The fisher is one of the largest members of the Mustelid family, which includes the weasels, mink, otter, and skunk. Fishers exhibit what is referred to as sexual dimorphism (physical differences) in body size between males and females. Adult male fishers weigh 8 to 16 pounds and measure approximately 3 feet from head to tail. Adult female fishers average 20% smaller than males, weighing 4 to 6 pounds and measuring approximately 2 1/2 feet in length. In both males and females, the tail accounts for approximately 1/3 of the total body length. The fisher exhibits the typical “weasel” shape with a long, slender body, short legs, and furred tail. They have a pointed face (although not as pronounced as fox or coyote) with large, rounded ears set close to the head. They are well adapted for climbing and have sharp, retractable claws similar to those of domestic cats. Their coloration is generally a rich brown to black with grizzled grayish coloring on the head and shoulders and the darkest coloring occurring on the rump, tail, and legs. Individuals may also have irregular white patches of fur on their chest and lower abdomen.
Fishers breed in February to March and exhibit a reproductive strategy referred to as delayed implantation. The adult female breeds within days of giving birth, and the fertilized egg remains dormant for the next 10 to 11 months. The fertilized egg then implants in the uterine wall and development begins. The young kits are born 1 to 2 months after implantation occurs. Female fishers produce 1 litter each year consisting of 1 to 4 kits with an average litter size of 3 kits. Fishers reach sexual maturity at 1 year of age and females produce their first litter at the age of 2.
The young are born helpless, blind, and sparsely furred. Maternal dens, located in a tree cavity high in a large tree, are used for the first 8 to 10 weeks. Once the kits become mobile, they are moved from the maternal den to one on or below the ground. It is believed that the maternal den functions to protect the helpless young from aggressive male fishers and ground predators. The female nurses the kits until they reach 4 months of age at which time solid food is presented to them. By 5 months of age the kits are approximately the same size as the adult female and have begun to learn how to kill prey. The young remain with the female until late summer or early fall at which time they disperse to begin their solitary lives as adult fishers.
Fishers are shy and elusive animals that are rarely seen even in areas where they are abundant. They can be active day or night and tend to exhibit crepuscular (dawn and dusk) and nocturnal activity in the summer and diurnal (daytime) activity in the winter. They remain active year round and do not hibernate. Their preferred habitat is mixed forest with heavy canopy cover as they tend to avoid traveling in large open areas. They commonly use hollow logs, stonewalls, tree cavities, and brush piles as resting sites.
Fishers are omnivorous. Their primary foods include small rodents, squirrels, rabbits, birds, eggs, fruit, porcupines, and carrion. They will also opportunistically prey on poultry and domestic cats. Although they are proficient climbers, most of their hunting takes place on the ground.
Reference: Massachusetts Dept. of fish and Game (http://www.mass.gov/dfwele/dfw/wildlife/living/living_with_fisher.htm)