23. What is the species’ total population size in terms of number of mature individuals? How were population estimates derived and are they reliable? Are there other useful measures of population size and what are they?
In the absence of figures, terms such as common, abundant, scarce can be of value.
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There is insufficient data to reliably define the total population size of C. leucas. In international regions of similar environment and impact, local populations of C. leucas have fallen up to 99.9% due to overfishing and environmental degradation (O’Connell et al, 2007). Further, the average size of caught specimens has markedly reduced, indicating a lower proportion of mature individuals (Wintner et al, 2002).
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24. Does the species occur in a number of smaller populations? How many? For each population give the locality, numbers and trends in numbers and tenure of land (include extinct populations). Can these be considered to be subpopulations and why?
Subpopulations are defined as geographically or otherwise distinct groups in the population between which there is little demographic or genetic exchange.
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There is insufficient data to indicate the presence of distinct subpopulations of C. leucas in Australian waters, however due to the extensive size of the species’ natural range, it is likely that there is little genetic exchange between geographically distanced individuals. Further genetic studies of bull shark populations would assist in understanding these dynamics further.
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25. Provide details on ages of the following:
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a. sexual maturity;
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Sexual maturity for both sexes appears to vary throughout their range. Estimates for sexual maturity for both male and female C. leucas range from 6-14 years (Camhi et al, 2007), 9-10 years (Cruz-Martinez et al, 2004), and 20 years (Last and Stevens, 2009).
Males mature at 157-226cm and females at 180-230 cm (Simpfendorfer & Burgess, 2005).
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b. life expectancy;
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Life expectancy data varies across their range. Last and Stevens (2009) report that the species has a life expectancy of up to 50 years in South Africa. Simpfendorfer and Burgess (2005) estimate that females live up to 16 years and males to 12 years from a study in Lake Nicaragua. The author is not aware of any life expectancy study that has been undertaken in Australia.
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c. natural mortality.
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Heupel and Simpfendorfer (2008) note that juvenile C. leucas appear to have a relatively low natural mortality rate according to capture/recapture records over their first 18 months. There is insufficient data to estimate natural mortality rates of adult age cohorts, however as an apex predator in most of its range, it is likely that natural mortality for adults is low.
Simpfendorfer & Burgess (2005) reported that a research study estimated the natural mortality of C. leucas to be 0.166 year-1 based on a maximum age of 27 years.
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26. Reproduction
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For animals: provide overview of breeding system and of breeding success, including: when does it breed; what conditions are needed for breeding; are there any breeding behaviours that may make it vulnerable to a threatening process?
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Scientific research reports a wide range of information on the reproduction of C. leucas.
Depending on locality, bull sharks reach maturity at an age of between 6 – 15 years (the lower range for males), and a length of approximately 200cm for both sexes. C. leucas is a viviparous species that gives birth to 1 – 22 live young, dependent on locality (Cruz-Martinez et al, 2004). The gestation period for this species is between 10 – 11 months (Calliouet et al, 1964; Camhi et al, 2007). Breeding is temperature dependent – temperate areas have a late spring summer breeding period, while in tropical latitudes it is recorded that females can be caught at various stages of prenatal development throughout the year.
Last and Stevens (2009) report that females normally give birth in estuaries and river mouths and the young remain in the river for up to 5 years; litter sizes range from 1-13 and the gestation period is 10-11 months.
Simpfendorfer and Burgess (2005) report that reproduction is by placental viviparity, and most litters are between 6-8 pups. Birth normally occurs in late spring and summer, although in warmer areas (e.g. Nicaragua) breeding may occur year-round.
Juveniles are independent from birth. Low rates of mortality among juvenile cohorts are been recorded by Simpfendorfer et al (2005).
Bull sharks are highly dependent on estuarine environments as pupping and nursery grounds. Females move into these areas during parturition, however it is uncertain whether mating occurs there as well (Snelson, 1984). As these environments are also very important for human activities, there is potential that this breeding behaviour increases the potential for conflict. Beyond this, other factors of breeding success are poorly understood.
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27. What is the population trend for the entire species?
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What data are there to indicate past decline in size (if available, include data on rate of decline over past 10 years or 3 generations whichever is longer)?
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Numerous author report long term declines in C. leucas. O’Connell et al (2007) found high significant declines in populations in heavily fished and environmental degraded estuaries in Lousiana, United States – it was estimated that the species had declined by 98.6% to 99.9% since 1953. Wintner et al (2002) likewise reported an 85% decline in populations of C. leucas over a period between 1966 and 1977 in South Africa, with even further declines since. There is insufficient data to indicate similar trends in Australian waters, however it is likely that due to similar conditions and pressures, it could be assumed that similar declines are at least possible.
The IUCN states that the population trend is unknown (Simpfendorfer & Burgess, 2005).
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What data are there to indicate future changes in size (if available, include data which will indicate the percentage of decline over 10 years or 3 generations whichever in 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)?
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There are no published data readily available that indicate future changes in the size of C. leucas populations. However, due to the proximity of human development with C. leucas’ preferred habitat, it is likely that significant portions of C. leucas’ habitats, particularly riverine and estuarine systems but also marine environments, have suffered a decline in quality (Cavanagh et al, 2003). In addition, C. leucas is frequently caught as bycatch in industrial fishing practices as well as being targeted as a sport fish and also exploited for its skin, liver oil and flesh, and prized for its fins in the shark-fin trade (Simpfendorfer & Burgess, 2005). These are therefore two of the main threats facing the bull shark which suggest that the population may decline in future unless these threats are addressed.
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28. Does the species undergo extreme natural fluctuations in population numbers, extent of occurrence or area of occupancy? To what extent and why?
Extreme fluctuations can be said to occur in a number of taxa when population size or distribution area varies widely, rapidly and frequently, typically with a variation greater than one order of magnitude (i.e. a tenfold increase or decrease).
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Extreme fluctuations can be said to occur in a number of taxa when population size or distribution area varies widely, rapidly and frequently, typically with a variation greater than one order of magnitude (i.e. a tenfold increase or decrease). Though there is insufficient data to show that C. leucas undergoes extreme natural fluctuations in population numbers, this species is typified by slow growth rates, low mortality rates and a relatively low reproductive rate. Therefore it is highly unlikely that the species is characterized by extreme natural fluctuations.
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29. What is the generation length and how it is calculated?
Generation length is the average age of parents of the current cohort (i.e. newborn individuals in the population). Generation length therefore reflects the turnover rate of breeding individuals in a population. Generation length is greater than the age at first breeding and less than the age of the oldest breeding individual, except in taxa that breed only once. Where generation length varies under threat, the more natural, i.e. pre-disturbance, generation length should be used.
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In this examination generation length is the average age of parents of the current cohort (i.e. newborn individuals in the population). Generation length therefore reflects the turnover rate of breeding individuals in a population. Generation length is greater than the age at first breeding and less than the age of the oldest breeding individual, except in taxa that breed only once. Where generation length varies under threat, the more natural, i.e. pre-disturbance, generation length should be used. Utilising data from Smith et al (1998), in which C. leucas is conservatively given an age of maturity of 15 years and a maximum reproductive age of 27 years, it can be calculated as the average age of reproductive potential.
Therefore if generation length is the average age of parents of the current cohort, this would be estimated at 21.
(15+27)/2 = 21 years.
However there is data to indicate that Australian populations become sexually mature at larger sizes (Frisk et al, 2000), and as size is usually correlated with age, it is possible that this generation length is conservative.
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30. Identify important populations necessary for the species’ long-term survival and recovery? This may include: key breeding populations, those near the edge of the species’ range or those needed to maintain genetic diversity.
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There are no data readily available that identifies populations of importance for this species.
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31. Describe any cross-breeding with other species in the wild, indicating how frequently and where this occurs.
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There is insufficient data regarding the occurrence of cross-breeding with other species in the wild. To date this behaviour has not been observed or recorded.
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32. Departmental Use only:
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