C. leucas is a cosmopolitan species found in most northern and southern temperate, subtropical and tropical waters in all continents except Antarctica and Europe. See Figure 2 for the global distribution map. It is capable of entering freshwater river systems indefinitely, and has been found at least 3000km inland in the Amazon River and Mississippi River, as well as in the freshwater Lake Nicaragua in Central America and the hypersaline St. Lucia Estuary in South Africa (Jensen, 1976; Simpfendorfer et al, 2005; Thorson, 1972).
The species’ worldwide distribution has resulted in multiple descriptions and numerous common names for the species (including Zambezi Shark, Swan River Shark and Lake Nicaragua Shark) from throughout its range (Simpfendorfer & Burgess, 2005).
Simpfendorfer & Burgess (2005) note that the species is native to the following countries:
Australia, Bangladesh, Bolivia, Botswana, Brazil, Columbia, Costa Rica, Ecuador, Fiji, Gambia, Guinea, Guyana, India, Islamic Republic of Iran, Iraq, Malawi, Mexico, Mozambique, Nepal, New Caledonia, Nicaragua, Peru, Philippines, Senegal, South Africa, United Republic of Tanzania, United States, Venezuela, Zimbabwe.
18.Give an overview of the global population’ssize, trends, threats and security of the species outside Australia.
The population trend of C. leucas is unknown (Simpfendorfer & Burgess, 2005). There are trends to suggest that globally, bull shark populations are under threat by commercial fisheries. C. leucas is a commonly caught shark species in a large number of countries both as bycatch and a targeted species. The demand for shark products has increased in recent years, and thus C. leucas is threatened by an increase of fishing effort and a reduction of selectivity in the fishing process, as well as bycatch mortality in unrelated fisheries.
Additionally, due to its proclivity for estuarine and riverine environments as nursery grounds frequently neighbouring human development, C. leucas is threatened by anthropogenic habitat modification and shark control programs (Dudley, 1997). Despite their tolerance of a wide variety of water conditions, O’Connell et al (2007) reported an effective extirpation of bull sharks due to habitat pollution in Louisiana, USA. Myers et al (2007) collate United States data regarding C. leucas population trends, indicating an estimated decline of 67% - 99% in some regions of the eastern coast.
19.Explain the relationship between the Australian population and the global population, including:
What percentage of the global population occurs in Australia;
As there is no data defining populations of C. leucas either globally or Australia wide, it is not possible to define the percentage of the global population of the species is found in Australian waters.
Is the Australian population distinct, geographically separate or does part or all of the population move in/out of Australia’s jurisdiction (give an overview; details in Movements section);
Preferring shallow waters, C. leucas is rarely observed in open oceanand thus Australian populations of the species are likely distinct from those that have been geographically isolated by deeper waters. However there is insufficient data to positively define the movements of C. leucas on an international scale and we are not aware of any studies underway to ascertain this data.
Do global threats affect the Australian population?
The Australian population is likely subject to the same threats nationally as it is worldwide, particularly those brought about by human impact occurring in costal and offshore areas. These threats particularly impact on C. leucas due to the location of their nursery areas in estuarine and freshwater systems, making them vulnerable to pollution and habitat modification (Simpfendorfer & Burgess, 2005).
Whilst Illegal, Unregulated and Unreported (IUU) fishing levels have not been quantified, IUU fisheries in Australia, particularly in regions bordering south east Asia, may have a deleterious impact on Australian populations of C. leucas.
Surveys and Monitoring
20.Has the species been reasonably well surveyed?
Provide an overview of surveys to date and the likelihood of its current known distribution and/or population size being its actual distribution and/or population size.
C. leucas has been inadequately surveyed quantitatively in Australian waters. Due to this dearth of required data, there is a great deal of uncertainty regarding actual numbers. Anecdotal evidence from recreational and commercial fishers is common, however cannot be reliably accounted for accuracy. There have been limited surveys undertaken - Thorburn et al (2004) is a survey of elasmobranches in the freshwater reaches of the Fitzroy River catchment in Western Australia which while useful in its insight, is limited in its geographical range. Werry et al (2009) examined the value of varied coastal habitats to the species in the Gold Coast/Nerang river region using acoustic tagging.
The nominees are also aware of a NSW DPI program to tag and release bull sharks in Sydney harbour that has been underway since Summer 08/09. It is thought that only one shark was caught in 2009, and since then none have been caught.
21.For species nominated as extinct or extinct in the wild, please provide details of the most recent known collection, or authenticated sighting of the species and whether additional populations are likely to exist.
Not applicable
22.Is there an ongoing monitoring programme? If so, please describe the extent and length of the programme.
There are no ongoing monitoring programs of bull shark populations in Australia.
Yüklə 233 Kb.