Dugong (Dugong dugon)
Could become eligible for listing as Vulnerable.
Australian conservation status:
National: Listed as a Marine and Migratory species under the Environment Protection and Biodiversity Conservation Act, 1999.
New South Wales: Listed as Vulnerable under the Threatened Species Conservation Act, 1995.
Northern Territory: Listed as Protected Wildlife under the Territory Parks and Wildlife Conservation Act, 2001.
Queensland: Listed as Vulnerable under the Nature Conservation Act, 1992.
Western Australia: Listed as Specially Protected under the Wildlife Conservation Act, 1950
International conservation status:
-
Listed on Appendix I of the Convention of International Trade in Endangered Species of Fauna and Flora (CITES).
-
Listed as Vulnerable on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species.
-
Listed on Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS).
Although there remains large uncertainty about accurate population figures, a decline in dugong numbers since the 1970s is apparent (Preen, 2000). Marsh et al. (2005) reports on four decades of decline in dugong abundance in Queensland waters through a 40-year assessment of dugong catch per unit effort (CPUE) data in Queensland’s shark control nets, which found that the CPUE in 1999 was only 3% of that in 1962, which indicating a large decline in dugong populations over that period. Population decline measured in Queensland waters between the 1960s and mid-1990s would, if robust and extrapolated to the entire population, qualify the dugong for listing as “Critically Endangered” worldwide (Marsh, 2008).
The dugong is a long-lived mammal with a lifespan of 50-60 years and a low reproductive rate, the minimum pre-reproductive period for the dugong is roughly 9-10 years for both sexes, with one calf every 3-7 years (Marsh et al. 1984). Mammals with these life history parameters (long-lived, low reproductive rate, long generation time and large investment in each offspring) must sustain a high survival rate to maintain population numbers and are vulnerable to human induced mortality. Population simulations suggest that dugong populations are unlikely to increase by more than 5% per year, with sustainability relying on high levels of adult survivorship. This model was simulated using optimal life history parameters including low natural mortality rates and no human-induced mortality. Therefore, the slightest decline of adult dugong survivorship may see a population crash (Marsh e. al. 1984).
Boat strikes pose a direct threat to dugong populations. The shallow water habitat of the dugong subjects the species to high contact with human populations, and further constrains dugongs ability to avoid boats by diving (Hodgson & Marsh, 2007; Maitland et al., 2006). There are also concerns that frequent boat activity can displace dugongs from their preferred habitats (Hodgson & Marsh, 2007). Hodgson (2004) showed that it's likely dugongs initiate their response to approaching vessels appears as a function of distance rather than speed, meaning that when a boat approaches quickly the response may be too slow to avoid an impact.
One of the largest populations and feeding grounds of dugongs in Australia is found in the Townsville/Cardwell area. The Hinchinbrook region provides habitat for approximately half of all dugongs in the Great Barrier Reef south of Cooktown. Oyster Point is sheltered by the Hinchinbrook Channel and retains the world’s largest dugong population. As boat traffic in this area has increased since the 1970’s, the incidence of boat strike on dugongs has also increased and the areas in which they commonly aggregate has decreased (Preen, 2000). Boat traffic in the Hinchinbrook region is increasing due to development and increasing tourist activities, and is likely to continue to grow. This predicted increase in boat traffic can be expected to result in an increase in boat strikes (Preen, 2000).
Moreton Bay is home to the southernmost distribution of dugong on east coast of Australia. It is also an area of high boat usage adjacent to Brisbane and a region of high tourism activity. Dugong population size has increased in this area since the cessation of hunting for oil to a most recent estimate of 500 mammals. This small population size, combined with seagrass beds in close proximity to Brisbane leaves this dugong population especially vulnerable to continued anthropogenic effects.
The Department of Environment and Resource Management (DERM) of Queensland formerly kept an annual stranding and mortality database for dugongs, which included data on mortality due to boat strike. These annual reports, the most recent of which is Biddle et al. (2011), show the known level of dugong mortality as a result of boat strike between the years of 1996-2010, with a summary of this data being shown in the following table (Table 1). It can be reasonably assumed that the actual mortality rate is significantly higher, due to not all incidences being detected, further incidences not being reported, and many instances not being able to be officially assessed and have cause of mortality confirmed.
Table 1: Dugong mortalities caused by boat strike in Queensland waters for the period 1996-2010
Year
|
1996
|
1997
|
1998
|
1999
|
2000
|
2001
|
2002
|
2003
|
2004
|
2005
|
2006
|
2007
|
2008
|
2009
|
2010
|
Mortalities
|
3
|
4
|
2
|
0
|
3
|
4
|
7
|
3
|
5
|
4
|
2
|
2
|
2
|
3
|
4
|
A more detailed table, including data on all known causes of dugong mortality, is listed in the appendices as Figure 1. When taking into account natural causes of mortality (such as disease and ill health), incidences where the cause of mortality could not be determined, and incidences which were unable to be confirmed, a more accurate picture of the proportion of additional impact on dugong populations boat strike has can be determined (Table 2). With a total of 46 dugong mortalities attributed to boat strike, the process was notably the leading cause of anthropogenic mortality in 7 of the 15 years data has been recorded.
Table 2: Proportion of anthropogenic mortalities of dugongs attributable to boat strike for the period 1996-2010
Year
|
1996
|
1997
|
1998
|
1999
|
2000
|
2001
|
2002
|
2003
|
2004
|
2005
|
2006
|
2007
|
2008
|
2009
|
2010
|
Total confirmed mortality
|
16
|
20
|
9
|
25
|
33
|
18
|
15
|
14
|
15
|
10
|
5
|
10
|
10
|
21
|
34
|
Natural causes
|
3
|
7
|
4
|
10
|
23
|
11
|
5
|
6
|
2
|
3
|
0
|
2
|
3
|
10
|
16
|
Anthropogenic
|
13
|
13
|
5
|
15
|
10
|
7
|
10
|
8
|
13
|
7
|
5
|
8
|
7
|
11
|
18
|
% anthro. boatstrike
|
23.1
|
30.8
|
40.0*
|
0.0
|
30.0
|
57.1*
|
70.0*
|
37.5*
|
30.8*
|
28.6
|
40.0*
|
25.0
|
28.6*
|
27.3
|
22.2
|
* = boat strike caused more than or as many deaths than any other anthropogenic activity recorded
Data collected from carcasses in Moreton Bay suggests that the majority of boats involved in strikes on dugongs are not small recreational boats powered with outboard motors, but rather larger recreational and commercial vessels with twin propeller configurations (Limpus, 2002). Particularly at risk are regionally important Dugong populations in extensive shallow areas close to areas of high boat traffic (Marsh et al., 2002). On the urban coast of Queensland, areas of high recreational use such as the Hinchinbrook Island area, Cleveland Bay, Hervey Bay and Moreton Bay are of greatest concern (Marsh et al., 2002). Hodgson (2004) discusses mortality data and possible threat abatement measures for several priority areas of dugong habitat.
The plight of the dugong, as recognised through Marine Species and Migratory Species listings under the EPBC Act, as a protected species under New South Wales, Queensland, Northern Territory and Western Australian law, and international obligations with the species listed on Appendix I of CITES, Appendix II of the CMS and as Vulnerable on the IUCN Red list, clearly illustrates the high sensitivity of this species and the resulting need for a proactive and vigorous management approach. The above demonstrates that the threat posed by boat strike to Australia’s dugong population could be a significant contributing threat causing it to become eligible for listing under the EPBC Act as Vulnerable. Not only is the current known rate of boat strikes on dugong a cause for concern, but also the projection of that rate into the future, as boat traffic increases due to the growth in coastal human populations in key areas.
Australian snubfin dolphin (Orcaella heinsohni)
Could become eligible for listing as Vulnerable.
Australian conservation status:
National: Listed as a Cetacean under the Environment Protection and Biodiversity Conservation Act, 1999.
International conservation status:
-
Listed as Orcaella brevirostris on Appendix I of the Convention of International Trade in Endangered Species of Fauna and Flora (CITES).
-
Listed on Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS).
In 2005 the Irrawaddy dolphin (Orcaella brevirostris) was discovered to be two separate species, with one species retaining the original name and the other described as the Australian snubfin dolphin (Orcaella heinsohni). In Reeves et al. (2008), it is cited that Australian snubfin dolphins inhabit coastal, shallow waters of the tropical and subtropical zones of Australia, and possibly some parts of New Guinea (Beasley et al., 2005). In Australia, they occur from Broome, Western Australia, north and east to the Brisbane River, Queensland. The range along the northern Australian coast and New Guinea is poorly documented (Parra et al., 2002).
As many national and international conservation agreements and pieces of legislation are yet to assess the Irrawady and Australian snubfin dolphins as separate species, various conservation statuses do not yet accurately reflect these species individual situations. It logically follows however, that when a threatened population is split, the status of the remaining populations become even more critical and vulnerable to anthropogenic threats. The effects of ongoing loss of only a small proportion of individuals from such small and possibly geographically isolated populations can be as severe as extinction over a relatively short time-frame (Thiele, 2010).
Thiele (2010) studied injuries in a population of 161 snubfin dolphins in Roebuck Bay, on the Western Australia Kimberley Coast, of which 124 had suitable images for determining whether an injury was present and its likely cause. Instances of boat strike were inferred from blunt trauma marks or propeller cuts, and it was found that there was evidence of vessel strike alone for 12 individuals (9.7% of those with suitable photographs) , and of vessel strike and fishing gear combined in 14 individuals (11.3% of those with suitable photographs). Therefore, of the 124 Australian snubfin dolphins in Roebuck Bay for which imaging was available, approximately 26.1% had evidence of being struck by boats.
These figures represent conservative figures of interactions as snubfin dolphins that have died following a boat strike are unlikely to be found in an area of such high tidal fluxes, with the high incidence of non-fatal interactions suggesting mortality is likely to occur (Thiele, 2010). Thiele (2010) goes on to hypothesise that the high rate of vessel related snubfin dolphin injuries is likely attributable to the concentrated and active socialising behaviour of the species, during which they appear much less aware of their surroundings and may be unable to react quickly enough to avoid approaching boats.
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