Definitions

whaler shark

Australian blacktip shark

The Australian blacktip shark 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 fins, 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.

Taxonomy and Related Species

C. tilstoni is distinguished as a separate species in 1950 by Whitley. Previously, it has been described as C. limbatus (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 form 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.

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 fin. 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.

Size and Growth

The maximum size of C. tilstoni is uncertain, because of confusion with C. limbatus 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. Teleost 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 cephalopods, 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 prawns. .

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, 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 gillnet fishery that operated from 1974 - 1986 off northern Australia. The species was caught for its meet 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 to 50 nautical miles 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

Threats

Of the 1025 chondrichthyan 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.

References

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