Only a small number of the predatory species are definitely known to engage in unprovoked attacks on humans. The largest and most feared of these is the great white shark, which may reach 20 ft (6 m) in length and is probably responsible for more such attacks than any other species. Other sharks reputed to be especially dangerous are the tiger and blue sharks and the mako. Sharks are extremely sensitive to motion and to the scent of blood. Swimmers in areas where dangerous varieties occur should leave the water quietly if they are cut; spearfishing divers should remove bleeding fish from the water immediately. In some places bathing areas are guarded by nets. A number of substances have been used as shark repellents, including maleic acid, copper sulfate, and decaying shark flesh, but their effectiveness is variable. An electrical repellent device, exploiting the shark's sensitivity to electrical fields, has been developed in South Africa. Sharks usually circle their prey before attacking. Since they seldom swim near the surface, an exposed dorsal fin is more likely to be that of a swordfish or ray than that of a shark.