majority of tree mortality occurred within 5 years of fire [63,77,118]. In at least some cases, snag recruitment can be low [111], and fire-created snags may be comparatively small [63,65] or short lived [77,92]. Longevity of snags created by fire is addressed in these sources: [32,92,111]. Recruitment of large snags is dependent on the retention of large trees [65] and, according to a review, "loss of larger snags could take decades to recover" [95].
Fisher may also be affected, at least in the short term, by the reduction in canopy cover due to fire. In the Sierra Nevada, fire-treated areas had significantly lower canopy cover in Sequoia-Kings Canyon National Park (P<0.0001) than unburned sites, and fire and/or mechanical harvest treatments had significantly (P=0.0086) decreased canopy cover at Blodgett Forest Research Station compared to unburned sites. Despite the significant effects of fire on canopy cover, of the 3 fire-only treatments, only the late-season burn resulted in significant (P≤0.05) impacts on fisher habitat suitability [123]. In addition, the effect of fire on canopy cover is likely short lived [123,140]. A review suggests that even in the absence of canopy and understory cover, the large amounts of coarse woody debris present in some recently burned areas may provide the cover necessary for fishers [35]. However, Native Americans of northern Alberta did not burn mature forests due to the absence of fisher and other fur-bearers in sites with no canopy [79]. According to a review, differences between unburned and burned sites such as decreased canopy cover can result in altered snow characteristics [81].
The extent to which a fire would affect fishers would likely depend on several factors including fire size, frequency, timing, uniformity, and severity. Substantial impacts of fire on fishers are not likely unless prescribed burns are implemented on a large scale or wildfires cover fairly large areas. Most prescribed burns, and even small wildfires, only affect a fraction of a fisher's home range [84,95]. In a study of the effect of fire on fisher habitat suitability in the central Sierra Nevada , the largest treatment units were 74 acres (30 ha) [123]. This is <6% of the average female fisher home range size in the southern Sierra Nevada [141], where some of the smallest home ranges are reported (see Home range/density). Conversely large wildfires could have substantial impacts on fishers within a burned area due to loss of resting structures, decreases in cover, and reduced habitat connectivity (see Landscape/scale effects).
Frequent fires are likely to have negative impacts on fisher habitats. Changes to habitat structure and stand composition due to relatively short fire intervals are likely detrimental (see Preferred Habitat and Cover Requirements). For instance, in Washington and Oregon conversion from grand fir forests to drier, more open ponderosa pine stands would probably decrease habitat suitability [122]. Decreasing the frequency of prescribed burns could reduce impacts on individual fishers [123] and may provide temporally continuous foraging habitats [84].
Timing of the fire will probably influence the degree to which fishers are affected. Although early season fires may have greater detrimental direct impacts on fishers (see DIRECT FIRE EFFECTS ON ANIMAL) than fires outside the denning period, they may have less impact on fisher habitat. For instance, late-season fires resulted in greater reductions in canopy cover and fisher habitat suitability than early season burns in Sequoia-Kings Canyon National Park [123]. Also, a review [84] notes that early season fires generally have fewer detrimental effects on wildlife habitat than late-season fires. In addition, burning in cool, wet conditions has been recommended to reduce fire damage to snags [44].
Patchy fires will likely have less of an effect on fishers in the short term, and may benefit fishers in the long term. Since fishers can be isolated by relatively small expanses of an unsuitable environment (see Landscape/scale effects), areas that burned at low severity or were not burned in a patchy fire may allow for greater connectivity across a landscape. In addition, in at least some portions of their range fishers may be less impacted or benefit from patchy burns that produce diverse habitats with high interspersion (see Landscape/scale effects). The vegetation mosaic created by the 1910 wildfires in north-central Idaho provided favorable American marten habitat [70], and a review states that fishers are dependent on fire-created mosaics due to the reliance of their herbivore prey on open foraging areas [136]. Arranging burns across the landscape could reduce impacts on individual fishers [123], improve the spatial availability of foraging habitat [84], and, if travel corridors such as riparian habitats are left unburned, provide connectivity across the landscape [84,106].
More severe fires probably have greater negative impacts on fishers than less severe fires. In Idaho, the young forest that fishers selected in winter retained several structures after fire, including large-diameter trees, snags, and logs [66]. According to a review, small, low-severity fires in which some of the canopy remains may not be detrimental to American martens [102]. Reduced loss of canopy cover, coarse woody debris, and snags also suggests that low-severity fires would have less of an impact on fishers.
Fire Ecology: Fishers typically occur in forests with moderate to long fire-return intervals. According to reviews, most fisher habitats have mixed-severity or stand-replacement fire regimes, with the majority of fires occurring in summer [5,45,128]. Over time, environments with mixed-severity FIRE REGIMES may have more coarse woody debris than those with low- or high-severity regimes [2]. Fishers in south-central British Columbia occur in a habitat where large-scale fires burn most stands about every 125 years [132]. Information on the fire ecology of areas occupied by fishers, including the boreal forests of Boundary Waters Canoe Area, Minnesota [61], and the Sierra Nevada [126] is available.
The following table provides fire-return intervals for plant communities and ecosystems where fishers occur. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire-Return Interval Range (years) silver fir-Douglas-fir Abies amabilis-Pseudotsuga menziesii var. menziesii >200 grand fir Abies grandis 35-200 [5] maple-beech Acer-Fagus spp. 684-1,385 [39,128] maple-beech-birch Acer-Fagus-Betula spp. >1,000 silver maple-American elm Acer saccharinum-Ulmus americana <5 to 200 sugar maple Acer saccharum >1,000 sugar maple-basswood Acer saccharum-Tilia americana >1,000 [128] birch Betula spp. 80-230 [117] Atlantic white-cedar Chamaecyparis thyoides 35 to >200 beech-sugar maple Fagus spp.-Acer saccharum >1,000 black ash Fraxinus nigra 128] tamarack Larix laricina 35-200 [93] western larch Larix occidentalis 25-350 [6,21,41] yellow-poplar Liriodendron tulipifera <35 [128] Great Lakes spruce-fir Picea-Abies spp. 35 to >200 northeastern spruce-fir Picea-Abies spp. 35-200 [45] southeastern spruce-fir Picea-Abies spp. 35 to >200 [128] Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to >200 [5] black spruce Picea mariana 35-200 conifer bog* Picea mariana-Larix laricina 35-200 red spruce* Picea rubens 35-200 [45] whitebark pine* Pinus albicaulis 50-200 [1,4] jack pine Pinus banksiana 39,45] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-340 [20,21,119] Sierra lodgepole pine* Pinus contorta var. murrayana 35-200 Jeffrey pine Pinus jeffreyi 5-30 western white pine* Pinus monticola 50-200 Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47 [5] interior ponderosa pine* Pinus ponderosa var. scopulorum 2-30 [5,18,78] red pine (Great Lakes region) Pinus resinosa 3-18 (x=3-10) [38,49] red-white pine* (Great Lakes region) Pinus resinosa-P. strobus 3-200 [39,60,80] pitch pine Pinus rigida 6-25 [29,62] pocosin Pinus serotina 3-8 eastern white pine Pinus strobus 35-200 eastern white pine-eastern hemlock Pinus strobus-Tsuga canadensis 35-200 eastern white pine-northern red oak-red maple Pinus strobus-Quercus rubra-Acer rubrum 35-200 [128] aspen-birch Populus tremuloides-Betula papyrifera 35-200 [45,128] quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [5,57,86] black cherry-sugar maple Prunus serotina-Acer saccharum >1,000 [128] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [5,7,8] coastal Douglas-fir* Pseudotsuga menziesii var. menziesii 40-240 [5,87,107] Pacific coast mixed evergreen Pseudotsuga menziesii var. menziesii-Lithocarpus densiflorus-Arbutus menziesii <35-130 [5,37] California oakwoods Quercus spp. <35 [5] oak-hickory Quercus-Carya spp. <35 northeastern oak-pine Quercus-Pinus spp. 10 to <35 [128] white oak-black oak-northern red oak Quercus alba-Q. velutina-Q. rubra <35 [128] Oregon white oak Quercus garryana <35 [5] California black oak Quercus kelloggii 5-30 [93] northern red oak Quercus rubra 10 to <35 [128] redwood Sequoia sempervirens 5-200 [5,47,116] western redcedar-western hemlock Thuja plicata-Tsuga heterophylla >200 [5] eastern hemlock-yellow birch Tsuga canadensis-Betula alleghaniensis 100-240 [117,128] eastern hemlock-white pine Tsuga canadensis-Pinus strobus x=47 [39] western hemlock-Sitka spruce Tsuga heterophylla-Picea sitchensis >200 mountain hemlock* Tsuga mertensiana 35 to >200 [5] *fire-return interval varies widely; trends in variation are noted in the species reviewStatus: According to reviews, fishers are apparently recovering in much of the northeastern United States but remain vulnerable in the West [24,99,121]. A 2003 distribution of the fisher in the Pacific states is provided by Aubry and Lewis [14].
Trapping: According to reviews, fishers are quite vulnerable to trapping [99,102]. In addition to possible population declines [67,102], trapping may also change age distributions and sex ratios [53,71]. Recommendations for maintaining stable populations include closing trapping seasons for other furbearers as well as fishers and closely regulating and monitoring trapped fisher populations [22,99]. The history, methods, and effects of fisher trapping are summarized in [24,99,102].
Michael Schwartz, Rocky Mountain Research Station.Timber harvesting: Forest management practices recommended to mitigate effects on fisher are summarized in several reviews and include minimizing the size [64,66,99,102] and extending the rotation [3,94] of shelterwood and clearcuts, increasing interspersion of mature and harvested areas, [3,10], avoiding harvests in riparian areas [3,64,106], using uneven-aged techniques [3,30,66,99,102], conserving and minimizing damage to snags and live denning trees [3,28,31,90,101,106], cutting cavity trees or snags outside of the March to June denning period [90], and retaining coarse woody debris [3,31,64,66,102,106]. In addition, salvage logging after fires in western forests is discouraged [34], as is planting sites with ponderosa pine in northwestern California, since it results in a dry vegetation community that may not provide quality fisher habitat [30].
Information on denning tree [28,32,33,34,43,124] and coarse woody debris management [32,33,114] is available.Fishers are associated with areas of high cover and structural complexity (see Cover Requirements) in large tracts (see Landscape/scale effects) of mature and old-growth forests. Other site characteristics that can be important include presence of nearby water, slope, elevation, and snow characteristics.
Much of the information regarding fisher habitat is based on occurrence of fishers compared to availability. It is important to note that preference does not necessarily equate to superior habitat [30]. As a measure of preference, time spent in available habitat assumes that individuals have equal access to available habitats, which is unlikely (review by [35]). In addition, Powell [97] describes the case where foraging techniques for various prey have different efficiencies, resulting in relatively less time spent in the habitat of the prey associated with the more efficient foraging technique.
Fishers generally avoid early and/or prefer late successional stages, but in some cases they use fairly young forests extensively. For example, in predominantly grand fir (Abies grandis) and subalpine fir (Abies lasiocarpa) forests of north-central Idaho, stands subjectively categorized as pole-sapling age or younger were rarely used in summer or winter. In summer, mature forest and old-growth were preferred, but in winter young grand fir forests were preferred [66]. In Douglas-fir, lodgepole pine, and hybrid white spruce forests of south-central British Columbia, early seral stands (<10 years) were used by transient fishers significantly (P<0.05) less than expected, and young (41-80 years) forests were used more than expected based on availability in the landscape [131]. In northwestern California, fishers were associated with old-growth Douglas-fir forest [108]. However, use of varying succession classes by fishers in conifer stands of northwestern Montana was in accordance with availability [110].
In some locations, fishers prefer areas near water. In a relatively hot and dry California study area comprised primarily of Sierra mixed conifer, ponderosa pine (Pinus ponderosa), red fir (Abies magnifica), and montane hardwood habitats, there were significantly (P<0.05) more resting sites (51.9%) than random sites (27.8%) within 330 feet (100 m) of water [140]. In coniferous forests of north-central Idaho, several factors demonstrated the fisher's preference for riparian areas. Summer fisher locations were significantly (P<0.0001) closer (223 feet (68.0 m)) to water than random sites (400 feet (121.9 m)). In addition, fishers preferred grand fir/arrowleaf ragwort (Abies grandis/Senecio triangularis) riparian habitat in both summer and winter [66]. Similarly, fisher rest sites in northwestern Connecticut occurred in forest wetland habitat significantly (P<0.05) more often than expected (17%) in both winter (39%) and summer (29%) [69].
In some areas, fishers prefer comparatively steep slopes. In a mixed-wood forest of central Alberta, the average slope of fisher locations (5.8°) was significantly (P=0.002) greater than the average slope of random locations (1.9°) [17]. The average slope at resting sites (49.8%) in 2 California study areas, one predominantly comprised of Douglas-fir, white fir (Abies concolor), Oregon white oak (Quercus garryana), and tanoak (Lithocarpus densiflorus) stands and the other primarily Sierran mixed conifer, ponderosa pine, red fir, and montane hardwoods habitats, was significantly (P<0.05) steeper than random sites (42.6%) [140]. In coniferous forests of north-central Idaho, level slopes and benches were used (38%) much less than expected based on availability (71%) [66].
Fishers occur in a wide range of elevations, but generally prefer relatively low-elevation sites. In Maine, fishers were studied in an area that ranged from 0 to 1,210 feet (0-370 m) in elevation [10]. In the southern Sierra Nevada, fishers have been reported as high as 8,000 feet (2,438 m) [26]. In 75-year-old, even-age hardwood, mixed, and coniferous forests of White Mountain National Forest and surrounding areas of New Hampshire, use of varying elevations differed significantly (P<0.01) from expected. Based on availability fishers were expected to occur on sites from 1,100 to 2,000 feet (329-607 m) in elevation 264.7 times, and were expected on sites above 2,000 feet (≥608 m) 197.3 times. However, they preferred the low sites, occurring 363 and 99 times on low and high sites, respectively [67]. West of the Cascade Range crest in Washington, 87% of sighting and trapping records from 1894 to 1991 were from <3,300 feet (1,000 m), and none were from elevations >5,900 feet (1,800 m). However, east of the crest 70% of records were from sites >3,300 feet (1,000 m) and 18% were from sites 5,900 to 7,200 feet (1,800-2,200 m) in elevation. [13].
Fishers are likely affected by snow characteristics such as depth and consistency. Ninety-nine percent of the area in California where fishers were detected but American martens (Martes americana) were not was in the <5-inch (<13 cm) average snowfall zone; 1% was in the 5- to 9-inch (13-23 cm) average snowfall zone; and none was in the >9-inch (>23 cm) average snowfall zone [72]. In the low boreal region of southeastern Manitoba, significantly (P<0.005) fewer fisher tracks were observed during midwinter, when deep, soft snow conditions prevailed. Method of travel was also affected by snow conditions, with fishers walking in midwinter instead of bounding or galloping, and traveling on snowshoe hare (Lepus americanus) and their own trails more than they did during the thin snow cover of early winter and the crust conditions of late winter [104]. In addition, the stronger selection for low elevations west of the Cascade crest compared to the east (see previous paragraph) could have been due to the rain shadow, resulting in less snow on the east side compared to similar elevations on the west side [13]. However, in predominantly grand fir and subalpine fir cover types of north-central Idaho, fishers did not appear influenced by snow characteristics. Use of various elevations did not change over the seasons, and fishers preferred young forests in winter: a rather open habitat with large amounts of deciduous shrubs [66]. In addition, in an area of northwestern Connecticut with maximum snow depths less than 10 inches (<25 cm), there was no evidence that snow affected the fisher's habitat preferences [69].
Home range/density: Fisher home range size and density exhibit substantial variation, although male home ranges are larger than those of females. Small average female home range sizes (1,300 acres (527.5 ha), n=8) occurred in the southern Sierra Nevada [141]. Home ranges in the hardwoods and mixed-woods of southern Quebec were also comparatively small, with female (n=7) home ranges averaging 5.4 km² and male (n=3) home ranges averaging 9.2 km². Based on these estimates fishers occurred at a density of 2.7 fishers/10 km², while estimates from tracking and radio-collaring resulted in density estimates of 3.0 fishers/10 km² [53]. In conifer and mixed forests of south-central Maine, the median home range was 12.2 km² for females (n=5) and 25.5 km² for males (n=6). Density estimates in this area were 1 fisher/2.8-10.5 km² in the summer and 1 fisher/8.3-20.0 km² in winter [11]. Male home ranges in 2 California study areas, one predominantly comprised of Douglas-fir, white fir, Oregon white oak, and tanoak stands and the other primarily Sierran mixed conifer, ponderosa pine, red fir, and montane hardwoods habitats, were significantly (P<0.0001) larger (9,700 acres (3,934.5 ha), n=6) than female home ranges (2,400 acres (980.5 ha), n=15) [141]. In conifer forests of Idaho, fishers had very large home ranges. Average "year-long" estimates of home range size were 40.8 km² and 82.6 km² for females and males, respectively [66]. Powell and Zielinski [99] provide a thorough review of home ranges throughout the fisher's distribution, including notes on the methods used for calculation.
Landscape/scale effects: Fishers require large, well-connected habitat patches. In addition to their large home ranges, fishers can apparently be excluded from nearby habitat by relatively small expanses, possibly as short as 6 to 12 miles (10-20 km), of unsuitable environment [12,16]. In Douglas-fir forests of northwestern California, fishers were sensitive to habitat fragmentation. Fishers were detected in 70% of stands where <10% of the perimeter was clearcut edge and in less than 20% of stands where >75% of the stand's perimeter was clearcut edge. Fisher detections decreased markedly in stands <250 acres (100 ha) in area [108]. In mixed-wood forest of central Alberta, fishers used continuous (>2 km²) forest blocks significantly (P<0.01) more than expected based on availability, and the woodlots (independent forested blocks ≤2 km²) used were significantly (P<0.01) larger (x=0.4 km²) than random woodlots (x=0.18 km²) [17].
Although edges between fisher habitats and open areas are apparently avoided [108], edges between various types of fisher habitat are often used. In 75-year-old, even-aged hardwood, mixed, and coniferous forests of the White Mountain National Forest, fishers occurred within 98 feet (30 m) of an edge (change in species composition, height or density class) significantly (P<0.01) more often than expected (observed=60, expected=42.5) in winter. The trend was the same in summer (observed=130, expected=114.6), but was not significant (P≥0.05) [67]. In a study in Douglas-fir, lodgepole pine, and hybrid white spruce forests of south-central British Columbia, Weir and Harestad [131] concluded that the fine-grained nature of the early and later seral stages provided sustainable fisher habitat. Likewise, fisher habitat use in central Maine suggested that areas with high interspersion of many forest types provided optimal habitat [10]. In addition, a review states that diverse forest communities would better conserve mustelids than a homogenous mature forest due to the variability of mustelid responses to conditions such as snow accumulations, microclimate conditions, prey availability, and predator densities [102].
Fishers select habitat on several scales. In predominantly upland hardwood habitat of upper peninsular Michigan, fishers used mixed pine (P. banksiana, P. resinosa, and P. strobus) habitat in accordance with availability at a fine scale (the area around fisher tracks), but used it more than expected based on its availability at a coarser scale (the general study area). Fishers selected (P<0.001) dense, lowland forests as rest sites at both these scales [97]. In forests dominated by Douglas-fir, lodgepole pine, and hybrid white spruce in south-central British Columbia, fishers displayed selectivity at the stand, patch, and element (resting, maternal den, and natal den habitat) scales [132]. In Douglas-fir forests of northwestern California, fishers were sensitive to fragmentation at the plot, stand, and 2,500-acre (1,000-ha) block scales [108].
Reviews state that the fisher breeding cycle begins with mating starting in March. For females this is a few days after giving birth, since fishers exhibit delayed implantation lasting about 10 months. Active pregnancy typically begins in February and lasts until March or early April, when fishers give birth to an average of 2 to 3 kits [85,96,99]. Captive females trapped in Maine had an average initial litter size of 2.7 kits, with litters ranging from 1 to 4 kits [84]. Several years of observations showed the average litter size of 12 females in coastal Maine varied from 2.0 to 2.2 kits. Females gave birth from 3 March to 1 April [89]. In north-central Massachusetts and southwestern New Hampshire, the birth of 20 litters occurred from 4 March to 24 March with the median on 17 March [101]. According to reviews, males are reproductive beginning in early March [85,99]. During this time they often leave their home ranges in search of mates [11,16,17]. Fishers are likely polygamous [40].
Females den with their altricial kits for several weeks [40]. In Maine the denning period ranged from 8 to 12 weeks, with an average of 71 days, and ended as late as 1 June. Individual dens were used for 2 to 12 weeks, with a median of 22 days [9,89]. In north-central Massachusetts and southwestern New Hampshire, maternal dens were used until 3 June [101]. Kits in south-central Maine stayed with their mothers until they were about 150 days old, with males exhibiting more variation than females. Kits occurred within their mothers' home range from August to January. During this period mothers and young showed no attraction to or avoidance of each other. Two female kits dispersed in January [12,89]. Field and genetic evidence from an untrapped population in Oregon suggests males disperse further from their natal home range than females [16]. For information on development of young fishers, see these sources: [52,96]. According to reviews, males apparently can breed after a year, but the extent to which this actually occurs is unknown. Females also reach sexual maturation at a year. Due to delayed implantation they do not give birth to their first litter until they are 2 years old [40,85,99]. A review states that fishers can live to about 10 years [99]. Average annual survival rates over a 6-year period in a trapped fisher population in coastal Maine were 0.74 for adult females and 0.33 for juveniles [89]. In the upper peninsula of Michigan, the survival rate of fishers from June to December was 0.889, with 95% confidence intervals ranging from 0.500 to 0.985 [22].
Fisher denning rates are often lower than pregnancy rates. Reviews note cases of pregnancy rates ≥95% [9,24,99]. However, denning rates are typically much lower. For instance, in south-central Maine denning rates varied from 0% to 100% (n=12 females) over several years. The average annual recruitment was 0.7 to 1.3 kits/female. This recruitment rate and the estimated survivorship suggested the population was declining [9,89]. Factors influencing reproductive success of female fishers are addressed by McMahon and deCalesta [84].
Fishers exhibit sexual dimorphism, with females much smaller than males. Average female weight (4.3-4.9 lbs (1.97- 2.24 kg)) was substantially less than average male weight (7.9-9.5 lbs (3.6-4.3 kg)) in New Hampshire [67], northwest Connecticut [69], Vermont [127], and the southern Sierra Nevada of California [26]. The ratio of average male mass to average female mass for fishers collected throughout British Columbia was 1.64:1.00 [133].
Although fishers have large home ranges and are capable of moving long distances, in at least some instances their dispersal ability may be rather poor. In Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), and hybrid white spruce (Picea engelmannii ÃÂ P. glauca)- dominated forests of south-central British Columbia, translocated fishers moved extensively before establishing their home ranges. The average area traversed was 442.5 km² for females and 1,438.0 km² for males. The majority of fishers traveled more than 100 km before establishing a home range [131]. In Oregon, dispersal distances greater than 31 miles (50 km) were observed. However, lack of gene flow to a population approximately 31 miles away suggests there is no dispersal between the populations [16]. In mixed second-growth coniferous and deciduous forest of south-central Maine, the greatest natal dispersal distance observed was 14 miles (23 km) [12]. There are several possible reasons for the variability in fisher dispersal distances, such as the presence of recently vacated territories nearby [12] and the degree of habitat connectivity (see Landscape/scale effects).
For detailed information on fisher reproductive behavior such as territoriality, parental care, and denning see these sources: [40,89,96,99]. For information on periods of activity and regular movements of fisher, see: [9,40,67,96].
