The Inventory of Fisher Populations and Reproductive Dens in the Bridge River Watershed


Fishers (Martes pennanti) belong to the weasel family and are carnivores living in forested areas. They are an essential part of healthy ecosystems. This species is considered at risk according to the Identified Wildlife Management Strategy and is blue-listed in the province of British Columbia where the population was evaluated at an average of 12,2 fishers per 1000 square kms.  The main cause of their population decline is probably the loss of suitable habitat. They are dependent on uncommon components found in late-successional forests and need particular tree-cavities for their reproductive dens. These essential and rare characteristics make them very sensitive to forest destruction or damage that can be caused by insect infestations, hydro-electric or oil and gas projects and forestry activities. As such, high focus is made to preserve fishers, and the species is considered rank 2 of the Goal 3 of the provincial Conservation Framework aiming to preserve the diversity of native species and ecosystems.

Background and Objectives of the Research Project

The Bridge River Watershed in British Columbia includes medium to high suitable habitat for fishers. However the flooding of the Bridge River Valley probably generated the destruction of most appropriate tree-cavities, considerably reducing the availability of reproductive dens for the species. Moreover the carrying capacity of the area for fisher population may also have been affected by the flooding and  wildfires that have occurred in the area. To better evaluate the effects of the flooding of the Bridge River Watershed, a survey of fisher population size, density and distribution and of suitable tree-cavity abundance is necessary. The potential habitat capacity of the area surrounding the flooded area should also be assessed.

The inventory research project extends over several years. The data collected through this survey would improve the  knowledge about the fisher population and enhance the understanding of the impacts of hydroelectric or other projects on the fisher population. This would provide information for management decisions and for BC Hydro and the St’át’imc Nation to elaborate habitat recovery actions and sustainability efforts.

Methods, Materials and Study Area

The assessment area was situated in the Bridge River Watershed within the traditional territory of the St’át’imc Nation. The area included various biogeoclimatic subzones which were considered to have potential for fisher habitat. The subdivisions of the area, 48-20 km2 grid cells, corresponded to the size of the foreseen home range size of female fishers. Both surveys done in 2011 and 2013 applied the  Resources Information Standards Committee (RISC) standards.

In 2011 track transects were used for detected/not detected studies during winter. Trappers with registered traplines in the surveyed area and other local residents were consulted for advice and information in order to focus on the more suitable areas to sustain fisher population. As riparian areas present more interests for fishers, focus was made on those areas. The transects were chosen by a combination of orthophotos and grid cell data. Studies of the fisher tracks encounters in the recent snow were done through the use of  snowshoes, snowmobiles and 4×4 trucks depending on the accessibility of the transect.

In 2012-2013, a genetic-based capture-mark-recapture (SMR) study was conducted. The DNA-based survey followed the methodology of Mowat and Paetkau. Hair and follicles were sampled using hair traps and non-invasive techniques, and habitat data were collected from January to April. DNA extraction and genetic analysis of the samples were conducted to establish species and individual identity. On this basis, the fisher population size was assessed. Wildlife Habitat Areas (WHAs) were described using those results in combination with previous information. Recognition and tagging of those areas were made on the field before GIS mapping and results were submitted to the Ministry of Environment in Kamloops. The results were also used for communication efforts.

Results of the Study

In 2011, 72 fisher tracks, 92 marten tracks and 14 wolverine tracks were observed. Tracks were encountered in a variety of habitats in diverse biogeoclimatic zones but always in pole-sapling or older aged forests. The dry-cold sub-zone presented the more abundant fisher tracks (60 %). Fisher and wolverine tracks were encountered at low elevation of the area and martens at higher elevations.

In 2012, hair samples were found in 40% of the sample sites and enabled researchers to identify 8 fishers from the 16 recorded. Five of them were only recorded at a single sample site whereas two males were detected at 4 sites and one female at 3 sites. Most of them (81 %) were found in the Interior Douglas-fir (IDF) biogeoclimatic zone. Fishers were detected at 1141m elevation on average. American marten, red squirrel, northern flying squirrel and wolverine samples were also collected. American martens were recorded at 1309 m elevation in average and in more various biogeoclimatic zones. Using the TIRM model, the fisher population was estimated at 14 individuals in the assessment area and the density was evaluated to be 18.1 fishers/1000 km2 throughout the whole area. As a result the authors proposed five WHA candidates, one in Yalakom, two close to Marshall Lake and two close to Gun Lake.


The inventory of winter 2010-2011 ensured the presence of fishers in at least 50% of the grid cells studied in the Bridge River Watershed. The method used to detect the presence or absence of fishers was effective but was highly dependent on proper weather conditions for collecting data (new snow). Fishers tracks were more abundant in low-elevation zones close to Carpenter Lake. On the basis of the results of this survey, inventory of the size and distribution of fisher population and of the distribution and density of cavity-trees was considered useful as fishers were proved to be well distributed and sufficiently present in the area to guarantee the purpose of further inventories. During the first year of the project the multiple communication actions initiated were found successful in raising the awareness of fisher preservation importance and in creating and developing partnerships with communities, First Nations, organizations, trappers and government.

The fisher population size estimated during the survey of 2012-2013 in the Bridge River Watershed was high compared to results of other studies in British Columbia but lower than in eastern North America. That might be due to various factors including environmental aspects and the use of different approaches, methodologies and calculations. The approach used for this survey was based on samples collected and did not consider the individuals that might have died, dispersed or that were non-resident and did not contribute to the sustainability of the local population. The program CAPWIRE, used for this study and not for the other studies, also seemed very promising for low-density population estimation. However the recapture rate was under the average advised and probably altered the precision of the population density estimation. Improvement of recapture rate could be made by increasing the number of sampling sessions and focus sampling in more fisher abundant areas. The non-invasive CMR method used for this inventory was economical, quick and efficient which probably diminished stress, capture bias and the time needed for sampling. The problem of genotyping bias usually associated with non-invasive methods was very likely low in this case.

The results of the survey corroborated previous predictions  about the carrying capacity level of the different biogeoclimatic zones studied. However the impact of including massive numbers of biogeoclimatic zones with poor-carrying capacity for the estimation of fisher population density is unknown. The data collected during the three years suggested that high-elevation high-snow depth zones constitute poor suitable habitat for fishers as their movement is limited in that kind of habitat. Correlated to that, the low-elevation zones affected by activities such as hydro-electric projects would have presented high-capability habitat for fishers thanks to low-snow, high-prey and high tree-cavity abundance. Therefore, the estimated density would have been likely different if the assessment area would only have encompassed the IDF biogeoclimatic zone.

As the fisher population density estimation was only based on data from the winter 2012 and as other studies observed that density estimation varied greatly over several years, prolonged monitoring of the assessment area would be necessary to evaluate or adjust our estimation.

The results of this survey will help evaluate the sustainable level of resource extraction activities and improve previous information of the province especially for the IDF biogeoclimatic zone which contains high-capability fisher habitat. However the interpretation of the results must be done carefully as the estimation was done on only one year’s data, was subject to low precision and because the data collected were insufficient for appropriate sustainable harvest levels. Furthermore the effects of recent habitat damages such as beetle infestation and fire would probably keep affecting the fisher population after the end of the study.


In addition to the inventory realized to estimate fisher population density, a den-tree inventory is recommended to improve knowledge about the presence and density of those critical habitat structures. Those information would help to identify areas where recovery and enhancement actions are the most suited. Improvements of fisher habitat will also benefit other endangered species.

Conversation plans for fisher habitat are necessary especially in areas impacted by resource extraction. Four major strategies are recommended to enhance the supply of den-trees according to the type of landscape and activities. Those strategies are  forest retention, mechanical alteration, artificial den boxes and fungal inoculation. Further studies are required to determine  the best parameters for each of those strategies.

The communication efforts to involve communities and other decision-makers should be pursued as it might positively influence the behaviour and decisions of land managers in favour of the preservation of fisher habitats in the Bridge River Watershed.


The long term goal of this project was to collect data and information about fisher population in the Bridge River watershed to help develop conservation and recovery stewardship plans for fisher habitat and establish sustainable harvest levels. The inventory provided useful information about fisher population density in the area and proposed Wildlife Habitat Areas that should be created to serve as preserving patches. Partnerships have been created and should continue to be developed with communities, First Nations, organizations and government to enhance the feasibility of those stewardship plans.