In the food hierarchy of the sea, large sharks often swim at the top. However, even the fierce great white sharks have enemies in the water. Killer whales that weigh around 5 tons (4,535 kilograms) have eaten great whites before [source: Cawardine]. Crocodiles, seals and other sharks may also prey on younger sharks.
By far, sharks' most threatening predator is humans. Shark meat has become increasingly popular as food, and manufacturers also use shark cartilage in some medicines. In 2007, the United States exported 4,160 tons of shark and dogfish meat [source: National Marine Fisheries Service]. Although the United States imported 32 tons of shark fin in 2007, it, along with Canada, New Zealand, Australia and South Africa, has banned the fishing tactic of catching sharks, slicing off their fins and tossing them back in the water [source: National Marine Fisheries Service].
Because of the success of commercial shark fishing and the problems with catching sharks as a byproduct of other fishing ventures, certain species populations have dwindled recently. Some shark experts have estimated that the overall number of sharks in the water has dropped as much as 90 percent [source: Kelly]. They also fear the decline because sharks reproduce at a much slower rate than other fish because most species carry their pups like mammals. At the same time, commercial shark fishing advocates argue that the business profits fishermen and also protects people from deadly shark attacks.
In reality, the widespread concern about shark attacks is overblown in relation to the incidence rate. People die each year from gruesome shark-related injuries, but only 30 of the nearly 400 species have ever made contact with humans [source: Florida Museum of Natural History]. Of those 30 or so species, shark specialists consider around a dozen as potentially harmful to people. Because of the higher concentration of muscle in humans' bodies, sharks will sometimes take a bite and release a victim since they prefer fattier foods [source: Klimely].
For more information on sharks and other sea life, read the links on the next page.