Pacific sleeper sharks are designed for stealth. Their eyesight is probably poor, but good eyesight is not necessary since they have an exceptional “sixth sense" to detect very slight electromagnetic fields. Muscle activity, even the beating of an animal’s heart or the movement of its diaphragm, emits an electrical signal that the sharks use to detect, locate and attack their victims. Under cover of darkness prey would be less likely to detect a sleeper shark coming up from the depths. Prey emits electromagnetic signals that guide the shark right in. For sleeper sharks, darkness is not a deterrent to detecting prey, but instead a cover or camouflage.
Pacific sleeper shark teeth are quite different in the lower jaw compared to the upper jaw. The upper jaw has small, sharp, conical teeth much like those in halibut. These are used to seize and hold prey. The teeth in the lower jaw are interlocking, forming a serrated blade used for slicing. Sleeper shark bite marks resemble large three-quarter moons or slices.
Sleeper sharks attack suddenly and without warning. A harbor seal might be floating on the surface of the ocean trying to catch its breath when it is attacked from below by a 400-pound (181-kg) sleeper shark. A bite to its midsection and the seal is eviscerated and struggling for its life. At minimum, the shark gets a large chunk of skin and blubber, likely enough to cause the seal soon to die. However the shark will finish the job by ripping the seal to bits, eating and digesting the entire animal. Smaller animals such as adult chum salmon or black cod usually are eaten whole.
Halibut and black cod struggling on a fisherman’s long line (bottom-set line with hundreds of baited hooks) attract sleeper sharks. The struggling fish emit signals that the sharks can detect from long distances. The sleeper sharks bite chunks out of the halibut. When sharks try to eat the cod whole, the same hook that caught the cod may hook the sharks. The struggling sleeper sharks tangle and damage commercial fishing gear, forcing many fishermen to change fishing areas.