In B.C., the normal lifespan of wild fishers is usually less than 8 years, and females produce 3-4 litters of kits over their lifetime. Fishers give birth to 2-3 kits in late March to early April and natal dens are always found in cavities of standing trees. At birth, fishers are blind, deaf, and have only a sparse covering of fine hair. Fisher young stay in the den for the first 8 weeks of their life, during which their eyes and ears open and they begin to eat solid food.
Fishers need large, standing trees with heart rot cavities for their reproductive dens. Access holes (5-10 cm wide by 7-15 cm tall) to the cavity are generally located in the tree more than 1.5 m above the ground to almost at the top. High quality denning stands are typically dense (large diameter trees located close together), complex (many tree species, many age layers, logs, etc), and old (greater than 80 years).
Another important type of habitat, resting habitat, offers fishers protection from predators, thermal cover, and opportunities for prey detection while resting. Fishers rest primarily in trees and most rest trees are old, decayed and display abnormal growths. Rust brooms in spruce are the structures most often used for resting by fishers in B.C., but they also rest on large limbs and in cavities of black cottonwood, trembling aspen and other tree species.
Ground-based rest sites are usually associated with complex piles of large woody debris (>30 cm diameter) but can also be found in animal burrows. Ground-based rest sites are used more in winter when temperatures are very cold (less than -10˚ C) and deep snow provides an insulating layer.
For more photos and description of denning and resting habitat take a look at the Pictorial Guide of Important Fisher Habitat Structures in British Columbia.