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Carcharhinus tilstoniAustralian blacktip shark(Also: Blacktip shark; Tilston's whaler shark; Whitley's blacktip shark)

By Megan Vineyard

Geographic Range

Australian blacktip sharks (Carcharhinus tilstoni) are native to Australia. As a coastal species restricted to the continental shelf, they are found in the northern half of Western Australia, Northern Territory, Queensland, and New South Wales. This species is also present in the Great Barrier Reef, and are found as far southeastward as Sydney, New South Wales. (Ceccarelli, et al., 2013; Harry, et al., 2013; Stevens, et al., 2000)

Habitat

Australian blacktip sharks are a coastal, oceanic species. They inhabit waters at depths of 0m to 150m (average = 50 m). They are most common in shallow coral reefs along the Australian coast. They can also be found in brackish waters. They migrate vertically in the water column, usually near the sea bed during the day and near the surface at night. (Harry, et al., 2013; Kailola, et al., 1993; Last and Stevens, 1994; ; Morgan, et al., 2012a)

  • Range depth
    0 to 150 m
    0.00 to 492.13 ft
  • Average depth
    50 m
    164.04 ft

Physical Description

Adult Australian blacktip sharks earned their common name because of the black tips on their pectoral, dorsal, and caudal fins. Their first and second dorsal fins and their caudal fin is otherwise bronze or gray. These sharks are white and or yellow-white on their ventral side, and this white extends to the pelvic and anal fins. These sharks possess a long snout. Their teeth can be up to 10mm long and they have 32 to 35 teeth in the upper jaw and 29 to 31 teeth in the lower jaw. Males weigh 32kg on average (range 29kg to 52kg). The length of adult males varies from 65cm to 200cm, and adult females similarly reach adult lengths of 68cm to 200cm. Females weigh an average of 32kg but can reach weights up to 63kg. Despite similarities in length ranges, adult females are typically larger than males.

Both male and female pups at birth are about 60cm long and weigh an average of 1.37kg. Pups have the same color scheme as adults.

Australian blacktip sharks are nearly identical to common blacktip sharks (Carcharhinus limbatus), and identification of live specimens in the wild is nearly impossible. They are separated by two features that can only be completed with deceased sharks or by tissue samples: counting the vertebrae (Australian blacktip sharks have 182 or fewer, common blacktips have more than 182), and DNA analyses. There are differences in the size at maturity, but age must be determined for this to be a useful metric. (Davenport and Stevens, 1988; Ebert, et al., 2013; Harry, et al., 2019; Harry, et al., 2013; Johnson, et al., 2016; Johnson, et al., 2019; Morgan, et al., 2012b; Morgan, et al., 2012a)

  • Sexual Dimorphism
  • female larger
  • Range mass
    29 to 63 kg
    63.88 to 138.77 lb
  • Average mass
    32 kg
    70.48 lb
  • Range length
    65 to 200 cm
    25.59 to 78.74 in

Development

Australian blacktip sharks mate in March, and employ internal fertilization through the release of sperm from males’ claspers. Development of the embryo is internal and visible in the ova by April. The average litter size is 3 (range 1 to 6 pups). Pups are fed through the placenta and live birth occurs in January after a gestation period of 10 months. They are approximately 60 cm long at birth. Growth is rapid for this species' first year of life. They grow 17 cm or more their first year. At 5 years of age their growth rate is 8 to 10 cm per year. These sharks exhibit indeterminate growth.

Females reach sexual maturity at 6.1 years of age at lengths of ca. 115 cm. Males reach maturity at 5.2 years wen they are ca. 110 cm in total length. Both sexes can reach or are thought to exceed lengths of 200 cm. (Davenport and Stevens, 1988; Stevens and Wiley, 1986)

Reproduction

Australian blacktip sharks breed once each year between February and March. Male sharks have claspers for internal fertilization, and females then store the sperm. In the case of spot-tail sharks (Carcharhinus sorrah), females did not select for males on the basis of any physical feature or behavior. This suggests that members of the genus Carcharinus simply seek any available mate. (Dareen, 2020)

Australian sharks are viviparous, with females breeding once every year. Because females are capable of storing sperm in their oviducal glands, these sharks breed in February and March, with ovulation occurring in March. Fertilization is internal, as male sharks possess claspers to release sperm. After a 10-month gestation period, females give birth to an average of 3 pups (range 1 to 6). Birthing rates peak in January, but birthing can occur between late November and early February. Pups are born 60cm long and weigh 1.32 to 1.40kg. Pups are immediately independent after birth.

Females reach maturity at about 6.1 years old when they are about 115cm in length. Males mature at 5.2 years old and at ca. 110cm in length. (Davenport and Stevens, 1988; Harry, et al., 2013; Stevens and Wiley, 1986)

  • Breeding interval
    Australian blacktip sharks breed once each year
  • Breeding season
    Australian blacktip sharks mate and breed between February and March
  • Range number of offspring
    1 to 6
  • Average number of offspring
    3
  • Average gestation period
    10 months
  • Average time to independence
    0 days
  • Average age at sexual or reproductive maturity (female)
    6.1 years
  • Average age at sexual or reproductive maturity (male)
    5.2 years

Females migrate inshore along Australian coasts to birth pups. Cleveland Bay along northern Queensland is a seasonal primary nursery for pups of Australian blacktip sharks. Pups remain in the nursery only for a few months after birth before leaving the nursery. Males provide no parental investment beyond the act of mating. Females don't provide additional investment after birthing the pups, pups are immediately independent after birth. (Simpfendorfer and Milward, 1993)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

The longest known lifespan in the wild for Australian blacktip sharks is 12 years. Expected lifespan in the wild is not reported, though for other members of the genus, such as common blacktip sharks (Carcharhinus limbatus), their expected lifespan is 10 to 12 years.

Although Salini et al. (1999) report diets of juveniles in captivity, but it appears they were only kept captive for a matter of weeks. Therefore, lifespan in captivity is unknown,

Causes of mortality or limitations of lifespan are unknown for Australian blacktip sharks. (Garcia, et al., 2008; Salini, et al., 1999; Whitney, et al., 2017)

  • Range lifespan
    Status: wild
    12 (high) years

Behavior

Migrating vertically each day, Australian blacktip sharks reside near the sea bed during the day and rise vertically to near the surface to feed at night. They are thought to be nocturnal, based on relative activity levels. They mate with internal fertilization causing males and females to come in direct contact. Most shark mating includes males biting females.

Although males do not migrate seasonally, pregnant females migrate inshore to nursery locations to give birth. That have been reported to come together in large groups, suggesting they are social, although the explanation or timing for this gathering was not given.

When sharks appear threatened or are aggressive towards other sharks, blacktip sharks might use their pectoral fins to touch the other sharks. Their pectoral fin will be pointed away or outward from the body. At rest, the pectoral fins are close to their body. Sharks will also fold pectoral fins close to their bodies if they are in a resting phase. (Gruber and Myrberg, 2015; Last and Stevens, 1994; ; Ritter, 2002)

Home Range

This is no reported home range for Australian blacktip sharks, and these sharks do not defend territories. (Simpfendorfer and Milward, 1993)

Communication and Perception

Sharks don't have vocal cords, so they use their eyes and changes with their body language through movement to communicate with other sharks. When sharks feel threatened or are aggressive sharks might stretch their pectoral fins out to touch the other sharks. At rest, the pectoral fins are close to their body.

Sharks have a strong sense of smell, being able to smell blood (prey) up to 1 km away. They can detect sounds as well, finding prey in this manner. Detection of sound may help blacktip sharks detect ill prey, using low frequency pulses.

Using ampullae Lorenzini, which is a set of organs sharks possess, they can also detect electrical signals from their surroundings. These are common in all sharks, a modification of fish lateral line systems.

When mating, male sharks bite female sharks to keep them stable during reproduction. This tactile sense is evident with females who have mating scars on them. (Gruber and Myrberg, 2015; Hart, 2020; Kalmijin, 1966; O'Connell, et al., 2012)

Food Habits

Australian blacktip sharks are carnivores, primarily consuming bony fish. No diet studies exist in the wild, but captive studies suggest their diets change with age and size. Salini et al. (1999) found that juvenile Australian blacktip sharks in captivity consumed 3.44% of their body weight per day. However, this study appeared to have lasted less than two weeks. So, long-term or seasonal rates are unreported.

Stevens and Wiley (1986) examined the stomach contents of adult Australian black-tipped sharks along the continental shelf of Australia called the North West Shelf. Here, they reportedly found that 89.5% of stomachs containing food had fish in them. Just 15.8% of stomachs with food included cephalopods (e.g. squid and octopus). (Salini, et al., 1999; Stevens and Wiley, 1986)

  • Animal Foods
  • fish
  • mollusks

Predation

No predator studies have been conducted for this species. Humans (Homo sapiens) fish for many different species of sharks including Australian blacktip sharks, catching them for their fins and meat. (Johnson, et al., 2019; Last and Stevens, 1994; ; Stevens and Wiley, 1986)

Ecosystem Roles

Australian blacktip sharks are apex predators and their primary prey consists mostly of bony fish and cephalopods (class Cephalopoda). Humans fish for Australian blacktip sharks for their meat and fins.

Parasites of these sharks include tapeworms (Platybothrium). Australian blacktip sharks also serve as hosts for a parasitic copepod (Gloiopotes huttoni) which belongs to the family Caligidae.

Common remora fish (Remora remora) are considered mutualists for Australian blacktip sharks. Common remora fish consume bacteria present on the skin of the sharks, as well as dropped food scraps. (Healy, 2003; Parker and Martini, 2014)

Mutualist Species
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Johnson et al. (2018) report that Australian blacktip sharks are caught for their meat and fins. Although not the most commonly shark captures, the locations where they are netted or caught on fishing lines the most are in the Gulf of Carpentaria, which includes coasts along the Northern Territory and Queensland.

From 1979-1986, a Taiwanese fishery was catching about 2800 metric tons (t) of Australian blacktip sharks per year. After the population collapses that followed, harvests are now reduced: 300-400 t in the Northern Territory, 192 t in Queensland, and 146 t in the Gulf of Carpentaria. (Harry, et al., 2013; Johnson, et al., 2019)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

Australian blacktip sharks have no reported negative economic impacts on humans.

Conservation Status

Australian blacktip sharks are listed as a species of "Least Concern" on the IUCN Red List. They have no special status on US Federal List, CITES appendices, or State of Michigan List.

The primary threat to this species is overfishing by commercial fisheries. Johnson et al. (2008) reported that in the mid-1980s, Australian blacktip shark populations decreased an estimated 93% over 30 years due to overfishing. In 1986, the Taiwanese fisheries believe responsible for overfishing were shut down and replaced by smaller fisheries that catch smaller quantities. Now, all Australian fisheries that target Australian blacktip sharks are managed. Another issue is confusion with blacktip sharks (Carcharhinus limbatus); fisheries reporting captures of either shark likely group them as one species. These shark look and behave in a similar manner, and hybrids are known to exist. The effect of hybridization for Australian blacktip sharks is unknown. Illegal fishing may also harm these sharks.

Conservation measures for Australian blacktip sharks include the presence of restricted areas for fishing and gear limitations to catch them. Recent studies in the Northern Territory of Australia suggest that these shark have recovered well with reduced fishing. They appear quite resilient among shark species, as they reach adulthood relatively quickly and reproductive output is consistent across years. (Johnson, et al., 2019; Parker and Martini, 2014; Whitney, et al., 2017)

Contributors

Megan Vineyard (author), Radford University, Sierra Felty (editor), Radford University, Bianca Plowman (editor), Radford University, Karen Powers (editor), Radford University, Victoria Raulerson (editor), Radford University, Christopher Wozniak (editor), Radford University, Genevieve Barnett (editor), Colorado State University.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

electric

uses electric signals to communicate

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

piscivore

an animal that mainly eats fish

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

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