At great distances, a shark may become aware of a wounded fish by detecting vibrations caused by its unusual swim pattern. When the shark moves closer, perhaps still hundreds of feet away, it may pick up the scent of the wounded fish's body fluids. Following the scent still closer to about two to three hundred feet away, the shark uses its eyesight to identify the prey and begin its final approach. At the last moment before the attack, at a few feet away, the shark covers or rolls its eyes back in its head and lets its electro-reception take over as a means of identifying its victim. One of a shark's major supersenses is vibration and pressure detection.
Sharks have what is known as a lateral line system .
This is a system of sensory pits that extend along a line stretching from behind the gill slits to the tail on both sides of the shark. These pits contain nerve endings, which are very sensitive to vibrations in the water. Sharks can hear sounds within a human's hearing range and, using their lateral line, they can also detect low frequency vibrations such as those made by struggling or wounded fish. The inner ears contain canals, which are probably used in motion detection and they also contain vibration sensors that work with the lateral line system. Sharks' nostrils are located on the underside of their snouts where they are used for smelling, not for breathing.