Dusky Shark Carcharhinus obscurus^a

Western Australian biological stock

The cross-jurisdictional western Australian Dusky Shark stock has components in Western Australia, South Australia and the Commonwealth. This biological stock is most abundant in waters between north-west and southern Western Australia (to 120°E longitude). Its abundance in South Australian waters is generally lower, and possibly more seasonal and sporadic^2,7. 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 the 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 fish. Collectively, these age classes accounted for 89 per cent of the observed catch during the 1990s^8,9. 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 data^1,10. 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 (1–2 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 standardised Dusky Shark catch-per-unit-effort 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^2,6,7. This evidence indicates that the biomass of the biological stock was likely to be recruitment overfished. However, for the period 2006 to 2012, an increasing trend in the effective catch-per-unit-effort data suggests improved recruitment rates and a recovering biological stock. Thus, following the comprehensive measures to mitigate cryptic mortality of older Dusky Sharks within all Western Australian–managed commercial fisheries, and to reduce catches of juveniles to less than the levels determined to be sustainable in the mid-1990s, current management arrangements are considered suitably precautionary to ensure that the biological stock continues 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, Dusky Shark was not identified and reported at the species level in the commercial catch logbooks until 2009. Observer data indicate that whaler sharks represent the second highest shark species catch in the Ocean Trap and Line Fishery (New South Wales), at 15 per cent of the overall catch¹¹. Insufficient information is available to determine status for any of the whaler shark species in New South Wales, including Dusky Shark ¹².

Dusky Shark is taken as a nontarget species by Commonwealth fishers in the Eastern Tuna and Billfish Fishery, but its biological stock status has not been assessed. Occasional very small catches (200 kg or less) of Dusky Shark have also been reported from the Coral Sea Fishery (Commonwealth). Research is currently under way to confirm the stock status of Dusky Shark.

Insufficient information is available to confidently classify the status of this stock, and the biological stock is therefore classified as an undefined stock.