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Bull sharks (Scientific name: Carcharhinus leucas) are regarded as among the most aggressive and unpredictable sharks and considered potential man eaters. Rarely reaching a length of more than 3.4 meters (10 feet) or a weight of more than 230 kilograms (510 pounds), they eat a wide variety of things: bony fish, invertebrates, birds, mollusks, dolphins, mammals and other sharks, including young bull sharks.
Bull sharks are also known as cub sharks and ground sharks. They have the highest testosterone levels measured in any animal, including lions and elephants. They have two spineless dorsal fins, five pair of gill slits, a stubby rounded snout and large, angular pectoral fins.
Bull sharks have lower spiked teeth designed to hold prey in place and lower serrated teeth that tear flesh apart. George Burgess, director of the International Shark Attack File at the University of Florida, said their teeth are designed to incise chunks out of their prey and shear off flesh.” They usually ambush their prey and are not afraid to attack on prey as large as it is,” he said.
The average lifespan of bull shark in the wild is 13.7 to 32 years depending on the source. Their average lifespan in captivity is 30 years. Their longevity appears to be related to geographic region. Studies of bull sharks in the southern Gulf of Mexico indicate that they can reach an age of 28 years for females and 23 years for males. In the northern Gulf researchers found that female bull sharks live to be 24.2 years and the oldest male was 21.3 years. Researchers found a female bull shark off the eastern coast of South Africa that was 32 years. Two bull sharks housed at the South African aquarium were known to be 29 years old. Another bull shark at the Okinawa Churaumi Aquarium lived to be 30 years old. [Source: Kristi Cascio, Animal Diversity Web (ADW), Max Planck Institute for Demographic Research]
Websites and Resources: Shark Foundation shark.swiss ; International Shark Attack Files, Florida Museum of Natural History, University of Florida floridamuseum.ufl.edu/shark-attacks ; Tracking Sharks trackingsharks.com, which records all global shark attacks; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
See Separate Article BULL SHARK ATTACKS ioa.factsanddetails.com
Habitat and Where They Are Found
Bull sharks are often found close to shore, sometimes in very shallow water. They live in temperate, tropical, saltwater and freshwater environments and are found in reefs, lakes, rivers, coastal areas and brackish water as well as in estuaries and intertidal areas. They are typically found at depths of 2.4 to 150 meters (7.87 to 492.13 feet) at an average depth of 30 meters (98.43 feet). [Source:Kristi Cascio, Animal Diversity Web (ADW) /=]
Bull sharks are widely distributed across the globe. They found along the coasts of the Africa, India, Australia, some Pacific and Indian Ocean islands, United States, Mexico, and Central and South America. On the west coast of North America they can be found from Baja California south to Ecuador. On the east coast of the United States their range extends from the Chesapeake Bay south through the Gulf of Mexico and further southward to southern Brazil.
Bull sharks also inhabit many estuaries and ocean inlets where saltwater meets freshwater, rivers. They give birth in estuaries, which serve as nurseries for the young sharks. The young remain in estuaries until temperatures drop seasonally, and the young move to the saltwater environs. Many juvenile sharks have been found in areas in southwest Florida, such as the Indian River Lagoon System, Charlotte Harbor, Caloosahatchee River, San Carlos Bay, and the lower Pine Island sound.
Freshwater Bull Sharks
Also known as river whalers and freshwater whalers, bull sharks live mostly in salt water but periodically venture into lakes and rivers as far away as 2,000 miles from the nearest salt water. They have been observed near St. Louis on the Mississippi River and 3,000 kilometers upstream in the Amazon. In 1937 two fishermen caught an 84-pound bull shark near Altin, Illinois, 1740 miles from the Gulf Of Mexico.
Bull sharks are the only big sharks that can go into fresh water and cope with great changes of salt to fresh water and are the only large shark species that can tolerate prolonged periods in freshwater environments.. When bull sharks enter rivers the roam at an average depth of just 2.4 meters. Their linear home ranges vary from 0.9-5.6 kilometers in rivers. They spend their day moving upstream and swim near the bottom of the river. At night they move downstream and swim at the top of the river.
Freshwater bull sharks are found in Lake Izabel in Guatemala and Lake Nicaragua. Scientists speculate the sharks entered the lakes through rivers or possible they date back to a time before the Ice Age when these lakes were part of the ocean. Sharks have also been reported in the Ganges in Pakistan and India and the Tigris and Euphrates in the Middle East. [Source: Nathaniel Kenney, National Geographic, February 1968]
Bull Shark Physical Characteristics
Bull shark females are larger than males. Females range in weight from 53 to 111 kilograms (117 to 245 pounds). The average weight for males is 95 kilograms (200 pounds; for females, 111 kilograms (245 pounds). They range in length from 193 centimeters for an immature male and 189 centimeters for an immature female to 228 centimeters for mature male and 242 centimeters for a mature female. At birth, bull sharks weigh 1.5- to 3.0 kilograms and are approximately 60- to 71 centimeters in length. Immature bull sharks of both sexes weigh around 53 kilograms. Females reach sexual maturity around at age 18; males around age 14.[Source: Kristi Cascio, Animal Diversity Web (ADW) /=]
According to Animal Diversity Web: Bull sharks have very short snouts and the adults are a light to dark gray on the dorsal side and white on the ventral side. Juveniles are a brownish gray color and have black tips on the pelvic, second dorsal, anal, and tail fins. Bull sharks teeth are a broad jagged triangle up top and a thin jagged triangle along the jawline. The shark’s placoid scales are overlapping, sharp, pointed triangles that effectively protect them. These scales are thought to be hydrodynamic and assist with efficient swimming./=\
Adult size varies depending on geographical region. For example, North American bull sharks are larger than those located in the Caribbean, Costa Rica, or Nicaragua. Females in North America are an average of 284centimeters and males are an average of 270 centimeters. Alternately, the largest female shark found in Costa Rica measured 251 centimeters and the largest male was 241 centimeters. ("Bull sharks", 2009; Castro, 1996; Clark and von Schmidt, 1965; Snelson, et al., 1984) /=\
Bull Shark Behavior, Perception and Communication
Bull sharks are diurnal (active during the daytime), nocturnal (active at night), motile (move around as opposed to being stationary), nomadic (move from place to place, generally within a well-defined range) and migratory (make seasonal movements between regions, such as between breeding and wintering grounds). They spend a large amount of time in backwater habitats and estuaries. Home ranges in oceanic environments have not been documented (in river see above). [Source: Kristi Cascio, Animal Diversity Web (ADW) /=]
According to Animal Diversity Web: Members of the genus Carcharhinus are considered very aggressive. Bull sharks are known to be so violent that many aquariums do not display them; they tend to kill everything in their environment. A study was conducted that studied the attack behavior of grey reef sharks. The sharks’ body starts to move in a spinning and winding motion. At the same time, its body moves in a back and forth motion. They swim in an almost disoriented way instead of a fluid motion. Their path of motion is in a circular pattern, with their snout pointing upward. It is thought that bull shark attacks may follow similar behavioral patterns.
Some bull shark are seasonally migratory. Those that inhabit the east coast of the United States, spend summers in northern latitudes and move southward again when waters cool. Sharks behavior may also vary according to habitat type. For example, sharks are active throughout the daytime in riverine systems, moving upstream. Their nocturnal (active at night), behavior returns them to their start point, returning downstream. Habitat choice in this species also seems to be linked to water temperature, and sharks will vacate cooler temperate waters when they are no longer optimal.
Bull sharks communicate with vision and chemicals usually detected by smelling. They also employ pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species) and sense using vision, sound, vibrations, electric signals and chemicals that can be detected with smell. Bull sharks have a keen sense of hearing. They are very efficient at detecting sound between 400-600 Hz but can not hear frequencies between 100-1500 Hz. This capability helps sharks detect potential prey at distances over six meters (20 feet) away. Reports on species within Carcharhinus indicate the use of their olfactory system to detect the opposite sex. Scientists suggest these sharks release a pheromone and the olfactory system is capable of identifying the smell.
Range of bull shark (Carcharhinus leucas)
Bull Shark Food, Eating Behavior and Predators
Bull sharks feed mainly on sea catfish such as hardhead catfish and Gafftopsail catfish as well as stingrays such as Atlantic stingray and bluntnose stingray (Say's stingray). Their diet can sometimes includes blue crabs, menhaden (sardine-like fish) and mullet. Bull sharks occasionally eat sheephead (a kind of fish), jacks, pompanos, weakfish, and even other bull sharks. [Source: Kristi Cascio, Animal Diversity Web (ADW) /=]
Bull sharks’ diet follow a general pattern as they mature. Younger ones feed on relatively small. fish. As they get older they start to consume sharks, rays, and skates and bottom-dwelling fish.
Bull sharks are known to occasionally eat smaller mammals, birds, mollusks, crustaceans, and even turtles. There have been recorded incidents of cannibalism. This usually occurs when at least six sharks are present and one of them — the prey — is injured.
Their main known predators of bull sharks are humans and other larger sharks Bull sharks are at the top of the food chain and have very few predators. Humans catch the for their fins and meat. It is not unusual to find bull shark accompanied by remora. If a shark is infected with parasites it changes its behavior to signal to the remora that it needs ready to be cleaned. The shark slows down its swimming speed and swims with its head raised. The remora will then approach and clean all around the shark’s body, gills, and inside the mouth. Through this symbiotic relationship the shark benefits because it rids itself of parasites and the remora benefits because it receives food.
Bull Shark Mating and Reproduction
Bull sharks are viviparous (they give birth to live young that developed in the body of the mother) and iteroparous (offspring are produced in groups). They engage in internal reproduction in which sperm from the male fertilizes the egg within the female and have a reproductive cycle that takes place every other year as they have a long gestation period,. The breeding season is from April to June. In tropical waters, they are known the breed year-round. [Source: Kristi Cascio, Animal Diversity Web (ADW) /=]
The gestation period of bull sharks ranges from nine to 12 months, with average period being 11 months. Females tend to give birth at the same time as the breeding season from April to June in most of the shark’s range. The number of offspring ranges from seven to 12, with the average number of offspring being 10. On average females reach sexual or reproductive maturity at age 18 years and males reach sexual or reproductive maturity at 14 to 15 years. Bull sharks often mate and use nursery areas in estuaries in order to provide protection for their young. Here young sharks are relatively safe from oceanic predators and have good supplies of food. There is no parental involvement in the raising of offspring. /=\
Little is known about bull shark mating behaviors other than that they they are polygynandrous (promiscuous), with both males and females having multiple partners. Within their genus,, blacktip reef shark and sand tiger shark females indicate they are ready to mate by swimming more slowly than they usually do with their tails raised in a more upward position and their snout pointing slightly down. The male then approaches the female and puts his snout below the female’s vent.
Both the gonads of males are of equal size and shape and functional. Immature gonads are very small and barely distinguishable. Mature gonads are larger and contain more vessels. Claspers of bull sharks are thicker towards the base and thinner towards the tip. Male sharks often bite during copulation, giving females nasty-looking scars. Researchers have found such scarring and bite marks on the on the female’s pectoral and pelvic fins and occasionally near the head. Males very rarely have these marks. /=\
Bull Shark Young and Development
According to Animal Diversity Web: Female bull sharks have one right active ovary that extends to the abdominal cavity. Usually 15 to 22 eggs are enlarged in the ovary; however, 10 or fewer undergo ovulation. The eggs that do not undergo ovulation are reabsorbed. Immature eggs are around 1-2 millimeters and those that mature are about 4-5 centimeters. Females sustain their young through a yolk-sac placenta. The female’s age can determine the size and shape of the oviducts. An immature female will have the same width throughout the oviduct and in general is very narrow. A mature female’s oviduct will be thicker towards the end of the oviducts and have more flexibility./=\
Towards the end of term, the embryos are around 50-75 centimeters long and are contained in elongated sections. They are born tail first. In order to contain all the eggs the uterine walls are stretched and very thin. After pregnancy, the uterine walls increase in thickness and decrease in size. Male bull sharks reach sexual maturity around 160-165 centimeters. This is partly determined by the size of their claspers. Female sharks mature around the size of 160-170 centimeters. This is partly determined by the size and condition of the reproductive organs.
The Growth rate is fast in young sharks, and slowly decreases throughout the shark's life. Growth was estimated at 18 centimeters in the shark's first year of life and 16 centimeters in the second year. Subsequently, they grow about 11-12 centimeters a year, slowly dropping to 9-10 centimeters per year. Female bull sharks are estimated to reach reproductive maturity around the age of 18 and males are estimated to reach maturity at age of 14-15. It is estimated that bull sharks can grow up to 340 centimeters, and that this growth is indeterminate.
Bull Sharks, Humans and Conservation
Bull sharks are sometimes found in aquarium but not so often as they tend to consume all of the other fish in the tank they are displayed with. Still the species has it strong points. It adapts fairly well to tank life and it aggressive disposition is a source of amusement to aquarium goers. Bull sharks are sometimes caught for sport. Mostly they are victims of accidental captures. In a 1978 study of the capture of bull sharks in commercial nets in the Gulf of Mexico, bull sharks comprised 11 percent of the biomass catch for sharks. [Source: Kristi Cascio, Animal Diversity Web (ADW) /=]
Humans utilize bull sharks for food and their body parts are sources of valuable materials. They are also an ecotourism draw. People buy and sell their liver oil, meat, and skin. The liver oil is used in beauty products. Shark meat is mostly sold in European countries. Sharkskin is sold to use as leather. Their fins are an ingredient of shark fin soup.
On the International Union for Conservation of Nature (IUCN) Red List bull sharks are categorized as “near threatened”. They have no special status according to the Convention on the International Trade in Endangered Species (CITES). Studies that measured decline in accidental captures of sharks in gillnets and beach seines from 1953-2003 suggests that there has been a decline in bull sharks in estuary habitats. At least some of presumed decline can be attributed due to the increase of human development in coastal areas, Because bull sharks’ nurseries tend to be in shallow estuaries, such development could presumably affect their populations. Several states in the southern U.S. have eliminated the use of gill nets in potential nursery areas.
Image Source: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated March 2023