Porcupine defence is directional. Quills cover the back, flanks and tail, and the animal responds to a threat by presenting its rear and tucking its head against a trunk or rock. The face, however, carries no quills. Fishers exploit this geometry by keeping in front, attacking the head repeatedly over what can be many minutes, and turning the porcupine only after it is disabled.
The fisher is not immune to quills, which is a common misunderstanding. Quills are found embedded in fisher carcasses, apparently without causing the widespread infection they would in many species, but the animal succeeds by tactics rather than invulnerability. It is one of very few predators that take porcupines with any regularity.
The fisher's other notable trait is descending trees head-first, which it does by rotating its hind feet close to 180 degrees so the claws still grip when pointed upward. The name is a misnomer inherited from European trappers, probably via a word for the polecat; fishers do not fish, and fish is a negligible part of their diet. Where fishers were removed, porcupines increased, though disentangling that from forestry change is difficult.