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Dusky Whaler (2020)

Carcharhinus obscurus

  • James Woodhams (Australian Bureau of Agricultural and Resource Economics and Sciences)
  • Matias Braccini (Department of Primary Industries and Regional Development, Western Australia)
  • Victor Peddemors (Department of Primary Industries, New South Wales)
  • Paul Rogers (SARDI Aquatic Sciences, South Australia)

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Summary

Dusky Whaler is a common, large-bodied whaler shark shark found throughout tropical and temperate waters of Australia.  In Western Australia stocks are recovering and there is a sustainable stock in eastern Australia.

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Stock Status Overview

Stock status determination
Jurisdiction Stock Stock status Indicators
Northern Territory Western Australia Recovering Catch, CPUE, direct estimates of fishing mortality, demographic analysis, risk assessment
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Stock Structure

Dusky Shark has a tropical to warm-temperate distribution and is found off the west, south and north coasts of Australia, mostly between latitude 18°S and 36°S [McAuley et al. 2007, Rogers et al. 2013a], and off the east coast, where the range of the species is currently undefined. Electronic and conventional tagging studies have shown Dusky Sharks move between South Australia and Western Australia [Huveneers et al. 2014, Rogers et al. 2013b, Simpfendorfer et al. 1999], and genetic analyses suggest there is restricted gene flow between Dusky Shark off eastern and western Australia [Geraghty et al. 2014]. Therefore, Dusky Shark in South Australian and Western Australian waters are considered to form a single biological stock (the Western Australia stock). This biological stock exhibits a high degree of ontogenetic segregation, with juveniles most common in temperate latitudes and adults in warmer northern latitudes [McAuley et al. 2007, McAuley et al. 2015].

Here, assessment of stock status is presented at the biological stock level—Western Australia and Eastern Australia.

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Stock Status

Western Australia

The cross-jurisdictional Western Australia Dusky Whaler stock has components in waters of 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 carcharhinid species composition in South Australia's commercial multispecies, multi-gear and multi-sectoral Marine Scalefish Fishery (MSF) is not resolved in log-books. Onboard sampling showed carcharhinid catches by the MSF largely comprised juvenile Bronze Whalers (C. brachyurus) [Rogers et al. 2013a, SARDI unpublished data]. . Presence of Dusky Whaler in catches in South Australian waters is seasonally and spatially variable [Jones 2008, Rogers et al. 2013a]. 

Catches of Dusky Whaler in Western Australia’s 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 one–two year old sharks. Collectively, these age classes accounted for 89 per cent of the observed catch during the 1990s [McAuley and Simpfendorfer 2003, Simpfendorfer et al. 2002]. The status of this stock was assessed using demographic modelling techniques, fishing mortality rates estimated from a tagging study in the 1990s, and contemporary catch and catch per unit effort data [McAuley et al. 2007, Simpfendorfer 2010]. A 2005 assessment confirmed that recorded catches of young juvenile sharks in the target fisheries have been sustainable since the mid-1990s. However, the assessment model also predicted that very low levels of fishing mortality (1–2 per cent per year) applied to sharks older than 10 years would result in recruitment overfishing [McAuley et al. 2007]. Previous assessments therefore concluded that the declining trend observed in the effective Dusky Whaler catch per unit effort (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 jurisdictions [Marshall et al. 2016, McAuley et al. 2015, Jones 2008, Rogers et al. 2013a]. The above evidence indicates that this stock was likely to be depleted and recruitment impaired. As a result, comprehensive measures to mitigate cryptic mortality of older Dusky Whalers 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 stock [McAuley et al. 2015], indicating that current management arrangements are suitably precautionary to ensure that the biological stock continues to recover [McAuley et al. 2015].

Recently, a stock assessment was conducted based on a risk-based weight of evidence approach using all available lines of evidence, including simulated biomass trajectories derived from a combination of demographic modelling and catch-only stock reduction analysis [Braccini et al. 2018]. This assessment estimated a “Medium” current risk level for the Dusky Whaler stock, with 46 per cent, 73 per cent and 100 per cent of the simulated current (2015–16) relative total biomass trajectories being above the target, threshold and limit biomass reference points, respectively [Braccini et al. 2018].

The above evidence indicates that the current level of fishing mortality should allow the stock to recover from its recruitment impaired state.

On the basis of the evidence provided above, the Western Australia biological stock is classified as a recovering stock.

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Biology

Dusky Shark biology [Geraghty et al. 2013, 2016 McAuley et al. 2007, Simpfendorfer et al. 2002]

Biology
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  (Western Australia)

Females 15.5 years, 2811 mm TL (Eastern Australia)

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Distributions

Distribution of reported commercial catch of Dusky Shark

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Tables

Fishing methods
Northern Territory
Commercial
Gillnet
Longline (Unspecified)
Recreational
Hook and Line
Management methods
Method Northern Territory
Commercial
Gear restrictions
Processing restrictions
Quota
Catch
Northern Territory
Commercial 162.00kg
Indigenous Undetermined but likely to be negligible
Recreational Undetermined but likely to be negligible

Commercial (catch) Western Australian (state) and Eastern Australia biological stock is for Dusky Shark only. However, South Australian catches are reported as undifferentiated whaler sharks and are therefore not presented.

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.

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.

Western Australia – Recreational (Management Methods) A recreational fishing from boat licence is required for recreational fishing from a powered vessel in Western Australia.

New South Wales commercial fisheries with less than seven active fishers are not presented due to the Privacy Act.

New South Wales – Commercial (catch) For the Ocean Trap and Line Fishery (New South Wales), only one business reported sufficient catch to suggest targeting.

New South Wales – Indigenous (Management Methods) https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing

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Catch Chart

Commercial catches of Dusky Shark - note confidential catch not shown

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References

  1. Australian Fisheries Management Authority, Tropical Tuna and Billfish Management Advisory Committee (TTMAC19), Meeting Minutes, Australian Fisheries Management Authority, Canberra.
  2. Blower DC, Riginos, C and Ovenden, JR. 2019. NeOGen: A tool to predict genetic effective population size (Ne) for species with generational overlap and to assist empirical Ne study design. Molecular Ecology Resources 19: 290-271.
  3. Blower, D. C. 2020. Estimating contemporary abundance, demography, and vulnerability to change for long-lived species with effective population size and population simulation. PhD thesis. School of Biological Sciences, p. 257. The University of Queensland.
  4. Braccini, M, Blay, N, Hesp, A, and Molony, B 2018. Resource Assessment Report Temperate Demersal Elasmobranch Resource of Western Australia. Department of Primary Industries and Regional Development. Fisheries Research Report No. 294 Department of Primary Industries and Regional Development, Western Australia. 149 pp
  5. Geraghty PT, Macbeth WG and Williamson JE 2016, Aspects of the reproductive biology of dusky, spinner and sandbar sharks (Family Carcharhinidae) from the Tasman Sea, Marine and Freshwater Research, 67: 513–525
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  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. 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.
  13. 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.
  14. 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.
  15. 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.
  16. 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.
  17. Rowling, K, Hegarty, A-M and Ives, M 2010, Status of fisheries resources in NSW 2008/09, New South Wales Industry and Investment, Cronulla.
  18. Ryan, K.L., Hall, N. G., Lai, E. K., Smallwood, C. B., Tate, A., Taylor, S. M. and Wise, B. S. (2019). Statewide survey of boat-based recreational fishing in Western Australia 2017/18. Fisheries Research Report No. 297, Department of Primary Industries and Regional Development, Western Australia
  19. Simpfendorfer C, Chin A, Rigby C, Sherman S, White W. 2017. Shark futures: a report card for Australia’s sharks and rays, Centre for Sustainable Tropical Fisheries and Aquaculture, James Cook University. Fisheries Research and Development Corporation Project 2013/009. Final Report 49pp.
  20. Simpfendorfer CA, McAuley R, Chidlow JA, Lenanton R, Hall N and Bastow T 1999. Biology and stock assessment of Western Australia’s commercially important shark species. Project 96/130 Fisheries Research and Development Corporation, Department of Fisheries of Western Australia, Perth.
  21. 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.
  22. 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.
  23. The National Recreational and Indigenous Fishing Survey. FRDC Project No. 99/158. New South Wales Fisheries.

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