Bull shark

• 1. Introduction

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  The bull shark, Carcharhinus leucas, has been found as far up the Mississippi River as Alton, Illinois. That is about 700 miles from the ocean. A shark that size, in a muddy American river, sounds like a mistake. Yet the bull shark is a requiem shark that turns up in warm, shallow water along coasts all over the world. It also turns up in brackish estuaries and in the lower reaches of rivers. People have given it a long list of local names, and it has a reputation for being aggressive. This documentary asks how an animal built for the sea manages to cross into fresh water. It asks how the species lives, hunts and raises its young in places where other sharks cannot follow. It also asks why a predator with such a fearsome name is now in trouble, and what people are trying to do about it.

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• 2. Salt and Fresh Water

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  The bull shark is the best known of 43 species of elasmobranch, spread across 10 genera and four families, that have been reported in fresh or brackish water. Elasmobranchs are the sharks, rays and their relatives. Most of them struggle in fresh water because their blood is normally at least as salty as seawater. They hold that saltiness by accumulating urea and trimethylamine oxide. A bull shark living in fresh water carries a significantly lower concentration of urea. Even so, its blood stays saltier than the water around it. Water therefore floods in across the gills by osmosis, while sodium and chloride leak out. To cope, the shark relies on four organs. Its rectal gland, which normally dumps excess salt, cuts back its activity so that sodium and chloride are conserved. The kidneys produce large amounts of dilute urine, yet they also pull solutes back into the blood. The gills probably take up sodium and chloride from the surrounding water. The liver makes urea as the salinity changes. Fresh water brings another cost. It is less dense than seawater, so a shark sinks more easily in it. Bull sharks caught in fresh water have lower liver densities than those from the sea, which may soften that penalty. The species is euryhaline, meaning it can adapt quickly to a wide range of salinities. It is also diadromous, moving between salt and fresh water with ease. Its relatives often cannot do this. The sandbar shark, a member of the same genus, has no ability to osmoregulate. The river sharks of the genus Glyphis share the freshwater talent. Yet the two groups sit in different branches of the shark family tree, so the skill appears to have evolved separately in each. One suggested explanation involves the last ice age. A population bottleneck may have cut the bull shark off from other elasmobranchs and favored genes for an osmoregulatory system. The earliest known fossil teeth come from the Early Miocene of Egypt and Peru. They become far more widespread from the Middle Miocene onward. The ability has limits. Experiments that kept bull sharks in pure fresh water found that they died within four years. One animal's stomach held only two small, unidentifiable fishes, so starvation may have been the cause. The sharks' main food lives in salt water.

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• 3. Rivers and Floods

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  A bull shark has swum 4000 km up the Amazon River to Iquitos in Peru. Others have been recorded in the Tigris since at least 1924, reaching as far as Baghdad. The species lives in the Ganges and Brahmaputra of West Bengal and Assam, and in the St. Lucia Estuary in South Africa, where the water is very salty. Rumors place it in the Cahora Bassa lake upstream on the Zambezi. In Brisbane, Australia, more than 500 bull sharks are thought to live in the river. During the 2010-11 Queensland floods, one was reportedly seen swimming through flooded streets. Several were sighted on a main street in Goodna shortly after the January 2011 flood peak. Even more live in the canals of the Gold Coast, and a large one was caught in the canals of Scarborough. In the warmer months they frequent Sydney Harbour. Storms push them into surprising places. After Hurricane Katrina, many bull sharks were sighted in Lake Pontchartrain. In July 2023, fishermen on the Atchafalaya River reported growing numbers. Bull sharks have also been found in the Potomac River in St. Mary's County, Maryland. The strangest home was a golf course lake at Carbrook, Logan City, Queensland. A flood of the Logan and Albert Rivers in 1996 trapped several bull sharks in the 51-acre lake. They stayed until 2013, when another series of floods let them leave. The golf course put the sharks on its logo and ran a monthly tournament called the Shark Lake Challenge. A study of six of these sharks recorded the longest residency of the species in such conditions. It showed they can live indefinitely in low-salinity water. Lake Nicaragua once seemed to hold its own species, the Lake Nicaragua shark, Carcharhinus nicaraguensis. In 1961, taxonomists compared specimens and decided it was a bull shark. Sharks tagged in the lake have since been caught in the open ocean, and the reverse has happened too. Some made the trip in as few as seven to 11 days. This wide range explains the pile of local names, among them Zambezi shark, informally zambi, and Ganges River shark. Others are Fitzroy Creek whaler, van Rooyen's shark, Swan River whaler, cub shark and shovelnose shark. The standard name comes from its stocky shape, broad flat snout and unpredictable temper. Costa Rica even printed the animal on its 2000 colones banknote.

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• 4. Size and Strength

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  A female bull shark averages 2.4 m long and typically weighs 130 kg. The slightly smaller male averages 2.25 m and 95 kg. A newborn can measure up to 81 cm. Compared with other requiem sharks of similar length, the bull shark is wider and heavier. Its back is grey and its belly is white. Its second dorsal fin is smaller than the first, and it has no interdorsal ridge. A length of 3.5 m is commonly reported as the maximum. A single record describes a female of exactly 4.0 m. A pregnant female of 3.25 m reached 450 kg, and the 4.0 m female could weigh over 600 kg. That would rank her among the largest of the requiem sharks. Some catches are startling. In early June 2012, members of the R.J. Dunlap Marine Conservation Program caught a female off the Florida Keys. She was believed to measure at least 8 ft and weigh 800 to 850 lbs. Off Fujairah in the United Arab Emirates, a pregnant shark of 347.8 kg and 3 m was caught in February 2019. A second, around 350 kg and about the same length, followed in January 2020. Unconfirmed reports say the very largest specimens might reach 900 kg. Strength matters as much as bulk. Measured weight for weight, the bull shark's bite force of up to 5914 N is the highest among all cartilaginous fishes investigated. That bite is the tool for a very flexible diet.

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• 5. Hunter and Prey

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  Bull sharks hunt in murky water, where prey has a harder time seeing them approach. The usual method is a bump-and-bite. After the first contact, the shark keeps biting and tackling until the prey can no longer flee. Fish and small sharks make up most of the diet, including other bull sharks, along with stingrays. The menu also covers turtles, birds, dolphins, land mammals, crustaceans and echinoderms. The shark mostly works alone. Now and then two bull sharks briefly pair up to make hunting and tricking prey easier. Like other sharks, they eat in short bursts. When food is scarce, they digest for a much longer period to avoid starvation. A bull shark under threat has a trick of its own. It will regurgitate the contents of its stomach. If the predator turns to eat the discarded food, the bull shark can slip away. Few animals threaten an adult. Larger sharks such as the tiger shark and the great white may attack bull sharks, but they typically target juveniles. Crocodiles can be a danger in rivers. Saltwater crocodiles have been seen preying on bull sharks in the rivers and estuaries of Northern Australia. A Nile crocodile was reportedly sighted eating one in South Africa. Humans are the largest threat of all.

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• 6. Nurseries and Young

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  Bull sharks mate in late summer and early autumn, often in bays and estuaries. Courtship has not been observed in detail. The male likely bites the female on the tail until she turns upside down, and mature females commonly carry scratches. A bull shark gestates for 12 months, then gives birth to between 1 and 13 live young. The pups swim free at birth at about 70 cm. The mother does not rear them. Instead they are born into flat, sheltered places such as coastal lagoons, river mouths and other low-salinity estuaries. Young bull sharks are not born tolerating high salinity, so they stay in fresh water until they can travel out. Researchers who studied the species found that age drives the choice of water. Newborns and very young sharks were mostly in the freshwater areas. Much older sharks were in saltwater areas, because their tolerance had grown. Adults travel into rivers to breed, which is thought to be a strategy that improves juvenile survival and overall fitness. In 2008, researchers tagged young bull sharks in the Caloosahatchee River estuary to see what moved them. The sharks traveled downriver together when conditions changed. The cause was not predators. The sharks were saving energy. They rode the changing tidal flow downstream and reduced the energy needed to osmoregulate. A male can begin reproducing around the age of 15 years, but a female cannot start until she is 18.

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• 7. Danger to People

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  In 1916, a string of attacks along the Jersey Shore killed swimmers, and bull sharks may have been involved. The suspicion rests on two fatal bites in brackish and fresh water. Many people believe those attacks inspired the novel Jaws. Its author, Peter Benchley, has said they did not. Bull sharks often dwell in very shallow water, and they live in many habitats. They are territorial and have no tolerance for provocation. For these reasons they may be more dangerous to humans than any other shark. They are one of three species, with the tiger shark and the great white, most likely to bite people. Larger bull sharks account for a large share of nearshore attacks. They may also be behind many attacks where the species was never identified. They have attacked swimmers around the Sydney Harbour inlets and humans off the coast of Florida. In India, bull sharks swim up the Ganges, Brahmaputra, Mahanadi and other rivers, and have bitten bathers. Many of those bites were blamed on the Ganges shark, Glyphis gangeticus, a critically endangered river shark. During the 1960s and 1970s, the sand tiger shark was also blamed. Few interactions in fresh water have actually been recorded. Even so, concern for beachgoers has shaped how people treat the species.

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• 8. Threats and Protection

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  The International Union for Conservation of Nature, in its IUCN Red List, classes the bull shark as vulnerable, and its numbers are falling. It has no protected status. The threats are many. Sharks get caught in fishing nets. People overfish them for oil, skin and meat. Pollution damages their habitat. In many parts of the world, including Australia and South Africa, shark-culling measures operate near beaches to prevent attacks on swimmers. These efforts are one reason the population keeps decreasing. Researchers have looked for gentler tools. One test involved the SharkSafe Barrier, which used rows of piping to create a continuous magnetic field plus visual stimuli to deter sharks. Researchers concluded it needed more improvement and testing before it could serve as a reliable safety measure. Vision offers another clue. Bull sharks can tell colors of underwater mesh netting apart, and they tended to avoid bright colors rather than shades that blended with the water. Bright yellow mesh was easily avoided. Other conservation work focuses on knowledge. The Nature Conservancy has been satellite tagging bull sharks to track their migrations and find their habitats. The goal is to show which areas most need protection.

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