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Freshwater Sharks

Most people think that sharks are only found in salt water. That's not true. There are 43 species of elasmobranch (skates, rays and sharks) in 10 genera and four families that penetrate freshwater environments in Australia, Southeast Asia, western Africa, eastern South America, Central America, and southeastern parts of North America. Included in this figure are six species of shark in the genus Glyphis (river shark) and the bull shark. This page describes the sharks that can be found in fresh water.

River Sharks

River sharks are medium-sized whaler sharks that generally all share the following physical characteristics:

  • Greyish to brown bodies lacking a distinctive color pattern (fairly uniform in color);
  • Short, broadly rounded snouts (the length of the snout is less than the mouth width);
  • Small, widely spaced nostrils;
  • Small, dark eyes;
  • Erect, broadly-triangular, serrated upper teeth;
  • Very short labial furrows (lip grooves), restricted to the jaw corners;
  • A broad dorsal fin with the mid-base closer to the base of the pectoral fins than those of the pelvic fins;
  • A much smaller second dorsal fin; and
  • An anal fin that is somewhat smaller than the second dorsal fin.

Very little is known about river sharks because of their secretive habits. They are believed to be viviparous, meaning they give birth to live young that have been nourished inside the mother shark. The only identified river shark species are:

  1. Ganges Shark (Glyphis gangeticus) found in the Hooghly-Ganges river system, West Bengal, India. This species has been often confused with the aggressive bull shark, a salt-water shark that can also tolerate fresh water for extended periods of time.
  2. Speartooth Shark (Glyphis glyphis) found in Borneo, New Guinea, and Queensland, Australia.
  3. Irrawady River Shark (Glyphis siamensis) found in the Ayeyarwady (Irriwaddy) River, near Yangon (Rangoon).
  4. Bizant River Shark (Glyphis, species "A") found in the tropical waters of the in the Adelaide River Northern Territory, Australia and the lower reaches of the Bizant River in Queensland, Australia, where it occurs along with the bull shark.
  5. Borneo River Shark (Glyphis, species "B") found in the Kinabatangan River of Sabah, in northern Borneo in Indonesia.
  6. New Guinea River Shark (Glyphis, species "C") found in New Guinea.

Bull Sharks

The bull shark (Carcharhinus leucas) is also known as the bull whaler, cub, Ganges, Nicaragua, river, Swan River Whaler, Zambezi, shovelnose, slipway grey, square-nose, and Van Rooyen's shark. It is known as the Zambezi shark or Zambi in Africa and Nicaragua shark in Nicaragua. The bull shark is a salt-water shark, but it can also tolerate fresh water for extended periods of time.

The ability of elasmobranchs to enter fresh water is limited because their blood is normally at least as salty (in terms of osmotic strength) as seawater, through the accumulation of urea and trimethylamine oxide. But, bull sharks have special adaptations in the way their kidneys function and special glands near their tails (National Geographic) that allow them to live up to about 6 years in freshwater without any negative effects on their health. These adaptations reduce the concentration of urea and trimethylamine oxide by up to 50%. But, bull sharks living in fresh water need to produce twenty times more urine than those living in salt water. There is evidence that bull sharks can breed in freshwater, but not as regularly as they do in estuarine and marine habitats.

The bull shark is common worldwide in warm, shallow waters along coasts and in rivers, lakes and occasionally streams. Bull sharks been found up to 1,750 miles (2800 km) up the Mississippi River (as far north as Illinois) and Atchafalaya River in the USA and 2,500 miles (4000 km) up the Amazon River in Peru. It has been found in Lake Nicaragua (Central America) and the Zambezi River (Africa).

In the Western Atlantic, bull sharks have been spotted off the coast of Massachusetts, USA and down to southern Brazil. On the Eastern side of the Atlantic the bull shark lives off the coast of Morocco and from Senegal to Angola. The bull shark is also found in the Indo-West Pacific where it inhabits the region from Kenya and South Africa to India, and further into the waters around Australia and Vietnam. On the Western side of the South and North American continents, the bull shark lives from southern Baja California in Mexico and down to Ecuador.

Young bull sharks are particularly found of fresh water and are sometimes found hundreds of miles from the ocean in rivers and streams. The adult bull sharks prefer the region where fresh water rivers empty into the sea, such as river deltas and estuaries (semi-enclosed coastal bodies of water with one or more rivers or streams flowing into it, and with a free connection to the open sea). Coastal lagoons, river mouths, and other low-salinity estuaries are common birthing and nursery habitats. Sometimes, hungry females travel to freshwater where there is less competition for food. And, some bull sharks complete their lifecycles in fresh water, as well.

Bull sharks get their name from their short, blunt snout, as well as their aggressive disposition and a tendency to head-butt their prey before attacking. They are much wider in comparison to their length than other sharks, and their snouts are wider than they are long. Bull sharks are extremely territorial and will attack anything invading their territory, including people.

Out of about 375 species of shark, the bull shark is only of three species known to attack people. Because they swim in very shallow waters where people swim, bull sharks are one of the most frequent attackers of people and are considered the most dangerous sharks in the world. Their disproportionally large jaws and large, triangular serrated teeth deliver a deadly, shredding bite. The other two species known to attack people are the tiger shark and great white. Because great whites and tiger sharks live in deeper ocean waters, they are less dangerous to people than the bull shark.

Females grow larger than males. On average, adult males are about 7 feet (2.1 m) long weighing 200 pounds (90 kg). Adult females are about 11.5 feet (3.5 m) long weighing 500 pounds (230 kg). Bull sharks are gray on the top half of their bodies and white underneath. Some have pale stripes on the sides of their bodies. The bull shark has two dorsal fins: the characteristic first dorsal fin, which is wide and has a triangular shape, the second of which is much smaller than the first. Young sharks can be distinguished by the dark edges on their fins (Australian Museum 1999, Enchanted Learning 2000).

Bull Sharks are viviparous, giving birth to live young after nourishing them for 10 to 11 months inside their bodies. The pups are born in litters of up to 13 and are around 28 inches at birth. A common breeding place for the Bull Shark is the brackish water where freshwater rivers meet the saltwater oceans. (Microsoft Encarta 1997, Enchanted Learning 2000). Mating and birthing occurs year-round in the warmest parts of the bull shark's range, with a peak in spring and early summer. Females often bear courtship scars from being nipped at and grasped by males.

Bull sharks are opportunistic ambush hunters that hunt both day and night, and they'll eat almost anything. They hunt by themselves, rather than in schools like some shark species, such as the hammerhead. Bull sharks mainly eat fish, but sometimes eat rays, turtles, echinoderms, birds, mollusks, dolphins and even other sharks (usually young sandbar sharks). Remains of everything from humans to hippopotami have been found in bull sharks' stomachs (Australian Museum 1999, Bilson and Bilson 1999).

The bull shark's beady eyes are set far forward on their thick snouts, unlike tiger and blue sharks, which have large eyes that make them excellent sight hunters. Compared with these sharks the bull shark's eyesight is quite poor. But, it doesn't matter because they often hunt in murky waters where visual acuity is less of a factor. However, like all sharks, bull sharks have a keen sense of smell and can detect erratic movements from long distances. When zeroing in on prey, bull sharks will use a "bump and bite" technique to initially investigate a target it's not certain about or a more deadly rush attack where it delivers maximum damage immediately.

The bull shark is currently listed by the World Conservation Union (IUCN) as "Near Threatened", but they are not on the endangered species list. However, they are unted for food, their hides and for oils. Researchers believe their populations are shrinking. But, the bull shark is not legally protected in any part of its range.

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