Home Range Characteristics of Fishers in California

1. What they did

Fishers are forest members of the weasel family. In California they now hold less than half the range they had in the early 1900s, in two remnant populations about 400 km apart: one in the northwestern mountains near the coast and one in the southern Sierra Nevada. Few ecological studies had been done on fishers in western North America, and the southern Sierra population had never been studied intensively.

The researchers set out to measure the size of fisher home ranges in both populations, compare males and females, and describe what kinds of forest those home ranges contained.

• Capture. They first set traps where fishers had been detected at track plate survey stations and where the habitat looked suitable. Fishers were caught in box traps baited with chicken and a scent lure, sedated, and fitted with radio collars.
• Relocation. They tried to find every collared fisher at least once a week, mostly by walking in on the radio signal to the place where the animal was resting. On the coast they also took compass bearings on the signal from a distance (remote triangulation), and they also located fishers from a fixed-wing aircraft about once a month.
• Measuring error. They tested how accurate their remote and aerial locations were by hiding test collars at known spots, and built that error into their calculations.
• Home ranges. Fishers followed for at least 10 months, with at least 10 rest locations and at least 20 locations in total, were “focal” animals. Their home ranges were drawn as the smallest outline enclosing all their locations (100% minimum convex polygon).
• Vegetation. They used mapped vegetation data to work out what share of each home range was each forest type, forest age (seral stage), tree size and canopy closure.

2. What they found

Animals followed

• They captured 33 fishers in the Sierra and radio-collared 23; they captured 31 on the coast and radio-collared 22.
• Twenty-one fishers became focal animals: 12 in the Sierra (4 males, 8 females) and 9 on the coast (2 males, 7 females). Each home range was based on 20 to 91 locations.
• Rest sites made up 42.8% of all locations (51% in the Sierra, 33% on the coast).

Home range size

• Males had much larger home ranges than females: on average about 3,935 hectares (ha) for males and 981 ha for females, about four times larger.
• Coast: females 1,498 ha, males 5,807 ha. Sierra: females 528 ha, males 2,998 ha.
• Female home ranges were almost three times larger on the coast than in the Sierra. With only two focal males on the coast, the authors compare the two areas using females only.
• Females were roughly estimated at about 5 per 100 km² on the coast and about 8 per 100 km² in the Sierra.

What was inside the home ranges

• Coast: mid-seral Douglas-fir (24.5%), mid-seral true fir (18.3%) and late seral Douglas-fir (14.0%) made up the largest shares. Oak-pine, white oak and grassland were scarce. Males and females had similar home range make-up.
• Sierra: Sierran mixed conifer (40.4%), ponderosa pine (32.9%) and montane hardwood (12.3%) made up most of the home ranges. Small trees (29 to 61 cm in diameter) covered 60.7%, and dense canopy (60 to 100% closure) 66.3%.
• In the Sierra, males’ home ranges held more high-elevation red fir and less ponderosa pine than females’. Females’ home ranges held more of the densest canopy (71.7% against 55.6%).
• Hardwood forest types were more common in Sierra home ranges than anywhere else fishers had been studied in western North America, with one possible exception in Alberta, Canada.

3. What they concluded

• As in earlier studies, males use much larger areas than females, which fits the mating system seen in other forest weasels: males range widely to keep track of where females are and whether they are ready to breed.
• The smaller home ranges in the Sierra are probably due to more productive habitat. Black oak gives fishers cavities to rest in and produces acorns eaten by their prey. The forest types where black oak is most common made up about 19% of female home ranges in the Sierra but only about 9% on the coast. Coastal females were only slightly heavier (2.02 kg against 1.93 kg), too small a difference, in the authors’ view, to explain the larger ranges.
• Home ranges in western North America tend to be larger further north, but local conditions also matter.
• Females may be more selective about habitat and settle in lower, more productive forest, while males cross high ridges to reach females.
• The analysis cannot tell whether it is the proportion or the total amount of a forest type that matters to fishers.

4. How tracking was or was not used

Tracking played a small part. Traps were first set where fishers had been detected at track plate survey stations, which record the footprints of animals that visit them. The paper does not describe how the tracks were identified or who identified them. The study itself relied on radio telemetry: field workers walked in on the signal of resting fishers to locate them, but they did not follow tracks.

5. Tracker or observer certification

Not reported. Who identified fisher tracks at the track plate stations is not described. This affects only where trapping started, not the home range results.

6. Other technologies, skills or innovations

• Radio collars (Telonics model 80 for females, model 125 for males), handheld receivers and antennas, and radio-tracking from fixed-wing aircraft.
• A field test of location error: test collars inside saline-filled bottles were hidden in tree hollows at 30 places, and four observers who did not know where they were tried to locate them.
• Repeated resampling of uncertain locations to give a mean and spread for each home range, rather than one figure.
• Differentially corrected Global Positioning System (GPS) receivers to map locations, and Geographic Information System (GIS) vegetation maps from aerial photographs and satellite images.

7. Implications for conservation (from the paper)

• The fisher has been petitioned three times for listing under the United States Endangered Species Act; in 2004 listing of the west coast population was found warranted. The southern Sierra population is one of the most genetically depauperate in North America.
• The ranges of forest types found in home ranges give local managers guidelines for planning for fisher habitat and judging the effects of vegetation management. They are descriptions of areas used by these fishers, not thresholds for occupancy, and they should be tested elsewhere.
• For example, coastal home ranges rarely held less than about 15% or more than 35% mid-seral Douglas-fir. In the Sierra it was rare for home ranges to have less than 15% Sierran mixed conifer, less than 5% of the largest tree size class, or less than 53% dense stands.
• The authors suspect these guidelines are conservative, because historical landscapes probably had more late seral forest and larger fisher populations. Recovery will also need attention to small-scale habitat features and to how landscapes affect links between populations.