Ben Achtenberg was able to snap a rare photograph of a fisher cat in Bussey Brook Meadow in the Arnold Arboretum on Halloween. Fishers are solitary hunters and not commonly seen. They primarily prey on snowshoe hares and porcupines, according to wikipedia. They also eat insects, nuts, berries, and mushrooms.
Scientists have found evidence of birds, small mammals and even deer, which would indicate they're also scavengers and will eat carrion. While not common, fishers have also been known to kill wild turkeys.