Bull shark
The bull shark, also known as the Zambezi shark in Africa and Lake Nicaragua shark in Nicaragua, is a species of requiem shark commonly found worldwide in warm, shallow waters along coasts and in rivers. It is known for its aggressive nature, and presence mainly in warm, shallow brackish and freshwater systems including estuaries and lower reaches of rivers. Their aggressive nature has led to ongoing shark-culling efforts near beaches to protect beachgoers, which is one of the causes of bull shark populations continuing to decrease. Bull sharks are listed as vulnerable on the IUCN Red List.
Bull sharks are euryhaline and can thrive in both salt and fresh water. They are known to travel far up rivers, and have been known to travel up the Mississippi River as far as Alton, Illinois, about from the ocean, but few freshwater interactions with humans have been recorded. Larger-sized bull sharks are probably responsible for the majority of nearshore shark attacks, including many incidents of shark bites attributed to other species.
Unlike the river sharks of the genus Glyphis, bull sharks are not true freshwater sharks, despite their ability to survive in freshwater habitats.
This shark appears in the image of the 2000 colones banknote from Costa Rica.
Etymology
The name "bull shark" comes from the shark's stocky shape, broad, flat snout, and aggressive, unpredictable behavior. In India, the bull shark may be confused with the Sundarbans or Ganges shark. In Africa, it is also commonly called the Zambezi River shark, or just "zambi".Its wide range and diverse habitats result in many other local names, including Ganges River shark, Fitzroy Creek whaler, van Rooyen's shark, Lake Nicaragua shark, river shark, freshwater whaler, estuary whaler, Swan River whaler, cub shark, and shovelnose shark.
Evolution
Some of the bull shark's closest living relatives do not have the capabilities of osmoregulation. Its genus, Carcharhinus, also includes the sandbar shark, which is not capable of osmoregulation.The bull shark shares numerous similarities with river sharks of the genus Glyphis, such as its ability to inhabit freshwater. However, the two genera have distinct taxonomic placements within the Carcharhinidae, with the bull shark being nested within the genus Carcharhinus, while the river sharks are sister to the genus Lamiopsis. This suggests that their similar physiologies convergently evolved.
The earliest fossil teeth of the bull shark are known from the Early Miocene of Egypt and Peru. They start to become much more widespread in geologic formations worldwide from the Middle Miocene onwards.
Anatomy and appearance
Bull sharks are large and stout, with females being larger than males. The bull shark can be up to in length at birth. Adult female bull sharks average long and typically weigh, whereas the slightly smaller adult male averages and. While a maximum size of is commonly reported, a single record exists of a female specimen of exactly. A long pregnant individual reached. The maximum weight of the long pregnant female can be over, ranking it among the largest of the requiem sharks. Bull sharks are wider and heavier than other requiem sharks of comparable length, and are grey on top and white below. The second dorsal fin is smaller than the first. The bull shark's caudal fin is longer and lower than that of the larger sharks, and it has a small snout, and lacks an interdorsal ridge.Bull sharks have a bite force up to, weight for weight the highest among all investigated cartilaginous fishes.
Exceptional specimens
In early June 2012, off the coast of the Florida Keys near the western part of the Atlantic Ocean, a female believed to measure at least and was caught by members of the R.J. Dunlap Marine Conservation Program. In the Arabian Sea, off the coast of Fujairah in the United Arab Emirates, a pregnant shark weighing and measuring long was caught in February 2019, followed by another specimen weighing about and measuring about the same in length, in January 2020. Unconfirmed reports suggest that the very largest, exceptional specimens can possibly weigh up to.Distribution and habitat
The bull shark is commonly found worldwide in coastal areas of warm oceans, in rivers and lakes, and occasionally salt and freshwater streams if they are deep enough. It is found to a depth of, but does not usually swim deeper than. In the Atlantic, it is found from Massachusetts to southern Brazil, and from Morocco to Angola.Populations of bull sharks are also found in several major rivers, with more than 500 bull sharks thought to be living in the Brisbane River. One was reportedly seen swimming the flooded streets of Brisbane, Queensland, Australia, during the 2010–11 Queensland floods. Several were sighted in one of the main streets of Goodna, Queensland, shortly after the peak of the January 2011, floods. A large bull shark was caught in the canals of Scarborough, just north of Brisbane within Moreton Bay. Still greater numbers are in the canals of the Gold Coast, Queensland. In the warmer months of the year, bull sharks frequent Sydney Harbour. In the Pacific Ocean, it can be found from Baja California to Ecuador.
The bull shark has traveled up the Amazon River to Iquitos in Peru and north Bolivia. It also lives in freshwater Lake Nicaragua, in the Ganges and Brahmaputra Rivers of West Bengal, and Assam in Eastern India and adjoining Bangladesh. It can live in water with a high salt content as in St. Lucia Estuary in South Africa. Bull sharks have been recorded in the Tigris River since at least 1924 as far upriver as Baghdad, and has been rumored to also inhabit the Cahora Bassa lake upstream of the Zambezi. The species has a distinct preference for warm currents.
After Hurricane Katrina, many bull sharks were sighted in Lake Pontchartrain. In July 2023 some local fishermen in the Atchafalaya River have reported increasing numbers. Bull sharks have occasionally gone as far upstream in the Mississippi River as Alton, Illinois. Bull sharks have also been found in the Potomac River in St. Mary's County, Maryland. From 1996 to 2013, a golf course lake at Carbrook, Logan City, Queensland, Australia was the home to several bull sharks. They were trapped following a flood of the Logan and Albert Rivers in 1996, and resided in the lake until 2013, when they disappeared after another series of floods. The golf course capitalized on the novelty, changing their logo to feature the sharks and hosting a monthly tournament called the "Shark Lake Challenge".
Behavior
Freshwater tolerance
The bull shark is the best known of 43 species of elasmobranch, across 10 genera and four families, to have been reported in fresh and/or brackish water. Other species that enter rivers include the stingrays and sawfish. Some skates, smooth dogfishes, and sandbar sharks regularly enter estuaries.The bull shark is diadromous, meaning they can swim between salt and fresh water with ease, as they are euryhaline fish—able to quickly adapt to a wide range of salinities. Thus, the bull shark is one of the few cartilaginous fishes that have been reported in freshwater systems. Many of the euryhaline fish are bony fishes, such as salmon or tilapia, and are not closely related to bull sharks. Evolutionary assumptions can be made to help explain this sort of evolutionary disconnect, with one being that the bull shark experienced a population bottleneck during the last ice age. This bottleneck may have separated the bull shark from the rest of the Elasmobranchii subclass and favored the genes for an osmoregulatory system.
Elasmobranchs' ability to enter fresh water is limited because their blood is normally at least as salty as seawater through the accumulation of urea and trimethylamine oxide, but bull sharks living in fresh water show a significantly reduced concentration of urea within their blood. Despite this, the solute composition of a bull shark in fresh water is still much higher than that of the external environment. This results in a large influx of water across the gills due to osmosis and loss of sodium and chloride from the shark's body. However, bull sharks in fresh water possess several organs with which to maintain appropriate salt and water balance; these are the rectal gland, kidneys, liver, and gills. All elasmobranchs have a rectal gland which functions in the excretion of excess salts accumulated as a consequence of living in seawater. Bull sharks in freshwater environments decrease the salt-excretory activity of the rectal gland, thereby conserving sodium and chloride. The kidneys produce large amounts of dilute urine, but also play an important role in the active reabsorption of solutes into the blood. The gills of bull sharks are likely to be involved in the uptake of sodium and chloride from the surrounding fresh water, whereas urea is produced in the liver as required with changes in environmental salinity. Recent work also shows that the differences in density of fresh water to that of marine waters result in significantly greater negative buoyancies in sharks occupying fresh water, resulting in increasing costs of living in fresh water. Bull sharks caught in freshwater have subsequently been shown to have lower liver densities than sharks living in marine waters. This may reduce the added cost of greater negative buoyancy.
Bull sharks are able to regulate themselves to live in either fresh or salt water. It can live in fresh water for its entire life, but this does not happen, mostly due to the reproductive needs of the shark. Young bull sharks leave the brackish water in which they are born and move out into the sea to breed. While is theoretically possible for bull sharks to live purely in fresh water, experiments conducted on bull sharks found that they died within four years. The stomach was opened and all that was found were two small, unidentifiable fishes. The cause of death could have been starvation since the primary food source for bull sharks resides in salt water.
In a research experiment, the bull sharks were found to be at the mouth of an estuary for the majority of the time. They stayed at the mouth of the river independent of the salinity of the water. The driving factor for a bull shark to be in fresh or salt water, however, is its age; as the bull shark ages, its tolerance for very low or high salinity increases. The majority of the newborn or very young bull sharks were found in the freshwater area, whereas the much older bull sharks were found to be in the saltwater areas, as they had developed a much better tolerance for the salinity. Reproduction is one of the reasons why adult bull sharks travel into the river—it is thought to be a physiological strategy to improve juvenile survival and a way to increase overall fitness of bull sharks. The young are not born with a high tolerance for high salinity, so they are born in fresh water and stay there until they are able to travel out.
Initially, scientists thought the sharks in Lake Nicaragua belonged to an endemic species, the Lake Nicaragua shark. In 1961, following specimen comparisons, taxonomists synonymized them. Bull sharks tagged inside the lake have later been caught in the open ocean, with some taking as few as seven to 11 days to complete the journey.
A study of six bull sharks confined to a stagnant golf course lake in Brisbane, Australia, from 1996 to 2013 uncovered their adaptability to low-salinity environments, marking the longest recorded residency for the species under such conditions, and demonstrating their ability to live indefinitely in low-salinity aquatic environments.