| Australian blacktip shark | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Carcharhinidae |
| Genus: | Carcharhinus |
| Species: | C. tilstoni |
| Binomial name | |
| Carcharhinus tilstoni (Whitley, 1950) | |
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| Range of the Australian blacktip shark[2] | |
| Synonyms | |
Galeolamna pleurotaenia tilstoniWhitley, 1950 | |
TheAustralian blacktip shark (Carcharhinus tilstoni) is aspecies ofrequiem shark, in thefamily Carcharhinidae,endemic to northern and easternAustralia. Favoring the upper and middle parts of thewater column, it can be found from theintertidal zone to a depth of 50 m (160 ft). Appearance-wise this species is virtually identical to thecommon blacktip shark (C. limbatus), from which it can be reliably distinguished only by its lowervertebra number and bygenetic markers. Generally reaching 1.5–1.8 m (4.9–5.9 ft) in length, it is a fairly stout-bodied, bronze-colored shark with a long snout and black-tipped fins.
Primarilypiscivorous, the Australian blacktip shark forms large groups of similar size and sex that tend to remain within a local area. It exhibitsviviparity, meaning that the unborn young are provisioned through aplacental connection. There is a well-defined annual reproductive cycle withmating occurring in February and March. Females bear one to six pups around January of the following year, after a 10-monthgestation period. The Australian blacktip shark is among the sharks most commonly caught by northern Australiancommercial fisheries. It is mainly valued for itsmeat, which is sold as "flake". This species was an important catch of aTaiwanesegillnet fishery that operated from 1974 to 1986, and of the Australian Northern Shark Fishery that continues to the present day. As current fishing levels are not thought to threaten this shark's population, theInternational Union for Conservation of Nature (IUCN) has assessed it asLeast Concern.
The Australian blacktip shark was described by AustralianichthyologistGilbert Percy Whitley in a 1950 issue of thescientific journalWestern Australian Naturalist. He named itGaleolamna pleurotaenia tilstoni in honor of Richard Tilston, assistant surgeon atPort Essington,Northern Territory. Thetype specimen is a 1.5 m (4.9 ft) long female caught from Van Cloon Reef inJoseph Bonaparte Gulf.[3] Later authors have regardedGaleolamna as asynonym of the genusCarcharhinus. This shark may also be referred to as blacktip whaler, Tilston's whaler shark, and Whitley's blacktip shark.[4]
After Whitley's initial description,C. tilstoni was generally regarded as synonymous withC. limbatus, thecommon blacktip shark. In the 1980s, additionalmorphological andlife history data again favored the recognition ofC. tilstoni as a separate species, which was eventually confirmed byallozyme studies performed by Shane Lavery and James Shaklee.[5] Severalmolecular phylogenetic studies based onallozymes,mitochondrial DNA, andnuclear DNA have found that the Australian and common blacktip sharks form a closely relatedclade with thegraceful shark (C. amblyrhynchoides) and thesmoothtooth blacktip shark (C. leiodon).[6][7][8] The interrelationships between them have not been fully resolved, but available data suggest thatC. tilstoni andC. limbatus are not the most closely related species within the clade despite their similarity.[7][9]
Physically, the Australian blacktip shark can only reliably be distinguished from the common blacktip shark by the number ofvertebrae (174–182 total, 84–91 before the tail inC. tilstoni, 182–203 total, 94–102 before the tail inC. limbatus). It has a moderately robust, spindle-shaped body and a long, pointed snout. The anterior rims of the nostrils are slightly enlarged into low triangular flaps. The large, circular eyes are equipped withnictitating membranes. The furrows at the corners of the mouth are barely evident. There are 32–35 upper and 29–31 lower tooth rows; each upper tooth has a slender, upright cusp and fine serrations that become coarser near the base, while the lower teeth are narrower and more finely serrated. The five pairs ofgill slits are long.[9][10]
The long and narrowpectoral fins are falcate (sickle-shaped) with pointed tips. The large firstdorsal fin is also falcate and originates over or slightly behind the rear of the pectoral fin bases. The second dorsal fin is moderately tall and positioned about opposite theanal fin. There is no midline ridge between the dorsal fins. A crescent-shaped notch is present on thecaudal peduncle just before the uppercaudal fin origin. The caudal fin is asymmetrical, with a strong lower lobe and a longer upper lobe with a ventral notch near the tip. The roughly diamond-shapeddermal denticles are placed closely together and slightly overlapping; each bears five to seven (three in juveniles) horizontal ridges leading to marginal teeth. The Australian blacktip shark is bronze above (gray after death) and whitish below, with a pale stripe on the flanks. Some individuals have black tips on all fins, while others have unmarkedpelvic and anal fins. It typically reaches 1.5–1.8 m (4.9–5.9 ft) long; the maximum length and weight on record are 2.0 m (6.6 ft) and 52 kg (115 lb).[9][10][11]
Inhabiting thecontinental shelf, the Australian blacktip shark is found fromThevenard Island inWestern Australia toSydney inNew South Wales.[2][10] Within its range, it co-occurs with the common blacktip shark; the ratio betweenC. limbatus andC. tilstoni was once thought to be 1:300, but recent genetic studies have found it to be closer to 50:50.[12] This species has been reported from theintertidal zone to a depth of 150 m (490 ft); larger sharks tend to occur in deeper water.[1] Though it occupies the entirewater column, it is most common close to the surface or in midwater.[10] Genetic data suggest Australian blacktip sharks across northern Australia are all members of a single population.[13] Tagging studies have found this species mostly moves only short distances along the coastline and rarely enters offshore waters. However, some individuals have been recorded traveling longer distances, up to 1,348 km (838 mi).[14][15]
The Australian blacktip shark is known to form large groups, segregated by size and sex.[9] It preys mainly onteleost fishes, includingponyfishes,grunters,tunas, andherring.Cephalopods are a secondary food source, which are particularly important around April.[16][17] This species also occasionally consumes smaller sharks, includingsnaggletooth sharks (Hemipristis pristis),spottail sharks (C. sorrah), andsharpnose sharks (Rhizoprionodon).[9] Their diets shift with age: smaller individuals feed on proportionately morebottom-dwelling fish, while larger individuals feed on proportionately more midwater fish and cephalopods.[18] Documentedparasites of this shark include thetapewormsFossobothrium perplexum[19] andPlatybothrium sp.,[20] and thecopepodPerissopus dentatus.[21]
Like other requiem sharks, the Australian blacktip shark isviviparous: once theembryos exhaust theiryolk supply, the depletedyolk sac develops into aplacental connection through which the mother provides sustenance. Females bear litters of one to six pups (average three) every year.Mating takes place in February and March, with the females storing thesperm untilovulation in March and April. After agestation period of 10 months, the young are born around January of the following year.[18] Near-term females move into shallow, coastal nurseries, such asCleveland Bay in northern Queensland, to give birth.[17] Such nursery areas appear to be widespread, as newborns have been recorded throughout the range of the species.[15] The newborns are relatively large, measuring about 60 cm (24 in) long. They grow rapidly in their first year, increasing in length by an average of 17 cm (6.7 in). The growth rate subsequently slows, averaging 8–10 cm (3.1–3.9 in) per year by age five. Both sexes attainsexual maturity at three to four years of age, at lengths of roughly 1.1 and 1.2 m (3.6 and 3.9 ft) for males and females, respectively. Themaximum lifespan is estimated to be 20 years.[18][22]
Hybrids between the Australian blacktip shark and the common blacktip shark, comprising bothF1 andbackcrossed individuals, have been discovered all along the eastern coast of Australia. Despite the widespread incidence of hybridization, there is no evidence that the two parental lineages are merging and the mechanisms preventingintrogression are unclear. This is the first confirmed case of hybridization amongcartilaginous fishes.[23]
Along with the spottail shark, the Australian blacktip shark is one of the most economically important sharks off northern Australia. It was historically fished for its meat andfins by aTaiwanesegillnetfishery, which began operating off northern Australia in 1974. The annual catch initially averaged around 25,000 tons (live weight), with approximately 70% consisting of Australian blacktip sharks, spottail sharks, andlongtail tuna (Thunnus tonggol). Following the establishment of the Australian Fishing Zone (AFZ) in November 1979, the Australian government progressively excluded Taiwanese gillnet vessels from large portions of Australian waters and imposedfishing quotas. The fishery eventually became uneconomical when Australia restricted gillnet lengths to 2.5 km (1.6 mi) in May 1986, and Taiwanese vessels ceased fishing in Australian waters that year.[15] At end of the fishery, Australian blacktip shark numbers were estimated to have been diminished by around 50%. Given its relatively high reproductive rate, its population has likely since recovered.[1]
Since 1980, the Australian blacktip shark has also been targeted by Australian commercial gillnetters andlongliners in the Northern Shark Fishery. It is additionallycaught incidentally by othercommercial fisheries targetingbony fishes orprawns.[15] The contemporary northern Australian shark catch is estimated to be between 100 and 900 tons (live weight) annually, of which most are Australian blacktip and spottail sharks.[1] The meat of this species is sold in Australia as "flake", though it may contain high concentrations ofmercury. The fins are exported to Asia, while thecartilage,liver oil, andskin may also be used.[10][15] Since fishing pressure on the Australian blacktip shark is now much lower than historical levels, theInternational Union for Conservation of Nature (IUCN) has listed it underLeast Concern.[1] However, of potential concern areIndonesian fishers in theArafura Sea, who are increasingly trespassing into the AFZ.[10][15]
