Minnesota Fisher Den Box Project

The Minnesota Fisher Den Box Project is focused on evaluating an emerging habitat management tool to address the declining MN fisher population. 


Fishers are obligate tree cavity users, meaning that they strictly only use tree cavities for their dens. They use cavities in both living and dead trees, and these dens serve as rest sites, but most importantly, are where female fishers give birth and raise their young. Young fishers,  which are called kits, depend on the shelter and protection that cavities provide until they are old enough to disperse and live on their own. The cavities that female fishers use for den sites are unique in that the openings are only large enough to allow the female to enter the den, while preventing larger males from getting inside and harming the kits.

The fisher population in MN has experienced nearly a 50% decline over the last two decades.  Research conducted by the Minnesota Department of Natural Resources determined that there are fewer mature trees and cavities now on the landscape, which is an issue that may be contributing to the observed decline in the fisher population due to there being less suitable habitat. As the largest MN mammal that depends on tree cavities, fishers require sizable cavities which are typically only found in the largest 2-3% of trees on the landscape. Large enough cavities can take a considerable amount of time to develop. For example, previous research has shown that it can take 80 - 100+ years for appropriate cavities to form in quaking aspen, a common tree species in MN. However, typical harvest rotations of MN forests are largely not long enough to allow for cavity formation in mature trees. This results in there being fewer den sites on the landscape, and an overall reduction in habitat quality.  

Our project idea is based on previous examples of fisher using artificial structures for rest and den sites. The fisher is a threatened species in the western United States and Canada, and faces similar issues of reduced cavity availability like in MN. A similar project that employed man-made den boxes in British Columbia was successful in attracting fishers to 50% of their deployed den boxes.  The British Columbia den box project has observed success with this approach of providing alternative den habitat to support fisher reproduction, and we hope for similar results here in MN. There have also been observations of fisher using man-made wood duck boxes as rest and den sites in MN, further suggesting that fisher may shift to other cavity-like structures when limited by poor habitat. 


  1. Build and install at least 80 artificial den boxes in Minnesota.
  2. Monitor visitation and use of den boxes by fishers and other wildlife.
  3. Determine what factors influence use of den boxes by fishers.
  4. Develop guidelines and best management practices for use of artificial den boxes for fishers, including an understanding of how thermal properties of den boxes compare to natural tree cavities. 
  5. Identify areas with low habitat suitability as potential sites for future fisher habitat management.


We built the artificial den boxes at the Natural Resources Research Institute (NRRI) with support of NRRI staff and facilities. Each box was assembled using a double layer of plywood containing foam insulation. Den boxes were coated with exterior-grade deck stain to maximize their durability and lifespan outdoors. (Please see the "Den Box Info" tab for specific details of box construction and building plans). We deployed den boxes at sites near Remer and Hoyt Lakes, MN. Sites were chosen based on forest age and cover type. We put boxes in habitats considered to be "good" and "moderate" for fishers; to better understand how habitat is influencing fisher presence. We assigned sites randomly within specific cover types and habitat qualities to avoid bias in where we placed boxes. 

Each den box has been monitored using a remote camera that captures photographs of fishers and other wildlife species that visit and use the box. We analyze these photos to quantify what species are present at boxes, how individuals are using the boxes (e.g., examining the box or nesting inside), and when boxes are being visited. This data will further our understanding of how and where to place future den boxes and artificial structures for fishers and potentially other MN forest species. 

Project Partners


Associated Labs