40.Describe any relevant daily and seasonal pattern of movement for the species, or other irregular patterns of movement, including relevant arrival/departure dates if migratory.
Data from international research shows that female bull sharks in temperate waters show seasonal movement patterns related to breeding requirements. In temperate and subtropical waters, pregnant females move into estuaries and river mouths during late spring early summer to give birth, and often move upstream after birth to take advantage of low competition for prey items (Pillans et al, 2004; Schmidt and Murru, 1994).
Unfortunately there is little data on the daily movements of adult bull sharks, however Snelson et al (1984) and Thorburton (2006) observe a marked increase of activity during night hours in marine, estuarine and freshwater conditions.
Depending on its locale, the species is known to be either highly migratory or relatively sedentary. In Nicaragua, C. leucas has been observed moving up and down freshwater rapids towards the Nicaragua Lake system, travelling 190km from oceanic conditions to freshwater lacustrine conditions over the period of 7 – 11 days (Thorston, 1971). Conversely, elsewhere C. leucas is considered a relatively sedentary species (Winter et al, 2005). With conflicting evidence from international sources, and insufficient data regarding the movement of C. leucas in Australian waters, it is infeasible to apply the same assumptions to local populations of bull sharks without further research.
41.Give details of the species’ home ranges/territories.
There is insufficient data regarding adult bull shark home range or territory, although the species is considered solitary and aggressively competitive (Bres, 1993).
Survey Guidelines
42.Give details of the distinctiveness and detectability of the species.
C. leucas is a considered a large, aggressive and inquisitive shark (Thorburn, 2007). As it is one of a very limited number of shark species to spend time in low salinity environments, it often has distinctiveness due to its locality alone. However, in environments where other species of genus Carcharhinus are found, accurate identification is often challenging, and it is surmised that many observations of C. leucas in open marine environments are actually misidentified (Thorburn et al, 2004). Adults have indistinct fin markings making them challenging to identify from distance (Last and Stevens, 2009). Juveniles of C. leucas are difficult to identify without tooth count examination.
The detectability of C. leucas is reliant on water conditions, however it willingly takes bait and is regarded as a relatively easy species to detect if searched for (Heupfel and Simpfendorfer, 2008).
43. Describe methods for detecting species including when to conduct surveys (e.g. season, time of day, weather conditions); length, intensity and pattern of search effort; and limitations and expert acceptance; recommended methods; survey-effort guide.
C. leucas studies have primarily used direct capture as a method of detecting the species in an environment. Due to their preference for coastal waters, capture primarily involves longline fishing (Simpfendorfer et al, 2005; Thorburn, 2004), seine netting and gill netting (O’Connell et al, 2007), depending on the importance on the survival of the shark. Surveys have also been conducted using examination of commercial bycatch records (O’Connell et al, 2007), and beach meshing records (Dudley, 1997; Krogh and Reid, 1996).
Section 2 - Threats and Threat Abatement
Threats
44.Identify past, current and future threats, to the species indicating whether they are actual or potential. For each threat, describe:
Primary threats to C.leucas can be identified into 3 broad categories: 1) environmental degradation and 2) fishing and 3) meshing practices.
1) As this species is highly dependent on estuarine environments for parturition and juvenile development, the environmental degradation caused by human induced habitat modification and pollution are likely to have more of an impact on this species in comparison to more pelagic species of shark. Canal developments in Florida (USA) and the Gold Coast (QLD, Australia) which have substantially altered the environment have been prolific in areas where C. leucas is found. (Simpfendorfer & Burgess, 2005). This is widely regarded as an actual threat.
2) 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). This is a global impact, but in Australia is most likely to occur in subtropical and temperate coastal environments where humans and bull sharks most likely interact. Much of this fishing is largely unmonitored despite best efforts. The impacts of overfishing practices on C. leucas populations overseas are well documented, and thus are considered an actual threat.
3) The preferred environment of bull sharks overlaps with human recreational activities. C. leucas is frequently recorded in shark meshing programs in Australia and South Africa (Cliff And Dudley, 1991; Green et al, 2009). Simpfendorfer and Burgess (2005) note that due to species identification issues in Queensland program until the early 1990s the impacts of the Queensland shark control program is unknown. Shark nets have arguably contributed to the decline of large shark species and disruption of breeding behaviour, and thus this practice is a potential threat to the species, especially in combination with the previous factors.
The species is also exploited by large aquariums as they adapt well to life in a tank. The increasing number of aquaria worldwide is increasing this demand, and it is not known what impact this will have on the wild bull shark population. (Simpfendorfer & Burgess, 2005).
what its effect has been so far (indicate whether it is known or suspected; present supporting information/research; does it only affect certain populations);
1) There is a paucity of data regarding the specific impact of environmental degradation on bull sharks in Australia. However it is widely believed that bull sharks, just like other estuarine species, are susceptible to habitat modification and environmental degradation due to human activities. Camhi et al (2007) and McCord and Lamberth (2009) indicate that habitat degradation impacts many species of sharks to the point of local population extirpation, and as bull sharks are more dependent on these environments than many other shark species, it is likely that this risk is exacerbated for this species. Hazin et al (2008) argues that habitat degradation may also lead to the displacement of shark populations into areas preferred by humans, leading to an increase in interactions.
2) Overfishing pressure is considered the most significant of threats regarding the conservation of C. leucas. The species is both a targeted and bycatch species in many fisheries in Australia and overseas (Camhi et al, 2007; Zhou and Griffith, 2008). Studies overseas have found that bull shark constitutes a significant proportion of sharks caught in both of these fisheries (O’Connell et al, 2007). However, the majority of shark capture is unrecorded or lacking in specificity, therefore the exact impact of fisheries is difficult to quantitatively ascertain. O’Connell (2007) suggests that direct fishing mortality, either through overharvesting or bycatch, is higher for apex predators in easily accessible near-shore habitats compared to more remote offshore areas.
3) Due to the relatively small movement range of bull sharks and a preference for shallow coastal and estuarine habitats, bull sharks are caught in shark meshing programs (Krogh and Reid, 1996).
what is its expected effect in the future (is there supporting research/information; is the threat only suspected; does it only affect certain populations);
1) In general, bull sharks will be impacted by habitat modification and degradation more than pelagic shark species due to proximity. Unfortunately there is insufficient data regarding the long term impact of habitat degradation on bull sharks, and due to their wide range this threat will be of higher significance in areas of human environmental interaction. It is expected that as human distribution increases further developments will result in increased interaction with C. leucas. 2) There is a considerable amount of data that suggests that most shark species that are impacted by fisheries will continue to be negatively impacted due to their low capacity for population rebound after consistent population decline (Camhi et al, 2007; Cavanagh et al, 2003). This holds true for bull sharks, which as previously mentioned, are more susceptible to all forms of fishing due to their preference for shallow estuarine habitats. This impact will likely spread over a wide area, impacting more populations of sharks, as demand for fisheries increases over time.
3) Shark meshing programs likely exacerbate the previous threats due to displacement due to habitat modification; however the threat of netting is likely to remain constant over time. Naturally, if more mesh programs are approved, their impact will increase.
what is the relative importance or magnitude of the threat to the species.
1) Habitat degradation is likely to be of high concern to C. leucas, as its natural range and need for low salinity nursery grounds makes it more susceptible to human impact through pollution and environmental modification than other more pelagic species.
2) Due to a lack of protection and little capacity of regulatory enforcement in global and local shark fisheries, fishing pressure is likely to be of very high importance. There is little indication that restrictions on shark fisheries have been effective given a rapid increase in demand for shark products.
3) Shark meshing programs are relatively localised impacts that while overall do not have a high level of impact magnitude, may lead to an exacerbation of the previous impacts.
45. If not included above, identify catastrophic threats, i.e. threats with a low predictability that are likely to severely affect the species. Identify the threat, explain its likely impact and indicate the likelihood of it occurring (e.g. a drought/cyclone in the area every 100 years).
Due to its broad distribution and habitat tolerance, C. leucas is unlikely to be directly impacted across its total population by a catastrophic effect as defined in this question. However, the possibility of food chain collapse due to any manner of events may have a deleterious effect on C. leucas populations. It could be argued that enhanced fishing pressure could emulate the effects of a catastrophic event (Hoenig and Gruber, 1990).
46.Identify and explain any additional biological characteristics particular to the species that are threatening to its survival (e.g. low genetic diversity)?
There is limited to no data on further biological characteristics of the bull shark that threaten its survival. Like most shark species, the bull shark maintains a relatively low fecundity and extended gestation period. There is arguably an osmoregulatory-related physiological need for juvenile bull sharks to remain in estuarine waters for extended periods of time (Heupel and Simpfendorfer, 2008; Whitehead, 2002) which increases the chances of individuals to be impacted by either degraded habitat conditions or fishing-related mortality.
Fishbase (www.fishbase.org) assigns very high vulnerability and very low resilience to C. leucas.
47.Identify and explain any quantitative measures or models that address the probability of the species’ extinction in the wild over a particular timeframe.
The nominees are not aware of a model such as this specifically outlined for C.leucas, however articles such as Myers and Worm (2005) highlight that in general, a reduction of mortality of 40%-80% is required for the recovery of elasmobranch populations in general.
Field et al. (2009) have assessed the susceptibility of sharks to extinction.
48. Is there other information that relates to the survival of this species that you would like to address?
The lack of research in Australia, continued high level tropical and temperate shark fisheries with no stock assessments, and the fact that species recording has been very poor result in there being little idea whether impacts on each species are also threats for the survival of C. leucas.
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