Australian blacktip shark
Encyclopedia
The Australian blacktip shark (Carcharhinus tilstoni) is a species
of requiem shark
, family
Carcharhinidae. It is one of the two (together with C. sorrah
) most abundant shark species in commercial gill-net catches off northern Australia . Black markings on the tips of both dorsal fin
s, and the pectoral and lower caudal fins distinguish C. tilstoni from most similar tropical whaler sharks . Even though often mistaken with the common blacktip shark - C. Limbatus
( which is found in seas all around the world), C. tilstoni is endemic to Australia, know form only one stock in northern Australia. It is commercially important in the NAGSF (Northern Australian Gillnet Shark Fishery). This species is not considered dangerous for humans.
(the common blacktip shark). C. tilstoni has been separated from C. limbatus
on the basis of differences in the enzyme systems, vertebral counts, size data and pelvic fin correlation . Field identification of C. tilstoni compared to the morphologically similar C. limbatus
is yet still difficult.
Another species, which is often related to C. tilstoni is the Spot–tail shark
(Carchahrinus sorrah
). Carchahrinus sorrah
has similar fin markings as C. tilstoni, but no ridge on the back between the dorsal fins. It is also known from a single stock off northern Australia . C. tilstoni and C. sorrah
have often been studied together, since they have many similar characteristics, inhabit the same area, and are the two most abundant sharks in commercial fisheries in north Australia.
. All fins (except sometimes the pelvic and anal fins) have black tips. Its dorsal fin
is erect and falcate, with a pointed apex. The first dorsal fin origin is usually over or just behind the pectoral fin insertions. The second dorsal fin is high with origin over or slightly before the anal fin’s origin. There is no interdorsal ridge. The teeth are slender, erect and serrated.
but appears to be about 180 cm total length (TL). The length at birth is 60 cm, with young sharks entering fishery soon after birth at a total length ~ 63 cm. Growth is relatively rapid in the first year of life: vertebral ageing indicated 17 cm growth in total length. By the time they are five years old, growth has declined to eight to 10 cm per year. Sexual maturity is reached early: at three to four years for both sexes, at about length of 110 cm and 115 cm for males and females respectively.
Compared to other shark species, C. tilstoni mature early. Research suggests that this might be due to the large size at birth relative to their size at maturity.
fish are important component of the diet of C. tilstoni and there are some indications of a change in feeding depth with shark size. The only distinct seasonal variation in its diet is in April, when cephalopod
s, rather than fish seem to be the main food
The study of the diet of these sharks is very important, since C. tilstoni constitutes a major fraction of the predator biomass and the diets of these sharks are known to contribute significantly to the natural mortality of valuable commercial prawn
s..
C. tilstoni exhibits placental vivipary
, meaning the mother nourishes embryos through a placental connection and produces live young .
fishery that operated from 1974 - 1986 off northern Australia. The species was caught for its meat and to a lesser extent, its fins. Until 1991, it was taken by Taiwanese longliners and utilized in the same way. From 1974 to 1978, the Taiwanese fished to within 12 nautical miles (22 km) of the Australian coast. With the declaration of the Australian Fishing Zone in 1979, foreign vessels were excluded from the Gulf of Carpentaria
and from 40 nautical miles (74.1 km) to 50 nautical miles (92.6 km) off the Wessel Islands
and Arnhem Land
coast. Further restrictions were introduced in 1986 in response to declining shark catch rates. The reduction of net lengths to 2,500 m rendered the Taiwanese fleet uneconomical, and despite permitted use of baited longlines, foreign fishing operations in northern Australian waters ceased by the end of 1986. A small domestic fishery also developed over this time and is still in operation to this day. C. tilstoni is primarily targeted for its flesh, which is sold under the marketing name “flake”, and the fins. The flesh has a relatively high mercury concentration
species found worldwide, approximately 300 are found in Australia and over 50% of these Australian species are endemic. Commercial, indigenous, recreational and game fishers target them, and sharks are a by-catch or by-product in at least 70 types of commercial fishing operations. C. tilstoni is one of seven commercial species that make up this fishery. Stock assessment for the shark resources in Queensland
has not been undertaken and the sustainability of the shark resource at the current harvest and effort levels both there and in the Northern Territory
(which are the only areas where C. tilstoni is found) are yet unknown. Shark fisheries are particularly sensitive to overfishing. Slow growth rates, low rates of reproduction and a close relationship between stock size and recruitment in shark population typically contribute to a rapid decline in numbers soon after exploitation begins. For a fishery to be viable over the long term, it must be managed effectively , and therefore it is of crucial importance for further research to be conducted, so that stock assessment can be properly made.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of requiem shark
Requiem shark
Requiem sharks are a family, Carcharhinidae, of sharks in the order Carcharhiniformes, containing migratory, live-bearing sharks of warm seas such as the tiger shark, the blue shark, the bull shark, and the milk shark.The name may be related to the French word for shark, "requin", itself of...
, family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Carcharhinidae. It is one of the two (together with C. sorrah
Spottail shark
The spottail shark is a species of requiem shark, family Carcharhinidae, found in the Indo-West Pacific oceans waters between latitudes 31° N and 31° S, from the surface to 140 m. Its length is up to about 1.6 m....
) most abundant shark species in commercial gill-net catches off northern Australia . Black markings on the tips of both dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s, and the pectoral and lower caudal fins distinguish C. tilstoni from most similar tropical whaler sharks . Even though often mistaken with the common blacktip shark - C. Limbatus
Blacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...
( which is found in seas all around the world), C. tilstoni is endemic to Australia, know form only one stock in northern Australia. It is commercially important in the NAGSF (Northern Australian Gillnet Shark Fishery). This species is not considered dangerous for humans.
Taxonomy
C. tilstoni is distinguished as a separate species in 1950 by Whitley. Previously, it has been described as C. limbatusBlacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...
(the common blacktip shark). C. tilstoni has been separated from C. limbatus
Blacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...
on the basis of differences in the enzyme systems, vertebral counts, size data and pelvic fin correlation . Field identification of C. tilstoni compared to the morphologically similar C. limbatus
Blacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...
is yet still difficult.
Another species, which is often related to C. tilstoni is the Spot–tail shark
Spottail shark
The spottail shark is a species of requiem shark, family Carcharhinidae, found in the Indo-West Pacific oceans waters between latitudes 31° N and 31° S, from the surface to 140 m. Its length is up to about 1.6 m....
(Carchahrinus sorrah
Spottail shark
The spottail shark is a species of requiem shark, family Carcharhinidae, found in the Indo-West Pacific oceans waters between latitudes 31° N and 31° S, from the surface to 140 m. Its length is up to about 1.6 m....
). Carchahrinus sorrah
Spottail shark
The spottail shark is a species of requiem shark, family Carcharhinidae, found in the Indo-West Pacific oceans waters between latitudes 31° N and 31° S, from the surface to 140 m. Its length is up to about 1.6 m....
has similar fin markings as C. tilstoni, but no ridge on the back between the dorsal fins. It is also known from a single stock off northern Australia . C. tilstoni and C. sorrah
Spottail shark
The spottail shark is a species of requiem shark, family Carcharhinidae, found in the Indo-West Pacific oceans waters between latitudes 31° N and 31° S, from the surface to 140 m. Its length is up to about 1.6 m....
have often been studied together, since they have many similar characteristics, inhabit the same area, and are the two most abundant sharks in commercial fisheries in north Australia.
Distribution
C. tilstoni is currently known only form the continental shelf of tropical Australia. Genetic studies and tagging indicate a single stock off northern Australia. It occurs from close inshore to a depth of about 150 m. It can be found throughout the water column, but mostly in midwater or near the surface. Sometimes C. tilstoni sharks form large aggregations .Appearance and coloration
C. tilstoni is a medium sized, long-snouted whaler shark with a bronzy to grayish dorsal coloration. The dorsal bronze surface fades to grey after death or in preservative. The ventral surfaces are pale. A pale stripe extends along each flank from the pelvic fin to below the first dorsal finDorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
. All fins (except sometimes the pelvic and anal fins) have black tips. Its dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
is erect and falcate, with a pointed apex. The first dorsal fin origin is usually over or just behind the pectoral fin insertions. The second dorsal fin is high with origin over or slightly before the anal fin’s origin. There is no interdorsal ridge. The teeth are slender, erect and serrated.
Size and growth
The maximum size of C. tilstoni is uncertain, because of confusion with C. limbatusBlacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...
but appears to be about 180 cm total length (TL). The length at birth is 60 cm, with young sharks entering fishery soon after birth at a total length ~ 63 cm. Growth is relatively rapid in the first year of life: vertebral ageing indicated 17 cm growth in total length. By the time they are five years old, growth has declined to eight to 10 cm per year. Sexual maturity is reached early: at three to four years for both sexes, at about length of 110 cm and 115 cm for males and females respectively.
Compared to other shark species, C. tilstoni mature early. Research suggests that this might be due to the large size at birth relative to their size at maturity.
Diet
The body form and dentition suggest that C. tilstoni is an active surface and midwater predator. TeleostTeleostei
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...
fish are important component of the diet of C. tilstoni and there are some indications of a change in feeding depth with shark size. The only distinct seasonal variation in its diet is in April, when cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s, rather than fish seem to be the main food
The study of the diet of these sharks is very important, since C. tilstoni constitutes a major fraction of the predator biomass and the diets of these sharks are known to contribute significantly to the natural mortality of valuable commercial prawn
Prawn
Prawns are decapod crustaceans of the sub-order Dendrobranchiata. There are 540 extant species, in seven families, and a fossil record extending back to the Devonian...
s..
Reproduction
This species has a distinctly seasonal reproductive cycle: females breed every year, with mating occurring in February–March, ovulation in March–April and parturition between late November and early February, with the peak parturition period in January. The gestation period is 10 months and the average litter size is three.C. tilstoni exhibits placental vivipary
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
, meaning the mother nourishes embryos through a placental connection and produces live young .
Fishery
C. tilstoni was the principal shark species taken by a Taiwanese gillnetGillnet
Gillnetting is a common fishing method used by commercial and artisanal fishermen of all the oceans and in some freshwater and estuary areas. The gillnet also is used by fisheries scientists to monitor fish populations. Because gillnets can be so effective their use is closely monitored and...
fishery that operated from 1974 - 1986 off northern Australia. The species was caught for its meat and to a lesser extent, its fins. Until 1991, it was taken by Taiwanese longliners and utilized in the same way. From 1974 to 1978, the Taiwanese fished to within 12 nautical miles (22 km) of the Australian coast. With the declaration of the Australian Fishing Zone in 1979, foreign vessels were excluded from the Gulf of Carpentaria
Gulf of Carpentaria
The Gulf of Carpentaria is a large, shallow sea enclosed on three sides by northern Australia and bounded on the north by the Arafura Sea...
and from 40 nautical miles (74.1 km) to 50 nautical miles (92.6 km) off the Wessel Islands
Wessel Islands
The Wessel Islands are a group of islands belonging to the Northern Territory of Australia. They extend in a more or less straight line from Buckingham Bay and the Napier Peninsula of Arnhem Land, and Elcho Island, to the northeast. Marchinbar Island is the largest of the group...
and Arnhem Land
Arnhem Land
The Arnhem Land Region is one of the five regions of the Northern Territory of Australia. It is located in the north-eastern corner of the territory and is around 500 km from the territory capital Darwin. The region has an area of 97,000 km² which also covers the area of Kakadu National...
coast. Further restrictions were introduced in 1986 in response to declining shark catch rates. The reduction of net lengths to 2,500 m rendered the Taiwanese fleet uneconomical, and despite permitted use of baited longlines, foreign fishing operations in northern Australian waters ceased by the end of 1986. A small domestic fishery also developed over this time and is still in operation to this day. C. tilstoni is primarily targeted for its flesh, which is sold under the marketing name “flake”, and the fins. The flesh has a relatively high mercury concentration
Threats
Of the 1025 chondrichthyanChondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
species found worldwide, approximately 300 are found in Australia and over 50% of these Australian species are endemic. Commercial, indigenous, recreational and game fishers target them, and sharks are a by-catch or by-product in at least 70 types of commercial fishing operations. C. tilstoni is one of seven commercial species that make up this fishery. Stock assessment for the shark resources in Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
has not been undertaken and the sustainability of the shark resource at the current harvest and effort levels both there and in the Northern Territory
Northern Territory
The Northern Territory is a federal territory of Australia, occupying much of the centre of the mainland continent, as well as the central northern regions...
(which are the only areas where C. tilstoni is found) are yet unknown. Shark fisheries are particularly sensitive to overfishing. Slow growth rates, low rates of reproduction and a close relationship between stock size and recruitment in shark population typically contribute to a rapid decline in numbers soon after exploitation begins. For a fishery to be viable over the long term, it must be managed effectively , and therefore it is of crucial importance for further research to be conducted, so that stock assessment can be properly made.