8.Give a brief description of the species’ appearance, including size and/or weight, and sex and age variation if appropriate; social structure and dispersion (e.g. solitary/clumped/flocks).
Carcharhinus leucas is a large, solidly built shark from the family Carcharhinidae with a blunt snout, small eyes, a relatively large second dorsal fin, no interdorsal ridge and broadly triangular serrated teeth. Dorsal surface is grey with a pale to white underbelly. (Last & Stephens, 2009).
See Figure 1 for a picture of C. leucas.
Juveniles are born at 55 – 80cm. Maximum length estimated at up to 340cm total length, with females slightly larger than males, although a female C. leucas was officially measured at 400cm total length with an estimated weight of over 600 kg (McCord and Lamberth, 2009; Last & Stephens, 2009). Mass at length can vary greatly for both males and females, with an estimated maximum weight of between 230 - 419kg (Camhi et al, 2007). Maximum age is estimated at 49 years, with sexual maturity occurring at approximately 14 - 20 years, and 160 – 230cm in length (Cruz-Martinez et al, 2004).
Little is known about the social behaviour of C. leucas, however it appears to be a primarily solitary or paired species which gathers to aggressively compete for plentiful food sources. The species gives birth to 1 -13 young in estuarine environments, and juveniles spend considerable periods in environments of low salinity, although this is likely a method of predator avoidance and/or increased food abundance rather than a physiological constraint (Pillans et al, 2005). Little is known about the movements of adult bull sharks in Australian waters, but there are reports of both sedentary and migratory behaviour internationally.
C. leucas is considered a very dangerous shark, due to its extremely aggressive nature, powerful jaws, broad diet, abundance and its preference for shallow, murky inshore habitats. The species is probably responsible for most of the attacks in and around Sydney Harbour (Last & Stevens, 2009).
9.Give a brief description of the species’ ecological role (for example, is it a ‘keystone’ or ‘foundation’ species, does it play a role in processes such as seed dispersal or pollination).
As one of the few species of shark that comfortably range between freshwater and hypersaline environments, C. leucas plays an important role as an apex and keystone predator in freshwater, estuarine and marine environments. Population declines of large sharks such as C. leucas have been correlated with detrimental ecological cascades from both environmental and economical perspectives (Myers and Worm. 2003).
Australian Distribution
10.Describe the species’ current and past distribution in Australia and, if available, attach a map.
C. leucas is a wide-ranging species that occurs in tropical and coastal regions of Australia, as far south as Sydney on the east coast around the northern coastline to Perth on the west coast. It prefers shallow waters, but has been found in waters to a depth of at least 150m (Camhi et al, 2007). As a euryhaline species, it is highly tolerant of salinity variations, and has penetrated far inland via river systems (Neer et al, 2005; Pillans et al, 2005).
Last and Stevens (2009) state that C. leucas has been reported from numerous freshwater systems in warm temperate and tropical Australia, including the Fitzroy, Ord, Adelaide, Daly, East Alligator, Mitchell, Normanby, Herbert, Brisbane, Clarence and Swan Rivers, as well as Lake Macquarie. It is rarely recorded in the sea off Australia, most probably due to confusion with other whaler sharks.
11.What is the extent of occurrence (in km2) for the species (described in Attachment A); explain how it was calculated and datasets used.
What is the current extent of occurrence?
The extent of occurrence of C. leucas has not been determined in terms of square kilometres. The Australian distribution of the species has been described as extending between Sydney (NSW) and Perth (WA) in coastal, estuarine, riverine and lacustrine habitats. In marine environments C. leucas occurs near the bottom from the surf zone down to a depth of at least 150 metres. (Last & Stevens, 2009)
Based on maps of hypothetical distribution, C. leucas occurs within an estimated 50% of Australia’s Exclusive Economic Zone (EEZ). Due to the inherent lack of data regarding extent of occurrence of C. leucas, a definitive extent of occurrence cannot be given.
What data are there to indicate past declines in extent of occurrence (if available, include data that indicates the percentage decline over the past 10 years or 3 generations whichever is longer)?
There is currently insufficient data available to indicate past declines in extent of occurrence.
What data are there to indicate future changes in extent of occurrence (if available, include data that indicates the percentage decline over 10 years or 3 generations whichever is longer (up to a maximum of 100 years in the future) where the time period is a continuous period that may include a component of the past)?
Due to insufficient data and the broad habitat tolerance of C. leucas, there is little to indicate that the potential extent of occurrence will change significantly in the future.
12.What is the area of occupancy (in km2) for the species (described in Attachment A); explain how calculated and datasets that are used.
What is the current area of occupancy?
C. leucas moves throughout the extent of occurrence described in Q11. Data do not exist on unoccupied habitats within the extent of occurrence. It is therefore difficult to determine the current area of occupancy for this species.
What data are there to indicate past declines in area of occupancy (if available, include data that indicates the percentage decline over the past 10 years or 3 generations whichever is longer)?
Due to insufficient data and broad habitat tolerances, there is little to indicate that the potential area of occupancy of C. leucas has changed significantly in the past.
What data are there to indicate future changes in area of occupancy (if available, include data that indicates the percentage decline over 10 years or 3 generations whichever is longer (up to a maximum of 100 years in the future) where the time period is a continuous period that may include a component of the past)?
There is insufficient data to indicate future changes in area of occupancy. Due to the broad tolerance of environmental conditions by C. leucas, it is unlikely that there will be future changes in area of occupancy. However, anthropogenic habitat modification and pollution may lead to a reduction of suitable estuarine and freshwater regions for juveniles (Hazin et al, 2008, O’Connell et al, 2007).
13.How many natural locations do you consider the species occurs in and why? Where are these located?
The term 'location' defines a geographically or ecologically distinct area.
C. leucas has a very broad range of habitat within its area of occupancy, and thus the species does not occur within any specific geographically or ecologically distinct marine environments. However, C. leucas does inhabit a number of freshwater/estuarine systems in Western Australia, including the Fitzroy river and its tributaries, and notably in the Parramatta River in Sydney (Green et al, 2009; Thorburn et al, 2004). It is likely that it capable of inhabiting freshwater reaches of rivers, although these are not necessarily distinct in nature.
14. Give locations of other populations: captive/propagated populations; populations recently re-introduced to the wild; and sites for proposed population re-introductions. Note if these sites have been identified in recovery plans.
Although C. leucas has been successfully kept in captivity elsewhere, there are no instances of captive populations of C. leucas in Australia. There has been no recent reintroduction of C. leucas populations to the wild. There are no current sites for proposed reintroductions. Despite local observations, Werry et al (2009) observed that canal development for waterfront real estate has not necessarily produced an environment favourable for adult bull sharks.
15. Is the species’ distribution severely fragmented? What is the cause of this fragmentation? Describe any biological, geographic, human-induced or other barriers causing this species’ populations to be fragmented.
Severely fragmented refers to the situation in which increased extinction risk to the taxon results from most individuals being found in small and relatively isolated subpopulations (in certain circumstances this may be inferred from habitat information). These small subpopulations may go extinct, with a reduced probability of recolonisation.
Due to the wide distribution of C. leucas, it is unlikely that the species’ distribution is severely fragmented. However, due to the cohabitation of sharks and humans in coastal environments, human induced fishing pressures may have some impact on genetic isolation. Unfortunately, data on the genetic distribution of C. leucas subpopulationsin Australia is currently insufficient or unavailable.
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