Carcharias taurus (Western Australia subpopulation)
|Scientific Name:||Carcharias taurus (Western Australia subpopulation)|
|Taxonomic Notes:||Synonyms = Odontaspis taurus (Rafinesque, 1810); Eugomphodus taurus (Rafinesque, 1810)|
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor(s):||Pollard, D., Gordon, I., Williams, S., Flaherty, A. & McAuley, R.|
|Reviewer(s):||Shark Specialist Group Australia & Oceania Regional Workshop, March 2003, participants (Shark Red List Authority)|
Grey nurse sharks have never been targeted in Western Australia (WA). The only significant source of mortality has been from incidental capture by a demersal gillnet fishery. Based on data from this fishery, the west coast population of C. taurus is assessed as Near Threatened because mean annual catches of 77 sharks, in conjunction with a stable CPUE, indicate that the subpopulation is larger and more stable than the eastern subpopulation, and aggregation sites are not known within the range of this fishery. However, these data are only available for 1989–1997, when the species was protected under the Endangered Species Protection Act and commercial catch reporting ceased. Due to the loss of this established index of abundance, the limited reproductive capacity of C. taurus and the precarious status of the eastern subpopulation, it is recognized that the western subpopulation still has the potential to become Vulnerable in the future. Even low levels of bycatch may lead to population declines in a species with such a low intrinsic rate of increase. There is a need to develop the means to monitor the abundance of these sharks in WA and conduct further research into their ecology.
|Range Description:||Because Carcharias taurus is extremely rare in the Northern Territory and South Australia, there is likely to be almost no genetic exchange between sharks on the east and west coasts and, for the purposes of this assessment, the Australian subpopulation is considered to consist of two subpopulations: the New South Wales (east coast) subpopulation, and the Western Australia (west coast) subpopulation. Information for the Western Australia subpopulation follows:
Despite a relatively large and active recreational diving industry in Western Australia (WA), no aggregation sites have been confirmed on the west coast, however, one site has been reported to exist off Perth. WA Department of Fisheries research data suggest that grey nurse sharks are more dispersed throughout temperate continental shelf waters than they are in eastern Australian waters.
Native:Australia (Western Australia)
|FAO Marine Fishing Areas:|
Indian Ocean – eastern
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Whilst a population estimate is not available for the west coast subpopulation, mean annual catches of 77 sharks in the WA demersal gillnet fisheries, in conjunction with a stable CPUE trend indicate that the population is larger and more stable than the eastern subpopulation.|
|Current Population Trend:||Stable|
|Habitat and Ecology:||Carcharias taurus generally occurs in warm-temperate and sub-tropical waters, ranging from the surf zone and shallow bays to approximately 200 m depth on the outer continental shelf. The species is most usually found on or near the bottom in reef areas but may occasionally occur in midwater or at the surface (Compagno 1984). In NSW and southern Queensland, C. taurus is most frequently sighted in or near sandy-bottomed gutters or in rocky caves, often around inshore rocky reefs and islands at depths of between 15 and 25 m (Pollard et al. 1996; Otway and Parker 1999).
Embryonic oviphagy and intra-uterine cannibalism occurs in this species and only two large pups are produced per litter every second year. As a result, annual rates of population increase are very low, greatly reducing its ability to sustain fishing pressure. Maximum size attained is 220 to 270 cm total length (TL) (males) and 300 to 320 cm TL (females). Age at maturity is 6 to 7 years (males) and 9 to 10 years (females). Female k = 0.11, Male k = 0.16 (Goldman 2002).
Currently the main threatening processes in Australian waters would appear to be commercial and recreational fishing (including shark control programs in NSW and QLD), in which this species is taken as a bycatch. It has also been suggested that excessive dive tourism activities (i.e., 'shark diving') may also have the potential to adversely impact upon populations of the grey nurse shark. Refer to the Recovery Plan for the Grey Nurse Shark (Carcharias taurus) in Australia (Environment Australia, 2002) for a fuller description of threats facing grey nurse sharks in Australian waters.
Unlike NSW and Queensland, grey nurse sharks have never been subjected to targeted fishing in WA. The only significant source of C. taurus mortality on the west coast has been from incidental capture by the demersal gillnet fishery that operates between Steep Point and the South Australia border. Relatively high quality grey nurse shark catch and catch rate data are available from this fishery. These data have been independently verified by estimating the fishery’s total catch from the CPUE recorded by scientific observers (McAuley and Simpfendorfer, draft report). However, these data are only available for the period July 1989 to December 1997, when the species was protected under the Endangered Species Protection Act and commercial reporting ceased. These data cover the eight year period immediately after the historical peak in demersal gillnet fishing effort and the period during which direct management adjustment reduced effort to 42% of its maximum level.
Catches of between 70-105 sharks per year (McAuley, unpublished data) indicate that grey nurse sharks were relatively abundant on the lower west coast of WA between 1989 and 1997 and CPUE of grey nurse sharks in the demersal gillnet fishery increased between 1989-1993 and then remained level until 1997, indicating that the population was stable.
WA Department of Fisheries (WA DOF) research records do not suggest that aggregation sites occur within the functional area of the WA demersal gillnet fishery. If such sites do occur within the fishery’s geographic boundaries, they are likely to be in areas of heavy reef, where gillnet vessels do not operate due to the risk of net entanglement. Additionally, there are several records of grey nurse shark occurrence in two significant regions outside of the fishery’s operational range, between Steep Point (26º 30’ S) and NW Cape (22º S), which has been closed to shark fishing since 1993 and, in deeper coastal waters (>100 m), where demersal gillnet vessels cannot operate due to their generally small size and the amount of expected damage to gear and catch caused by currents and predation. Both areas are likely to contain large amounts of suitable unfished habitat and are thought to offer significant refugia to this species in WA. However, there is also some concern regarding anecdotal reports that grey nurse sharks were more abundant in the 1960s and 1970s and that there may have been inshore aggregation sites that are no longer in existence.
Due to the limited reproductive capacity of C. taurus, the precarious status of the eastern subpopulation and the loss of an established index of abundance (commercial catch records), it is recognised that the western subpopulation still has the potential to become Vulnerable in the future. Therefore, developing a means to monitor the abundance of grey nurse sharks in Western Australia and further research into their ecology are necessary. Archival tagging of grey nurse sharks to provide data on distribution and migratory behaviour in Western Australia is expected to be undertaken in the next 12 months.
This species may well be reassessed over the coming year, as well as undergoing routine reassessments in the future.
Protection Status in Australia
- Protected Species in Commonwealth waters under the EPBC Act, 1999: West Coast Population - Vulnerable (since 1997).
- Protected Species in Western Australian waters under the Wildlife Conservation Act, 1950 (since December 1999).
- A National Recovery Plan for the Grey Nurse Shark (Carcharias taurus) in Australia was adopted for implementation by the Minister for Environment and Heritage June 2002.
Proposed Conservation Measures
Conservation measures can been found in the National Recovery Plan for the Grey Nurse Shark (Carcharias taurus) in Australia (Environment Australia 2002). The overall conservation objective is to increase Grey Nurse Shark numbers in Australian waters to a level that will see the species removed from the IUCN Red List.
|Citation:||Pollard, D., Gordon, I., Williams, S., Flaherty, A. & McAuley, R. 2003. Carcharias taurus (Western Australia subpopulation). The IUCN Red List of Threatened Species 2003: e.T44071A10854958.Downloaded on 21 November 2017.|
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