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Stock Status Overview
|South Australia||Western Australia||MSF||Transitional-recovering||Catch, CPUE , direct estimates of fishing mortality|
- Marine Scalefish Fishery (SA)
Dusky Shark occurs off the west, south and north coasts of Australia, mostly between latitude 18°S and 36°S1,2, and off the east coast, where the range of the species is currently undefined. Tagging studies have demonstrated Dusky Shark movements between the South Australian gulfs and Western Australian shelf and slope habitats3,4, and genetic analysis suggests restricted gene flow between Dusky Shark off eastern and western Australia5. Therefore, Dusky Shark in South Australian and Western Australian waters are considered to form a single biological stock (the Western Australian stock). This biological stock exhibits a high degree of ontogenetic segregation, with juveniles most common in temperate latitudes and adults in warmer northern latitudes1,6. Because of negligible recorded catches in Victoria and Tasmania and comparatively small catches in Queensland and the Northern Territory, the Western Australian biological stock is believed to be separated from the east coast biological stock (the Eastern Australian stock).
Here, assessment of stock status is presented at the biological stock level—Western Australian and Eastern Australian.
Dusky Shark catches have historically been reported together with catches of similar-looking and co-occurring whaler sharks. In this report, the term ‘Dusky Shark’ refers specifically to Carcharhinus obscurus, whereas the term ‘whaler shark’ refers to C. obscurus in combination with other whaler shark species, particularly Carcharhinus brachyurus.
The cross-jurisdictional Western Australian Dusky Shark stock has components in the Commonwealth, Western Australia and South Australia. This biological stock is most abundant in waters between north-west and southern Western Australia (to longitude 120°E). The catch composition in the South Australian state-managed Marine Scalefish Fishery is largely comprised of juvenile Bronze Whalers (C. brachyurus), with juvenile Dusky Sharks (900–3000 mm total length) representing less than 10 per cent of the catch. Its abundance in South Australian waters is seasonally and spatially variable2,7. Given that this stock’s primary distribution is off the south-west coast and that Western Australian catches have historically been several times higher than those in the other jurisdictions, the status of this biological stock is determined from the Western Australian stock assessment.
Catches of Dusky Shark in the Joint Authority Southern Demersal Gillnet and Demersal Longline Managed Fishery, and the West Coast Demersal Gillnet and Demersal Longline Fishery have historically consisted of neonate (young of the year) and 1–2 year old sharks. Collectively, these age classes accounted for 89 per cent of the observed catch during the 1990s8,9. The status of this stock has been assessed using demographic modelling techniques, fishing mortality rates estimated from a tagging study in the 1990s, and contemporary catch and catch per unit effort (CPUE) data1,10. The most recent assessment in 2005 confirmed that recorded catches of young juvenile sharks in the target fisheries have been sustainable since the mid-1990s. However, the model also predicted that very low levels of fishing mortality (one–two per cent per year) applied to sharks older than 10 years would result in recruitment overfishing1.
Previous assessments therefore concluded that the declining trend observed in the effective Dusky Shark CPUE series between the mid-1990s and 2004–05 indicated that breeding biomass had been gradually depleted by low, but poorly quantified, levels of extraneous fishing mortality, including fisheries operating in other jurisdictions2,6,7,11. The above evidence indicates that this stock was likely to be subjected to overfishing. As a result, comprehensive measures to mitigate cryptic mortality of older Dusky Sharks within all Western Australian managed commercial fisheries, and to reduce catches of juveniles to below the levels determined to be sustainable in the mid-1990s, have been introduced since 2006–07. An increasing trend in the effective CPUE from 2006–12, suggests improved recruitment rates and a recovering stock6, suggesting current management arrangements are suitably precautionary to ensure that the biological stock continues to recover6.
On the basis of the evidence provided above, the Western Australian biological stock is classified as a transitional–recovering stock.
Dusky Shark biology1,9,14
|Species||Longevity / Maximum Size||Maturity (50 per cent)|
|Dusky Whaler||Females: >40 years; 2 890 mm FL Males: >32 years; 3 560 mm TL (~2 920 mm FL)||Females: 27–35 years; 2 540 mm FL|
Distribution of reported commercial catch of Dusky Shark
|Hand Line, Hand Reel or Powered Reels|
|Hand Line, Hand Reel or Powered Reels|
|96 in MSF|
- Marine Scalefish Fishery (SA)
|Recreational||Undetermined, Undetermined but likely to be negligible|
a Commonwealth – Recreational The Australian Government does not manage recreational fishing in Commonwealth waters. Recreational fishing in Commonwealth waters is managed by the state or territory immediately adjacent to those waters, under its management regulations.
b Commonwealth – Indigenous The Australian Government does not manage non-commercial Indigenous fishing in Commonwealth waters, with the exception of the Torres Strait. In general, non-commercial Indigenous fishing in Commonwealth waters is managed by the state or territory immediately adjacent to those waters.
c New South Wales – Indigenous (management methods) Aboriginal Cultural Fishing Interim Access Arrangement - allows an Aboriginal fisher in New South Wales to take in excess of a recreational bag limit in certain circumstances, for example, if they are doing so to provide fish to other community members who cannot harvest themselves.
d New South Wales – Indigenous (management methods) Aboriginal Aboriginal Cultural Fishing Interim Access Arrangement - allows an Aboriginal fisher in New South Wales to take in excess of a recreational bag limit in certain circumstances, for example, if they are doing so to provide fish to other community members who cannot harvest themselves.
e Indigenous (management methods) Aboriginal Subject to the defence that applies under Section 211 of the Native Title Act 1993 (Cth), and the exemption from a requirement to hold a recreational fishing licence, the non-commercial take by indigenous fishers is covered by the same arrangements as that for recreational fishing.
f New South Wales – Commercial (catch) For the Ocean Trap and Line Fishery (New South Wales), only one business reported sufficient catch to suggest targeting.
g Commercial (catch) Western Australian (state) and Eastern Australian biological stock is for Dusky Shark only. However, South Australian catches are reported as undifferentiated whaler sharks and are therefore not presented in Table 3 or Figure 2.
Commercial catches of Dusky Shark
Effects of fishing on the marine environment
- Recent analysis of potential changes in ecosystem structure of finfish in the South and West Coast Bioregions of Western Australia16 found no evidence of any systematic change in species diversity or richness, or trophic index, indicating that this fishery is not having a measurable impact on the food chain or trophic structure.
- Demersal gillnets used to catch Dusky Shark in Western Australia are deployed infrequently, over a small proportion of the target fisheries’ operational area. The physical impact of the gear on benthic habitats is minimal6.
- Demersal gillnets are known to interact with threatened and protected species in areas where they are used to catch Dusky Shark. However, such interactions occur at a very low frequency, and have been assessed as posing low to negligible risks to these populations6.
Environmental effects on Dusky Whaler
- Climate change and climate variability have the potential to impact shark stocks in a range of ways, including influencing their geographic distribution (for example, latitudinal shifts in distribution). The impact of environmental changes on Dusky Shark stocks is unknown.
- 1 McAuley, RB, Simpfendorfer, CA and Hall, NG 2007, A method for evaluating the impacts of fishing mortality and stochastic influences on the demography of two long-lived shark stocks, ICES Journal of Marine Science, 64: 1710–1722.
- 2 Rogers, PR, Huveneers, C, Goldsworthy SD, Cheung, WWL, Jones KG, Mitchell, JG and Seuront, L 2013, Population metrics and movement of two sympatric carcharhinids: a comparison of the vulnerability of pelagic sharks of the southern Australian gulfs and shelves, Marine and Freshwater Research, 64: 20–30.
- 3 Huveneers, C, Rogers, P and Drew, M 2014, Monitoring shark species of conservation concern within the Adelaide metropolitan and Gulf St Vincent regions, final report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board, South Australian Research and Development Institute (Aquatic Sciences), SARDI publication F2013/000716-1, SARDI research report series 754, SARDI, Adelaide, 89.
- 4 Rogers, P, Huveneers, C, Goldsworthy, SD, Mitchell, JG and Seuront, L 2013, Broad-scale movements and pelagic habitat of the Dusky Shark Carcharhinus obscurus off Southern Australia determined using pop-up satellite archival tags, Fisheries and Oceanography, 22: 102–112.
- 5 Geraghty, PT, Williamson, JE, Macbeth, WG, Blower, DC, Morgan, JAT, Johnson, G, Ovenden, JR and Gillings, MR 2014, Genetic structure and diversity of two highly vulnerable carcharhinids in Australian waters, Endangered Species Research, 24: 45–60.
- 6 McAuley, R, Braccini, M, Newman, SJ and O’Malley, J 2015, Temperate Demersal Gillnet and Demersal Longline Fisheries Status Report, in WJ Fletcher and K Santoro (eds), Status Reports of the Fisheries and Aquatic Resources of Western Australia 2014/15, Western Australian Department of Fisheries, Perth, 261–272.
- 7 Jones, K 2008, Review of the fishery status for whaler sharks (Carcharhinus spp.) in South Australian and adjacent waters, report to the Fisheries Research and Development Corporation, FRDC project 2004/067, South Australian Research and Development Institute (Aquatic Sciences) publication F2007/000721-1, SARDI research report series 154, SARDI, Adelaide.
- 8 McAuley, R and Simpfendorfer, C 2003, Catch composition of the Western Australian Temperate Demersal Gillnet and Demersal Longline fisheries, 1994–1999, Fisheries research report 146, Western Australian Department of Fisheries, Perth.
- 9 Simpfendorfer, CA, McAuley, R, Chidlow, J and Unsworth, P 2002, Validated age and growth of the Dusky Shark, Carcharhinus obscurus, from Western Australian waters, Marine and Freshwater Research, 53: 567–573.
- 10 Simpfendorfer, C 1999, Demographic analysis of the dusky shark fishery in southwestern Australia, in JA Musick (ed.), Life in the slow lane: Ecology and conservation of long-lived marine animals, American Fisheries Society Symposium 23, Bethesda, Maryland, 149–160.
- 11 Marshall, L, Giles, J and Johnson, GJ 2016, Catch composition of a traditional Indonesian shark fishery operating in the MOU Box, northwestern Australia: Results of shark fin identification from Operation Snapshot (May 2015), 2016/001375, Australian Fisheries Management Authority, Canberra.
- 12 Macbeth, WG, Geraghty, PT, Peddemors, VM and Gray, CA 2009, Observer-based study of targeted commercial fishing for large shark species in waters off northern New South Wales, Cronulla Fisheries Research Centre of Excellence, New South Wales Industry and Investment, Cronulla.
- 13 Rowling, K, Hegarty, A-M and Ives, M 2010, Status of fisheries resources in NSW 2008/09, New South Wales Industry and Investment, Cronulla.
- 14 Geraghty PT, Macbeth, WG, Harry, AV, Bell, JE, Yerman, MN and Williamson, JE 2013, Age and growth parameters for three heavily exploited shark species off temperate eastern Australia, ICES Journal of Marine Science, 71: 559–573.
- 15 Ryan, KL, Wise, BS, Hall, NG, Pollock, KH, Sulin, EH and Gaughan, DJ 2013, An integrated system to survey boat-based recreational fishing in Western Australia 2011/12, Fisheries research report 249, Western Australian Department of Fisheries, North Beach.
- 16 Hall, NG and Wise, BS 2010, Development of an ecosystem approach to the monitoring and management of Western Australian fisheries, Fisheries research report 215, Western Australian Department of Fisheries, North Beach.