Table 1: Stock status determination for Dusky Shark
Jurisdiction |
Commonwealth, South Australia, Western Australia |
Commonwealth, New South Wales |
Stock |
South-western Australian
(JASDGDLF, MSF, SESSF WCDGDLF, WTBF) |
Eastern Australian
(ETBF, OTLF) |
Stock status |
↑ |
|
Transitional–recovering |
Undefined |
Indicators |
Demographic analyses, catch, CPUE |
|
CPUE = catch per unit effort; ETBF = Eastern Tuna and Billfish Fishery (Commonwealth); JASDGDLF = Joint Authority Southern Demersal Gillnet and Demersal Longline Managed Fishery; MSF = Marine Scalefish Fishery (South Australia); OTLF = Ocean Trap and Line Fishery (New South Wales); SESSF = Southern and Eastern Scalefish and Shark Fishery (Commonwealth); WCDGDLF = West Coast Demersal Gillnet and Demersal Longline Fishery (Western Australia); W TBF = Western Tuna and Billfish Fishery (Commonwealth)
In Australia, Dusky Shark primarily occurs off the south-west coast between latitudes of approximately 18°S and 36°S. Tagging studies have demonstrated Dusky Shark movements between South Australia and Western Australia; hence South Australia and Western Australia are thought to comprise a single biological stock. The species range off the east coast is currently
undefined. Although it has been suggested that Dusky Shark may comprise a single biological stock in Australian waters1, negligible recorded catches in Victoria, Tasmania and the Northern Territory suggest the existence of a separate biological stock on the east coast. Status reporting here is
based on the assumption of two biological stocks.
South-western Australian biological stock
This cross-jurisdictional biological stock has components in Western Australia, South Australia and the Commonwealth. Research from Western Australia indicates that Dusky Shark from this biological stock is most abundant in waters between north-west Western Australia and the south coast of Western Australia (to 120°E); this part of the biological stock is referred to from here on as
'the Western Australian part of the biological stock'. In South Australian waters, abundance is low and possibly sporadic2. Given that the commercial catch of Dusky Shark from the South Australian and Commonwealth parts of the biological stock is low compared with commercial catch from the Western Australian part of the biological stock, status classification is based only on the Western Australian biological stock assessment.
Stock assessments for the Western Australian part of the biological stock use and assess data from the two fisheries that catch Dusky Shark in Western Australia: the Joint Authority Southern Demersal Gillnet and Demersal Longline Managed Fishery and the West Coast Demersal Gillnet and Demersal Longline Fishery. Because of the size selectivity characteristics of the mesh sizes permitted in the West Coast Demersal Gillnet and Demersal Longline Fishery and its area of operation, Dusky Shark catches have historically consisted of 1–2-year-old individuals, which collectively accounted for 89 per cent of the observed commercial catch during the 1990s3. In view of this, and the fact that the species takes 30 years to reach maturity and may live for more than 50 years, the status of the South-western Australian Dusky Shark biological stock is mostly assessed using stochastic demographic modelling4–5.
The most recent demographic assessment was conducted in 2005. Subsequent assessments of biological stock status have relied on analyses of catch and effective catch per unit effort (CPUE) data from south of 28°S (latitude) to 120°E (longitude) off the south coast. The 2005 assessment confirmed that demersal gillnet and longline fishing mortality rates were likely to have been sustainable for cohorts of sharks born in 1994–95 and 1995–96. However, the model also predicted that even very low levels of fishing mortality (1–2 per cent per year) on sharks older than 10 years could result in population declines. Although targeted catches of adult Dusky Sharks by Western Australian vessels had been eliminated and incidental mortality minimised, previous assessments concluded that the declining trend observed in the effective CPUE series between the mid-1990s and 2004–05 could indicate that breeding biomass had been gradually depleted by fishing mortality generated by other fisheries in other jurisdictions6. This evidence indicates that the biomass of
this biological stock is likely to be recruitment overfished. However, for the period 2006–10, these indicators suggest a recovering biological stock.
Over the past six years, the effective CPUE has shown an increasing trend, with the mean of the
past four years now higher than the mean of the previous 20 years7. In addition, commercial catches of juveniles of this species have been further reduced since 2006–07, to half the quantity determined to be sustainable for cohorts of sharks born in 1994–95 and 1995–96. Therefore, along with the introduction of comprehensive measures to mitigate cryptic mortality of older Dusky Sharks within all Western Australian–managed commercial fisheries, the current management arrangements are considered suitably precautionary to ensure that fishing mortality is now at a level that should allow the biological stock to recover from its recruitment overfished state.
On the basis of the evidence provided above, the biological stock is classified as a transitional–recovering stock.
Eastern Australian biological stock
In New South Wales, whaler sharks (Carcharhinus spp.), including Dusky Shark (C. obscurus), have historically not been identified and reported at the species level in the commercial catch logbooks. Observer data indicate that whaler sharks represent the second highest shark species catch in the New South Wales Ocean Trap and Line Fishery (15 per cent of overall shark catch)8. Insufficient information is available to determine status for any of the whaler shark species in New South Wales, including Dusky Shark9 in their jurisdictions.
Dusky Sharks are taken as a non-target species by Commonwealth fishers in the Eastern Tuna and Billfish Fishery, but have not had their biological stock status assessed. Occasional very small catches (200 kg or less) of Dusky Shark have also been reported from the Commonwealth Coral Sea Fishery. Collaboration is currently under way between New South Wales Fisheries and the Queensland Department of Agriculture, Fisheries and Forestry to confirm the stock structure of Dusky Shark.
Insufficient information is available to confidently classify the status of this biological stock; as a result, it is classified as an undefined stock.
Table 2: Dusky Shark biology1,4,6–7,10–11
Longevity and maximum size |
40–55 years, >289 cm FL, 365 cm TL |
Maturity (50%) |
Females: 27–32 years; 251 cm FL |
FL = fork length; TL = total length
Figure 1: Distribution of reported commercial catch of Dusky Shark in Australian waters, 2010
- Dusky Sharks are mostly taken commercially by demersal gillnets (Western Australia) and by most hook fishing methods, including set lines (New South Wales) and surface longlines (South Australia). They are also taken in smaller quantities by fish trawl fisheries. To minimise commercial catches of larger whaler sharks (in particular, Dusky Shark), longline hook sizes were restricted and metal snoods were prohibited in these fisheries in 2006. Most of the very limited recreational and charter fishing catches of Dusky Shark are taken by rod and reel with bait from boats or the shore.
- A range of input and output controls have been applied to Dusky Shark across the jurisdictions:
- Input controls include limited entry, total allowable effort limits, gear restrictions and spatial zonation.
- Output controls include total allowable catches, commercial size limits, and recreational size and bag limits.
- Four West Coast Demersal Gillnet and Demersal Longline Fishery (Western Australia) vessels and 22 Joint Authority Southern Demersal Gillnet and Demersal Longline Managed Fishery vessels reported Dusky Shark catch during 2009–10. A total of 112 licensees reported whaler shark catches in South Australia during 2009–10. In the Ocean Trap and Line Fishery (New South Wales), 29 businesses reported catching Dusky Shark in 2010. (Vessel numbers cannot be accurately presented in South Australia and New South Wales. Licence and business numbers give a more accurate indication of effort.)
- In 2009–10, the total commercial catch of Dusky Shark (including bronze whaler) was estimated to be 413 tonnes (t), comprising 237 t in Western Australia, 160 t in South Australia, 8 t in New South Wales and 8 t in Commonwealth fisheries. Recreational catch in New South Wales was estimated to be 40–160 t for all whaler shark species9,12. Recreational catches for all shark species in the West
Coast Bioregion of Western Australia were estimated to be approximately 13.5 t7. Indigenous catch is not known.
Figure 2: Commercial catch of whaler shark in Australian waters, 2000–01 to 2010–11 (financial year)
Until recent years, in most jurisdictions Dusky Shark commercial catches have been reported together with catches of the similar-looking and co-occurring whaler sharks. For example, in Western Australia, before 2006–07, Dusky Shark commercial catches were reported together with smaller catches of Bronze Whaler Shark (Carcharhinus brachyurus). For consistency across
years, all catch data presented here represent whaler shark (i.e. Dusky Shark combined with other whaler sharks).
The total commercial catch of whaler shark in Western Australia in 2009–10 was 237 t, somewhat higher than that reported in 2008–09 (177 t). The majority of this catch (188 t) was from the Joint Authority Southern Demersal Gillnet and Demersal Longline Managed Fishery, and a lesser amount (49 t) was from the West Coast Demersal Gillnet and Demersal Longline Fishery (Western Australia). The catch of whaler shark in Western Australia has ranged from 148 to 237 t over the past three years (2007–08 to 2009–10), which reflects changes in fishing effort by these fisheries. There have been negligible reported landings of whaler sharks by other Western Australian–managed fisheries since all sharks and rays were commercially protected in 2006.
Total annual landings of whaler sharks in the Marine Scalefish Fishery (South Australia) increased from 23 t in 1983–84 to around 60 t in 1987–88. Total annual landings then ranged between 60 and
125 t between 1987–88 and 2008–09. In 2009–10, catches nearly doubled from those in 2008–09, from around 90 t to around 155 t; this represents the largest annual increase. However, the greatest decrease followed, with catches decreasing to around 86 t in 2010–11.
For the eastern Australian biological stock, the most significant commercial catches of Dusky Shark are in the Ocean Trap and Line Fishery (New South Wales) as a key non-target species
9. Shark
catch in New South Wales was not required to be reported at the species level until 1991 and, until July
2009, fishers were not required to report individual species catches from a suite of 52 shark species. Dusky Shark catches are unlikely to have exceeded 30 t per year; 2009–10 catch was reported as
7.9 t. Catch of Dusky Shark in the Eastern Tuna and Billfish Fishery (Commonwealth) in 2010 was 2.8 t. Catch in the Eastern Tuna and Billfish Fishery (Commonwealth) has ranged from around 0.5 t to around 6 t since 2000.
- A recent analysis of potential changes in the ecosystem structure of the south and west coasts of Western Australia13 found no evidence of any systematic change in species diversity richness or trophic level. This indicates that the Western Australian fisheries are not having a material impact on the food chain or trophic structure.
- The catch composition of demersal gillnets is highly targeted towards a small number of shark species; the majority of landings consist of only three targeted species of sharks3. Levels of bycatch are therefore relatively low.
- Demersal gillnets and longlines do not significantly impact on benthic habitats where the gear is set—this is usually away from reefs7.
- Demersal gillnets can potentially interact with seals and Australian Sea Lions14. Work is currently under way to understand the potential interaction rates.
- Climate change and variability have the potential to impact on fish stocks in a range of ways, including influencing their geographic distribution (e.g. latitudinal shifts in distribution, changes in distribution patterns of prey species)15. However, it is unclear how climate change may affect risks to sustainability of Dusky Shark.
a Dusky Shark catches have historically been reported together with catches of similar-looking and co-occurring whaler sharks. Throughout this chapter, 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.
b Department of Fisheries, Western Australia
c South Australian Research and Development Institute
d Department of Primary Industries, New South Wales