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Received: 11 January 2021
Revised: 24 October 2021
Accepted: 17 November 2021
Post‐release survival of translocated fishers:
implications for translocation success
Jeffrey C. Lewis
|Kurt J. Jenkins
|Patricia J. Happe
David J. Manson
|Paul C. Griffin
Washington Department of Fish and
Wildlife, P.O. Box 43200, Olympia,
WA 98504, USA
U.S. Geological Survey, Forest and
Rangeland Ecosystem Science Center, 600 E.
Park Ave., Port Angeles, WA 98362, USA
Olympic National Park, 600 E. Park Ave.,
Port Angeles, WA 98362, USA
Jeffrey C. Lewis, Washington Department of
Fish and Wildlife, P.O. Box 43200, Olympia,
WA 98504, USA.
David J. Manson, Lower Elwha Klallam Tribe,
2181 Lower Elwha Road, Port Angeles,
Paul C. Griffin, Bureau of Land Management,
2150 Centre Ave. Building C, Fort Collins,
As a vital tool for the conservation of species at risk, translo-
cations are also opportunities to identify factors that influence
translocation success. We evaluated factors associated with
post‐release survival of 90 radio‐tracked fishers (Pekania
pennanti) translocated from central British Columbia, Canada, to
the Olympic Peninsula of Washington, USA, from 2008 to 2011.
We hypothesized that the survival of translocated fishers would
be affected by the same factors that influence the survival of
resident, native fishers (i.e., sex, age, season, body condition),
and additional factors that were associated with the transloca-
tion process (e.g., duration of captivity, release date, yr of
release). Fisher survival was most strongly influenced by trans-
location year (i.e., release‐yr cohort), season, sex, and age class
of fisher; whereas duration of captivity, standardized body mass,
release date, and number of intact canines did not influence
survival. Survival was lowest for fishers released in cohort 2 in
2009 and during the breeding season (Mar–Jun), and was
greatest for juveniles and males. When combined across
release‐year cohorts, year 1 survival rates were greatest for
juvenile males followed by juvenile females, adult females, and
adult males. Sex and age‐related differences in survival of
translocated fishers were counter to those commonly reported
for established fisher populations, where adult females often
have the highest survival rates and juveniles the lowest. Pre-
dation (40%) and vehicle strikes (20%) were the most common
J Wildl Manag. 2022;1–18. wileyonlinelibrary.com/journal/jwmg
© 2022 The Wildlife Society. This article has been contributed to by US Government employees and their work is in the public
domain in the USA.
causes of known mortality among the 24 recovered fishers for
which cause of death was determined. We speculate that fe-
males face higher risks of mortality in translocated populations
because their small size makes them more vulnerable to pre-
dation and because adult females in resident populations are
less likely than males and juveniles to disperse. Our findings
support designing translocations that favor releasing a pre-
ponderance of female fishers in recognition of their lower sur-
vival rates and to ensure adequate breeders are established in
the population, and juvenile and young adult fishers to enhance
survival of both sexes. Releases conducted over multiple years
will minimize the impact of stochastic annual events that may
adversely affect survival in any given year. Persistence, wide-
spread distribution, and documented reproduction of fishers
within our study area for ≥6 years following the last releases
indicate that survival parameters we measured contributed to-
ward successful population establishment over the short term.
carnivore, endangered, fisher, forest, Pekania pennanti, survival,
The goal for most conservation translocations is the reestablishment of a self‐sustaining population (Seddon 2010,
Jachowski et al. 2016). Many translocations have failed to meet this goal and success is far from assured (Griffith
et al. 1989, Yalden 1993, Wolf et al. 1998, Miller et al. 1999, Fischer and Lindenmayer 2000). Despite the
importance and extensive use of translocations to conserve species at risk (Seddon et al. 2014), little is known about
factors that influence translocation success, including the survival of translocated individuals for many species. A
greater understanding of these factors is important to managers that must prioritize limited funding for con-
servation actions with the greatest likelihood of success.
Successful population reestablishment or augmentation through translocation clearly relies on survival of translo-
cated individuals (Armstrong and Seddon 2008). In the absence of information on anticipated survival rates in founding
populations, managers may be able to offset the effects of potentially low survival rates by releasing large numbers of
individuals (Devineau et al. 2010) or continuing releases until evidence of success is apparent. Although releasing large
numbers of individuals is associated with translocation success (Lewis et al. 2012,Powelletal.2012), many biologists
must rely on limited founder populations and best available information to conduct successful translocations.
In the contiguous United States, reintroductions have played a key role in restoring populations of the fisher
(Pekania pennanti), a mid‐sized carnivore of the family Mustelidae, which occurs only in the temperate and boreal
forests of North America (Powell 1993, Lewis et al. 2012). The extremely high prices paid for fisher pelts in the late
1800s and early 1900s (up to ~$350 US/pelt; Seton 1926, Bailey 1936, Grinnell et al. 1937, Dalquest 1948)
resulted in overexploitation and the extirpation of fishers from much of the southern portion of their range in the
northern United States and southern Canada (Strickland et al. 1982, Powell 1993). Fishers were effectively
eliminated throughout Washington, USA, by the late twentieth century (Powell 1993, Lewis et al. 2012), at which
time fishers were listed as an endangered species in Washington (Hayes and Lewis 2006). From the mid‐1900s to
2012, there were 35 documented translocations of fishers within the species' historical range; 25 (71%) of these
LEWIS ET AL.
attempts were successful (Lewis et al. 2012). As a result, the fisher is one of the most successfully translocated
carnivores (Breitenmoser et al. 2001, Powell et al. 2012).
Improvements in planning, implementation, and monitoring processes have led to greater rates of translocation
success in recent decades (Seddon and Armstrong 2016). Despite these improvements and large investments in trans-
location efforts, past (and recent) failures demonstrate that success should not be taken for granted (Griffith et al. 1989).
Because many translocations occurred prior to the availability of radio‐telemetry technology and the associated capacity
for biologists to monitor survival, few projects have quantified survival of translocated fishers. Thus, few insights have been
gained on factors associated with survival that could assist with future translocation programs. Many intrinsic attributes
(i.e., sex, age, body mass, canine condition) and extrinsic factors (e.g., season), however, may influence the survival of fishers
in resident populations (Douglas and Strickland 1987,Powell1993,Koenetal.2007,Buskirketal.2012,Sweitzeretal.
2016), providing the basis for evaluating survival in translocated populations. Moreover, survival of translocated fishers
may also be affected by the capture and translocation process itself (e.g., yr of release [cohort], date of release, number of
broken canine teeth), and the stress associated with captivity (Hartup et al. 2005,Teixeiraetal.2007, Franceschini et al.
As a first step toward restoring a viable population of fishers in Washington (Hayes and Lewis 2006), a partnership
of state, provincial, and federal agencies, and private organizations translocated 90 fishers from central British Columbia,
Canada, to Olympic National Park in northwestern Washington from 2008 to 2010 (Lewis et al. 2018). We monitored
the survival of released fishers to evaluate factors that may influence survival and to provide the first sex‐and age‐
specific survival rates for a translocated fisher population. Our specific objectives were to 1) evaluate intrinsic (e.g., sex,
age, body mass, canine condition), extrinsic (e.g., season), and translocation‐process factors (e.g., yr of release, release
date, duration of captivity) that could influence the survival of fishers released on the Olympic Peninsula; 2) estimate
sex‐and age‐specific survival rates for the founder population; 3) determine the causes of mortality among recovered
fishers; and 4) compare the survival rates and causes of mortality between this translocated population and resident
fisher populations in other parts of the species' range.
We conducted this study from January 2008 to December of 2011 in an area (14,412 km
;Figure1) that encompassed
most of the Olympic Peninsula in northwestern Washington. The Olympic Peninsula is bordered by the Pacific Ocean to
the west, the Strait of Juan de Fuca to the north, and Puget Sound to the east (Lewis et al. 2016). The study area was
delineated by the areal extent of telemetry locations obtained for all fishers released on the Olympic Peninsula. The
center of the Peninsula is dominated by the Olympic Mountains, including glaciated headwaters, steep upper drainages,
and lowland valleys that radiate outward in all directions from the mountainous core of the Peninsula. Elevations range
from sea level to 2,415 m at the top of Mount Olympus near the center of Olympic National Park. The mountainous
center of the Peninsula slopes to a pronounced coastal plain to the west and smaller plains to the north and east.
The Olympic Peninsula has a temperate maritime climate characterized by warm summers (21 Jun–21 Sep) and
cool winters (21 Dec–21 Mar; Peel et al. 2007). Annual precipitation ranges from 315–500 cm on the west slope of
the Olympic Mountains, whereas annual precipitation is typically <40 cm in the rain shadow of the Olympic
Mountains (the NE corner of the Olympic Peninsula; Gavin and Brubaker 2015). Eighty percent of the annual
precipitation on the Olympic Peninsula falls from October through March and most winter precipitation falls as rain
at elevations <305 m and as snow above 760 m.
The moist climate and broad range of elevations profoundly affect vegetation patterns throughout the Olympic
Peninsula, which are dominated by conifer forests. A large portion of the Olympic Peninsula is composed of
Olympic National Park, which contains about 2,850 km
of forest and is managed primarily as wilderness. Olympic
National Forest bounds much of the Park and encompasses mountainous terrain that is managed for multiple uses
and the development of late‐seral conifer forests under the Northwest Forest Plan (U.S. Department of Agriculture
SURVIVAL OF TRANSLOCATED FISHERS
Forest Service and U.S. Department of Interior Bureau of Land Management 1994). Lower elevation lands outside
the Olympic National Forest boundary are owned and managed by multiple ownerships and are intensively man-
aged for commercial timber production. United States Route 101 is the largest highway on the Olympic Peninsula,
and it encircles much of study area (Figure 1).
In addition to translocated fishers, the Olympic Peninsula study area is occupied by a diverse mammalian car-
nivore community, including American black bears (Ursus americanus), coyotes (Canis latrans), mountain lions (Puma
concolor), bobcats (Lynx rufus), river otters (Lontra canadensis), Pacific martens (Martes caurina), western spotted skunks
(Spilogale gracilis), long‐tailed weasels (Mustela frenata), and ermine (Mustela erminea; Happe et al. 2020). Primary prey
species of fishers on the Olympic Peninsula include snowshoe hares (Lepus americanus), mountain beavers (Aplodontia
rufa), and sciurids (Happe et al. 2021).
FIGURE 1 Study area and land ownerships on the Olympic Peninsula, Washington, USA, where we released
and monitored fishers from 2008 to 2011. Stars indicate general areas where we released fishers from 2008
LEWIS ET AL.
Capture, translocation, and radio‐telemetry
Hired contractors coordinated fisher trapping activities with British Columbia trappers, housed and cared for
captive fishers, and assisted with preparations for fisher translocation. Contractors provided trappers with box traps
(~25 × 30 × 81 cm; cage‐type; made of wire mesh) and specialized transport boxes (40 ×40 × 90 cm; made of ply-
wood) for safely moving fishers from a capture location to the captive facility and subsequently to the Olympic
Peninsula (Lewis 2006,2013). Trapping for fishers coincided with the commercial trapping season in British Co-
lumbia, which occurred from 1 November to 15 February (2007–2010).
Contractors retrieved fishers from trappers, transported them to the captive facility in central British Columbia,
and provided an individual housing unit for each fisher. Each fisher was provided straw bedding, a litter box, water
ad libitum, and a diet (raw meat and eggs) that promoted weight gain. While at the captive facility, each fisher was
chemically immobilized and examined by a licensed veterinarian. Ninety fishers (50 females, 40 males; Table 1) were
certified by the veterinarian as suitable for translocation to Washington. Each certified fisher was vaccinated for
rabies (IMRAB‐3; Boehringer Ingelheim, Ingelheim, Germany) and canine distemper (Purevax; Boehringer In-
gelheim), treated with ivermectin (Merck, Kenilworth, NJ, USA) and Dronsit (Praziquantel; Bayer AG, Leverkusen,
Germany) for parasites, equipped with a radio‐transmitter and a passive integrated transponder (PIT) tag, measured,
weighed, and photographed. We extracted 1 first premolar to age each fisher, and took blood and tissue samples to
assess disease exposure and for genetic analyses, respectively. We also assessed the condition of each fisher's
teeth; 82 of the 90 certified fishers had 3 or 4 intact canines, whereas 8 (7 females, 1 male) had only 2 canines as a
result of tooth loss or breakage prior to project initiation, or as a result of the translocation process. Among the
90 certified fishers, time in captivity ranged from 2 to 55 days (
= 21 ± 12 [SD] days). Five fishers (3 females,
2 males) that received special medical treatment were kept in captivity for 32 to 55 days (
= 42 ± 9 days).
TABLE 1 Number of fishers released and monitored on the Olympic Peninsula, Washington, USA, 2008–2011,
to assess survival by release‐year cohort, release dates, sex, and age class
Cohort (release dates) Age class
Release‐year cohort 1 (27 Jan 2008, 2
Juveniles 3 5
Adults 9 1
Total 12 6
Release‐year cohort 2 (21 Dec 2008, 17 Jan
2009, 23 Feb 2009)
Juveniles 6 11
Adults 14 0
Total 20 11
Release‐year cohort 3 (24 Dec 2009, 21 Jan
2010, 20 Feb 2010)
Juveniles 7 16
Adults 11 7
Total 18 23
Total fishers released 50 40
For females, juveniles were <1 year old, and adults were ≥1 year old. For males, juveniles were <2 years old, and adults
were ≥2 years old. We used the cementum annuli from an extracted premolar to age each fisher.
SURVIVAL OF TRANSLOCATED FISHERS
To facilitate post‐release monitoring, we equipped 82 (50 female, 32 male) fishers with a 40‐gveryhighfre-
quency radio‐collar (model MI‐2 with mortality sensor, Holohil Systems, Carp, Ontario, Canada). The antenna broke on
several of these radio‐collars (via normal fisher behaviors), which led to a corresponding loss of signal transmission
distance; therefore, we also evaluated satellite collars and implant transmitters on small samples of fishers as alter-
natives to radio‐collars. Specifically, we equipped 5 males (each weighing >4.5 kg) with 120‐gArgossatellite‐collars
(Kiwisat 202, Sirtrack, Havelock North, New Zealand) and 3 males with a 41‐g very high frequency transmitter (model
IMP/310/L with mortality sensor, Telonics, Mesa, AZ, USA) surgically implanted in the abdominal cavity.
From January 2008 to February 2010, we transported fishers from British Columbia and released them in
Olympic National Park. We transported fishers by truck from central British Columbia to Port Angeles, Washington
in 1 day, housed them overnight in their transport boxes, and released them the following morning at pre‐
determined release sites in Olympic National Park. We released fishers at 9 locations within the Park to facilitate
fisher occupancy of landscapes dominated by late‐successional conifer forest (Figure 1; Lewis 2006,2014).
The study area was largely unroaded (Figure 1); therefore, we used primarily aerial telemetry to locate and
monitor the survival status (alive or dead) of fishers, although fishers occupying areas near roads were occasionally
located on the ground by tracking from vehicles and on foot. We attempted to locate each fisher and determine its
survival status at least once per week. Inclement weather, the large size and rugged nature of the study area, the
relatively short transmission distance of the radio‐transmitters, and the extensive movements of many fishers, made
it impossible to locate fishers each week. Upon discovering a mortality signal, we recovered carcasses as soon as
was possible and sent them to Colorado State University (Veterinary Diagnostics Laboratory) or the University of
California at Davis (Veterinary Genetics Laboratory) for an extensive necropsy and cause‐of‐death determination.
We examined the survival status of 90 fishers over 48 months (Jan 2008–Dec 2011). For each release‐year cohort,
we evaluated survival status for 24 months following release. An individual fisher could have 1 of 3 survival‐status
designations in a specific month: alive, dead, or censored. We censored a fisher from survival analyses for any
month that we could not determine its status (i.e., not found); we censored some fishers only temporarily because
we subsequently relocated them alive or dead.
We structured known‐fate models in Program MARK (White and Burnham 1999) to examine their relationships to
intrinsic (i.e., sex, age, body mass, number of intact canines), extrinsic (season), and translocation‐process factors (i.e.,
release‐yr cohort, duration of captivity, release date). The known‐fatemodelinMARKallowedforstaggeredentry
(Pollock et al. 1989) of fishers into the population throughout the 3 winters when fishers were released (Table 2). We
structured known‐fate models that included individual covariates such as sex, age, body mass, number of days in captivity,
release date, and the number of intact canines (Table 2). We used a 1‐month interval as the time step in survival analyses.
We constructed 2 sets of candidate models based on a priori biological hypotheses to investigate the influence
of intrinsic, extrinsic, and translocation‐process factors on fisher survival. The first set included models that as-
sumed constant survival and models incorporating the main effects of release‐year cohort, season, age, and sex
(Table 2). In models that included a seasonal influence on survival rate, we grouped the months of the year into 4
biological seasons (Table 2). The age of each fisher was determined by laboratory analysis of cementum annuli
(Arthur et al. 1992) of a first premolar. Fisher ages ranged from 0 to 5 years; in candidate models that included a
linear effect of age, we incremented each fisher's age by 1 for the second year after its release. We included an
interaction term for a subset of models that included both the effects of season and age (season × age) and for a
subset of models that included both the effects of release‐year cohort and sex (release‐yr cohort × sex).
We used Akaike's Information Criteria adjusted for small sample size (AIC
) to assess support among the first set of
candidate models. We considered those with ΔAIC
scores <2 to be highly supported by the data (Burnham and Anderson
2002), and we used these highly supported models as the basis for developing the second set of candidate models.
LEWIS ET AL.
In the second set of candidate models, we evaluated improvement in model fit by adding an additional
covariate to each of the most highly supported base models from the first candidate set. The added covariates were
related to the translocation process or the condition of individual fishers and included duration of captivity, release
date proximity to the beginning of the breeding season (release date), standardized body mass, and the number of
intact canine teeth (canines; Table 2). Models that included standardized body mass, canines, or release date
included a linear effect of that variable in the logit link function, which reflected the assumption that any effect
linked to this variable at the time of release persisted throughout the next 2 years.
We used the Kaplan‐Meier estimator (Pollock et al. 1989) to estimate empirical survival rates and developed
95% confidence intervals following methods provided by Cox and Oakes (1984). To estimate seasonal and annual
survival rates, we calculated the product of monthly survival rates to produce a cumulative survival rate for a season
or an entire year.
The first candidate model set comprised 18 models that included only main‐effect variables (release‐year cohort,
season, sex, and age; Table 3). Among these 18 models, 3 were highly supported by the data (i.e., ΔAIC
< 2) and
collectively represented 75% of the total model weights (w
; Table 3). Release‐year cohort and season were included
in all 3 highly supported models, whereas sex and age were each included in 2 (Table 3).
TABLE 2 Intrinsic, extrinsic, and translocation‐process variables incorporated into survival models and grouped
as main and additional effects, for translocated fishers released and monitored on the Olympic Peninsula,
Washington, USA, 2008–2011. The hypothesized relationship of each of the additional effects is provided in
Release‐year cohort Cohort 1 (released from Jan to Mar 2008),
cohort 2 (released from Dec 2008 to Feb 2009), or
cohort 3 (released from Dec 2009 to Feb 2010)
Orientation season (from release to 28 Feb; year 1 only),
breeding season (1 Mar–30 Jun),
kit‐rearing season (1 Jul–30 Sep), or
fall‐winter season (1 Oct‐28 Feb)
Age Estimated age in years at the time of release. Age was increased by
1 year at the start of an individual's second year after release.
Sex Male or female
Duration of captivity Number of days from the capture date in British Columbia until the release
date for each fisher (negative)
Release date Timing of release, as indicated by the number of days prior to the beginning
of the breeding season (1 Mar) that the release occurred
Standardized body mass Body mass relative to sex‐specific means and standard deviations; computed as
(individual body mass−mean sex‐specific mass)/standard deviation of
sex‐specific mass (positive)
Canines Binary covariate signifying ≥3or≤2 (negative) intact canines
Dates for kit‐rearing and breeding seasons follow those provided by Powell (1993).
SURVIVAL OF TRANSLOCATED FISHERS
The second candidate model set included these 3 base models (highest ranking models from the first candidate
model set; Table 3) and models adding a covariate to each base model. In the second candidate set, no single model
stood out as the best model; rather, 8 of the 15 models were highly supported by the data (i.e., ΔAIC
< 2; Table 4).
The 3 base models were among the 8 highly supported models in the second candidate set. Sex was included in 6 of
the 8 models, and age was included in 5. When duration of captivity was added to the base models, it resulted in
small improvements in model support. In all cases, the model with duration of captivity acquired an 8–25% increase
in model weights compared to the corresponding base models (e.g., evidence ratios 1.08–1.25 as per Burnham and
Anderson 2002:72), indicating weak support for an effect of the duration of captivity. When we excluded data for 5
fishers that were kept in captivity for a longer period to allow for medical treatment and recovery, there was even
less support for models that included duration of captivity. The addition of release date, canines, or standardized
body mass to base models resulted in reductions in model support (Table 4).
Survival rates differed among release‐year cohorts, seasons, sexes, and age classes of fishers (Table 5; Figure 2).
Although confidence intervals overlapped slightly for year 1 estimates, survival rates tended to be highest for
fishers released in cohort 1 (0.88; 0.73–1.00 [95% CI]) and lowest for those released in cohort 2 (0.57; 0.38–0.76;
Figure 2A). Survival rates were lowest during the breeding season and highest during the orientation period
immediately following the releases (1 Dec to 29 Feb; Figure 2B). Survival rates for males (0.79; 0.63–0.94) tended
to be greater than those of females (0.62; 0.47–0.76) in year 1 (Figure 2C). Juveniles (0.78; 0.65–0.91) tended to
TABLE 3 Model results for models in candidate set 1 that examined the influence of release‐year cohort,
season, sex, and age (main effects) on survival of fishers released and monitored on the Olympic Peninsula,
Washington, USA, 2008–2011. Results include Akaike's Information Criterion adjusted for small sample size (AIC
relative changes in AIC
), and Akaike's model weights (ω
Model Number of parameters AIC
Cohort + season + sex 7 297.70 0.00 0.34
Cohort + season + sex + age 8 298.34 0.65 0.24
Cohort + season + age 7 299.03 1.33 0.17
Cohort + season 6 300.84 3.15 0.07
Cohort + season + sex + age + season × age 12 301.02 3.33 0.06
Cohort + sex 4 301.40 3.70 0.05
Cohort + age 4 302.24 4.55 0.03
Cohort 3 305.07 7.37 0.01
Cohort + age + cohort × age 7 306.13 8.43 0.00
Cohort + sex + cohort × sex 7 306.15 8.45 0.00
Season + age 5 308.99 11.30 0.00
Season + sex 5 309.26 11.56 0.00
Season + age + sex 6 309.49 11.80 0.00
Age 2 310.02 12.32 0.00
Season 4 310.22 12.52 0.00
Season + age + season × age 9 310.86 13.16 0.00
Sex 2 311.69 14.00 0.00
Null 1 313.13 15.43 0.00
LEWIS ET AL.
have higher survival rates than adults (0.59; 0.42–0.76) in year 1 (Figure 2D). When combined across release‐year
cohorts, year 1 survival rates were greatest for juvenile males (0.83), followed by juvenile females (0.67), adult
females (0.60) and adult males (0.33; Table 5). There was no discernable difference in survival rates of fishers
between the first and second year following release for any population segment (Table 5).
We recovered carcasses of 35 of 90 fishers released and monitored from January 2008 to December 2011. We
determined cause of mortality for 24 of the 35 recovered fishers (68.5%; Table 6); cause of mortality could not be
determined for the remaining 11 recovered fishers because of advanced decay or extensive scavenging of the
carcass (Table 6). Predation was the leading known cause of mortality, followed by vehicle strikes. Analysis of DNA
confirmed that predation by bobcat and mountain lion resulted in 2 fisher mortalities. Fifty‐two percent of females
died during the study (26 of 50 females) accounting for 74% of 35 documented mortalities, whereas 22.5% of males
died (9 of 40), accounting for 26% of the 35 mortalities (Table 6).
We monitored fisher survival to determine if survival rates were substantially lower than those reported for established
resident populations (Table 7) or if they appeared too low to support a self‐sustaining population. Prior to this study,
survival estimates for fishers had only been reported for established native populations (Table 7). Survival rates varied
among release‐year cohorts, seasons, and sex and age classes of fishers released on the Olympic Peninsula resulting in
disparate relationships to survival rates reported previously for established populations. The survival rates for fishers in
TABLE 4 Model results for models in candidate set 2 to assess factors influencing the survival of fishers
released and monitored on the Olympic Peninsula, Washington, USA, 2008–2011. We used the candidate models
in set 2 to examine the influence of adding 4 covariates (duration of captivity, release date, canines, and
standardized body mass), individually, to base models from candidate set 1 (indicated in parentheses) to assess
improvement in model support. Results include Akaike's Information Criterion adjusted for small sample size (AIC
relative change in AIC
), and Akaike's model weights (ω
Model Number of parameters AIC
Cohort + season + sex + duration of captivity 8 297.48 0.00 0.14
Cohort + season + sex (base model 1) 7 297.70 0.21 0.13
Cohort + season + sex + age + duration of captivity 9 297.96 0.47 0.11
Cohort + season + sex + age (base model 2) 8 298.34 0.86 0.09
Cohort + season + age + duration of captivity 8 298.58 1.10 0.08
Cohort + season + sex + release date 8 298.60 1.11 0.08
Cohort + season + age (base model 3) 7 299.03 1.55 0.06
Cohort + season + sex + age + release date 9 299.48 1.99 0.05
Cohort + season + sex + canines 8 299.49 2.01 0.05
Cohort + season + sex + body mass 8 299.70 2.21 0.05
Cohort + season + age + release date 8 300.07 2.58 0.04
Cohort + season + sex + age + body mass 9 300.34 2.85 0.03
Cohort + season + sex + age + canines 9 300.34 2.86 0.03
Cohort + season + age + canines 8 301.04 3.55 0.02
Cohort + season + age + body mass 8 301.06 3.57 0.02
SURVIVAL OF TRANSLOCATED FISHERS
TABLE 5 Kaplan‐Meier survival estimates (Swith 95% CIs) by release cohort, sex and age class, and years 1 and 2 following release for fishers released and monitored
on the Olympic Peninsula, Washington, USA, 2008–2011
Cohort 1 Cohort 2 Cohort 3 All cohorts
Population segment at time of release nS(95% CI) nS(95% CI) nS(95% CI) nS(95% CI)
Juvenile females 3 1.00 (1.00–1.00) 6 0.56 (0.04–1.00) 7 0.57 (0.21–0.94) 16 0.67 (0.42–0.92)
Adult females 9 0.89 (0.69–1.00) 14 0.43 (0.17–0.69) 11 0.62 (0.29–0.96) 34 0.60 (0.42–0.77)
All females 12 0.92 (0.76–1.00) 20 0.45 (0.22–0.68) 18 0.61 (0.36–0.85) 50 0.62 (0.47–0.76)
Juvenile males 5 1.00 (1.00–1.00) 11 0.78 (0.51–1.00) 16 0.79 (0.56–1.00) 32 0.83 (0.69–0.98)
Adult males 1 0.00 (0.00–0.00) 0 7 0.50 (0.00–1.00) 8 0.33 (0.00–0.86)
All males 6 0.83 (0.50–1.00) 11 0.78 (0.51–1.00) 23 0.77 (0.54–1.00) 40 0.79 (0.63–0.94)
Juvenile females 3 0.67 (0.23–1.00) 2 0.50 (0.00–1.00) 4 0.67 (0.00–1.00) 9 0.61 (0.28–0.94)
Adult females 6 1.00 (1.00–1.00) 6 0.40 (0.00–1.00) 5 0.75 (0.23–1.00) 17 0.71 (0.42–0.92)
All females 9 0.88 (0.66–1.00) 8 0.44 (0.00–0.88) 9 0.71 (0.28–1.00) 26 0.67 (0.46–0.88)
Juvenile males 4 1.00 (1.00–1.00) 7 1.00 (1.00–1.00) 9 0.75 (0.00–1.00) 20 0.90 (0.62–1.00)
Adult males 0 0 1 0.00 (0.00–0.00) 1 0.00 (0.00–0.00)
All males 4 1.00 (1.00–1.00) 7 1.00 (1.00–1.00) 10 0.63 (0.00–1.00) 21 0.84 (0.50–1.00)
LEWIS ET AL.
release‐year cohort 1 were similar to the highest survival rates reported for established resident populations that were
either subjected to light trapping pressure (1 fisher harvested over 4 years in a national park in MI [Belant 2007]; mean
annual state‐wide harvest of 130 fishers in MA, 1992–1994 [York 1996]) or no trapping pressure (CA [Jordan 2007];
Table 7). Survival rates for female fishers in release‐year cohorts 2 and 3 were lower than those reported for
any established resident population, whereas the survival rates for males in those cohorts were comparable to
reported estimates (Table 7). Survival estimates for adult males were based on only 8 individuals (7 of the 8 were from
FIGURE 2 Univariate main effects (±95% CI) of cohort (A), season (B), sex (C), and age class (D) on post‐release
survival of fishers released and monitored on the Olympic Peninsula, Washington, USA, 2008–2011. Sample sizes
are provided above the bars for the estimates in each graphic
TABLE 6 Cause of mortality for fishers released and monitored on the Olympic Peninsula, Washington, USA,
2008–2011. Cause of mortality was determined for 24 of 35 fishers that died and were recovered
Females (n) Males (n)
3 (18) All (50)
3 (23) All (40)
Predation 1 3 7 11 1 2 3 14 (40.0)
Unknown 1 6 7 1 3 4 11 (31.4)
Vehicle strike 1 3 1 5 2 2 7 (20.0)
Drowning 1 1 2 2 (5.7)
1 1 1 (2.9)
Cohort totals 3 14 9 26 1 3 5 9 35 (100.0)
This female died from injuries she suffered after being caught in a leg‐hold trap ~14 months after release.
SURVIVAL OF TRANSLOCATED FISHERS
release‐year cohort 3); consequently, there is poor precision in the survival rates estimated for adult males and the
estimated survival for both male age classes combined is heavily influenced by the survival of 32 males <2 years old. We
determined no clear cause for lower survival rates for cohort 2 fishers; therefore, we attribute it primarily to stochastic
variation related to unknown, unmeasured variables or to small sample sizes. Despite the variable relationships of
survival between the reintroduced population of fishers and other established populations, subsequent monitoring of
population occupancy patterns of fishers on the Olympic Peninsula indicated that the post‐release survival of fishers we
observed was sufficient for the population to persist and become widely distributed throughout the Peninsula 6 years
after the last releases (Happe et al. 2020).
We evaluated factors that managers could control to increase survival rates and translocation success. Of the factors
we examined, variation among cohorts in first‐year annual survival rates suggests the potential benefits of releasing
TABLE 7 Estimated survival rates for established fisher populations in North America (modified from Buskirk
et al. 2012)
Study Location Population segment Survival interval Survival rate (95% CI)
Sweitzer et al. (2016)
CA Males Annual 0.62 (0.54–0.70)
Females Annual 0.72 (0.67–0.78)
MI All fishers Annual 0.89 (0.50–0.99)
Sierra Nevada, CA Males Annual 0.88 (0.54–0.98)
Females Annual 0.88 (0.59–0.97)
Koen et al. (2007)
ON Males >9 months Annual 0.33 (0.18–0.60)
Females >9 months Annual 0.63 (0.47–0.86)
Adult males Annual 0.45 (0.24–0.83)
Adult females Annual 0.81 (0.72–0.91)
Males >9 months 2‐year 0.15 (0.04–0.50)
Females >9 months 2‐year 0.51 (0.34–0.78)
MA Adult males Annual 0.77 (0.63–0.95)
Adult females Annual 0.90 (0.80–1.00)
Juvenile males Annual 0.77 (0.46–1.00)
Juvenile females Annual 0.84 (0.65–1.00)
Krohn et al. (1994)
ME Adult males Trapping season
Adult females Trapping season
Juveniles Trapping season
Adults Non‐trapping season 0.89 (0.81–0.99)
Juveniles Non‐trapping season 0.72 (0.53–0.99)
Paragi et al. (1994)
ME Adult females Annual 0.65 (0.50–0.86)
Juveniles Annual 0.27 (0.14–0.50)
Fishers were not commercially trapped.
Fishers were recreationally trapped in the study area.
Fishers were translocated previously near this study area.
LEWIS ET AL.
TABLE 8 Causes and associated numbers of fisher mortalities for translocated and resident populations in North America (modified from Lofroth et al. 2010)
mortalities Predation Trapping
This study WA 35 14 1 7 2 11
Serfass et al. (2001)PA311
Fontana et al. (1999)BC832 3
Weir (1995)BC5 2 21
Heinemeyer (1993)MT1423 4 5
Roy (1991)MT14931 2
Total 79 29 (37%) 12 (15%) 8 (10%) 4 (5%) 4 (5%) 22 (28%)
Gabriel et al. (2015) CA 156 90 5 13 21 27
Weir and Corbould (2008)BC 9 13 1 3 1
Koen et al. (2007)ON28292 1 68
Aubry and Raley (2006)
OR 9 2 1 4 1
Higley and Matthews (2006),
Higley et al. (1998)
CA 12 8 2 1 1
Truex et al. (1998)CA249 221 10
York (1996)MA122431 11
Krohn et al. (1994) ME 50 2 40 2 3 1 2 1
Buck et al. (1983)CA74 3
Total 307 120 (39%) 58 (19%) 14 (5%) 7 (2%) 17 (6%) 38 (12%) 53 (17%)
Includes mortalities associated with poisoning, handling by researchers, radio‐collars, predation by domestic dogs, being trapped in cisterns or railroad cars.
Includes disease, drowning, emaciation or starvation, infections, accidents, and freezing.
This study involved a resident population of fishers that were translocated to Oregon from 1977–1981.
SURVIVAL OF TRANSLOCATED FISHERS
fishers over multiple years. There was no meaningful relationship between survival and duration of captivity, even when
we included fishers that were kept in captivity for longer periods of time to recover from medical treatments. The high
quality of care that fishers received while in captivity likely minimized stress while in captivity and promoted weight gain,
which may have minimized negative effects of captivity on post‐release survival. The lack of model support for the effect
of the number of intact canines on survival may be a function of the small sample of released fishers with only 2 intact
canines (n= 8) or that the ability to acquire prey is not diminished with a loss of 2 canines.
Survival rates were lower for females during the breeding season and for fishers in release‐year cohorts 2 and
3. Females that had not established a home range before or during the first breeding season or that made atypical
movements to search for mates, were likely at increased risk (Lewis 2014). The lower survival estimates for males
during the breeding season were consistent with their extensive breeding‐season movements and fighting among
males when competing for mates (Douglas and Strickland 1987, Lewis 2014). Atypical breeding season movements
by females, and the typical breeding‐season movements of males, likely increased the risk of mortality, especially
when those movements occurred in a novel environment (Lewis 2014).
The relative ranking of survival rates among sex and age classes of translocated fishersontheOlympicPeninsula
(i.e., survival of juveniles > adults; males > females) was the reverse of that typically observed in established native
populations (i.e., adults > juveniles; females > males). Translocated juveniles may be better than adults at exploiting and
surviving in novel environments because their habitat selection behaviors may be more substantially influenced by the
habitat qualities of the release area than the source area, whereas adults developed habitat selection behaviors that
were effective for the source area but may be less effective for the release area (Stamps and Swaisgood 2007). Further,
the relative lack of competing males in the reintroduced population may have increased survival, particularly for juvenile
males, relative to established resident populations where territorial defense may be more pronounced. Juveniles may
also have other age‐specific traits (e.g., learning capacity, predator avoidance behaviors, physical condition) that enable
them to better exploit novel environments than adults. Although no data are available from previous fisher translo-
cations for comparison, research findings from translocations of the Arabian oryx (Oryx leucoryx; Stanley‐Price 1989),
thegoldenliontamarin(Leontopithecus rosalia; Kleiman et al. 1991), and the North‐Island saddleback (Philesturnus
rufusater;Armstrongetal.2005) indicated that juveniles and younger individuals had higher initial survival rates than
adults. The smaller size of female fishers (adults and juveniles) makes them more vulnerable than males to predation by
mid‐sized carnivores (e.g., bobcats and coyotes; Wengert et al. 2014). Translocated adult females may also be at a
greater risk to mortality because they have more specific habitat requirements, and they face greater nutritional and
energetic demands associated with fetal development and kit rearing than juvenile females or males.
Predation comprised the greatest percentage of known mortalities among translocated fishers on the Olympic
Peninsula, whereas predation and trapping were among the most common sources of mortality in other populations
both translocated and resident (Table 8). Vehicle strikes were also an important cause of mortality for translocated
fishers on the Peninsula, but this was not the case for other reintroduced populations or for established native
populations (Table 8). Five of the 7 vehicle strikes occurred on the primary highway in our study area (U.S. Highway
101), which varies in width from 2 to 4 lanes and surrounds the core of the study area in proximity to large expanses
of fisher habitat in Olympic National Park and Olympic National Forest. Although secondary paved roads may also
present risks to fishers, as indicated by 2 females that were killed on paved roads with reduced speed limits, highways
appear to be a significant factor affecting post‐release survival. Rodenticide poisoning associated with illegal mar-
ijuana plantations has been identified as a substantial source of mortality for fishers in California (Gabriel et al. 2012,
2015); however, it was not (or expected to be) a source of mortality for fishers on the Olympic Peninsula.
The relatively low survival rates we observed for adult females indicates that a founder population biased towards
females (55–65%) could compensate for stochastic occurrences of poor survival among years, and also result in
greater reproduction and recruitment than a population with an even or male‐biased sex ratio. The relatively high
LEWIS ET AL.
survival rates for juvenile females indicates their value as founder individuals because they are unconstrained by the
need to den and produce young the first months after translocation, yet they can breed shortly after being released
and give birth to their first litter in year 2. Planning efforts that include multiple years of releases or contingencies
for obtaining additional founders can provide management flexibility during a translocation in the event of poor
survival or poor capture success in one or more years. To minimize the mortality rate of founders, identification of
recovery areas safer from known mortality sources would benefit restoration success. Our results indicate that
areas in proximity to major highways and high traffic volumes or those open to trapping may reduce survival and
restoration success. Further, ancillary studies that identify predation risk zones for fishers and tradeoffs with prey
availability would be very useful for identifying optimal restoration areas.
Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the
United States Government. This project was possible due to broad‐based support of many partners. We thank D. O.
Werntz for providing administrative support of project operations in British Columbia, and helping with fisher
preparations, transportation, and releases. We are grateful to H. M. Schwantje, E. C. Lofroth, and R. C. Wright for
their efforts to provide healthy fishers from British Columbia. M. and D. Evans worked tirelessly with trappers to
obtain and transport fishers, to provide housing for fishers, and to provide excellent care of each captured fisher.
H. L. Allen provided essential support, guidance, and logistical assistance in all aspects of this project. We also thank
P. A. Becker, G. Olsen, W. A. Michaelis, A. K. McMillan, G. A. Shirato, E. S. Gardner, M. G. Cope, I. N. Keren, K. G.
Mansfield, A. A. Duff, S. F. Pearson, R. A. Hoffman, C. Copass, K. F. Beirne, S. A. Gremel, and C. H. Hoffman for their
administrative, logistical, analytical, and field support. K. J. Maurice, D. B. Houston, B. L. Howell, K. B. Aubry, and
L. R. Davis provided valuable assistance with fisher handling, preparation, transporting and release activities.
D. Ravenel and S. P. Horton helped us recover fishers on Quinault Indian Nation lands and Washington Department
of Natural Resource lands. We thank M. W. Gabriel, G. M. Wengert, L. Munson, P. M. Gaffney, J. A. Bryan, T. R.
Spraker, and H. Van Campen for their assistance with necropsies, disease testing, and assessments of toxicant
exposures. We greatly appreciate the skilled and dedicated pilots that enabled us to fly and locate fishers, including
J. Well, R. Mowbray, C. S. Cousins, and M. E. Kimbrel. We have also benefitted greatly from the insights provided by
S. D. West, K. B. Aubry, J. J. Lawler, A. J. Wirsing, R. D. Weir, J. D. Sauder, R. A. Powell, K. B. Aubry, C. M. Raley, J. S.
Yaeger, L. R. Davis, and S. D. Piper. C. M. Thompson provided a helpful review of a previous draft of this manuscript.
This study was funded by the United States Fish and Wildlife Service, United States Geological Survey, National
Park Service, Washington Department of Fish and Wildlife, Doris Duke Foundation, Conservation Northwest, and
Washington's National Parks Fund.
Animal handling procedures met or exceeded the guidelines of the American Society of Mammalogists for the use of
wild mammals for research (Sikes et al. 2016) and were approved by the British Columbia Ministry of Environment
(permit WL07‐40389). Each fisher importation from British Columbia to Washington was approved by the United
States Fish and Wildlife Service. This approval required the inspection and health certification of each fisher by a
licensed veterinarian, an approved capture and transport permit from the British Columbia Ministry of Environment
(as noted above), and a pre‐approved importation declaration form (U.S. Fish and Wildlife Service form 3‐177).
CONFLICT OF INTEREST
The authors declare that there are no conflict of interest.
DATA AVAILABILITY STATEMENT
Data associated with this research has been published in the USGS ScienceBase Catalog (https://doi.org/10.5066/
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How to cite this article: Lewis, J. C., K. J. Jenkins, P. J. Happe, D. J. Manson, and P. C. Griffin. 2022. Post‐
release survival of translocated fishers: implications for translocation success. Journal of Wildlife
Management 1–18. https://doi.org/10.1002/jwmg.22192
LEWIS ET AL.