Gummy Shark Mustelus antarcticus

Southern Australian biological stock

There is a close relationship between the number of pups and both the number and length of females⁵ and so the Commonwealth assessment uses pup production as an indicator of biomass. The most recent assessment⁶ treats Bass Strait, South Australia and Tasmania as separate subpopulations, with no movement of animals between these regions and no density-dependent effects of one population on another. Gillnet closures off South Australia have been progressively implemented to reduce interactions with marine mammals (particularly Australian Sea Lions and dolphins), starting with voluntary closures in 2009–10, followed by mandatory closures in 2010–11⁸. These closures have reduced catch and catch per unit effort off South Australia. As a result of the large changes in operating conditions, the South Australian catch-per-unit-effort data after 2010 was considered to be a poor indicator and was not included in the model. The South Australian closures may have also reduced the catch per unit effort off Victoria, as inexperienced operators moved from South Australia to Victoria⁶. However, as these impacts were considered minor, catch and catch-per-unit-effort data up to 2012 from Victoria (and Tasmania) were retained in the model.

The base-case assessment estimated pup production to be above 48 per cent of the unfished level of pup production (1927) for all three Gummy Shark populations. Some model sensitivities that explored density dependence estimated pup production to be as low as 31 per cent (for the Bass Strait population), which is below the target reference point for the Commonwealth Harvest Strategy Policy, but above the limit reference point. These results indicate that the three populations examined (Bass Strait, South Australia and Tasmania) are not recruitment overfished⁹.

The Shark Resource Assessment Group’s recommended biological catch for the three subpopulations combined was 1836 tonnes (t) for the 2013–14 season ¹⁰. Total combined commercial catch for the three subpopulations in 2013 was 1661 t. This level of fishing mortality is not expected to cause this part of the biological stock to become recruitment overfished⁹.

The Western Australian component of the stock was assessed using an age-structured model developed in the mid-1990s to estimate total and breeding stock biomass¹¹. Since then, catches and catch per unit effort have been monitored in reference to those biomass estimates. A new assessment model incorporating movement rates within Western Australian waters is due to be completed by 2015. The most recent assessment⁷ concluded that reductions in demersal gillnet fishing effort since biomass was estimated to be 42.7 per cent of the unfished (1975) level, in 1997–98, should have ensured that biomass has remained above this level. Therefore, this part of the biological stock is not considered to be recruitment overfished.

These conclusions are supported by an increasing trend in standardised catch per unit effort between the mid-1990s and 2005–06. This is thought to be the result of reductions in demersal gillnet fishing effort in Western Australia from 1992 onwards, leading to increases in breeding stock biomass. A subsequent increase in catch per unit effort to unprecedented levels peaked in 2007–08 and was followed by a rapid decline to 2011–12 (the most recent year of available data). However, the catch per unit effort in 2011–12 was still at historically high levels, and this issue will be assessed through updated modelling for this part of the biological stock during the next 12 months. Despite the recent unusual spike and subsequent decline in catch per unit effort, current levels of fishing mortality are considered unlikely to cause this part of the biological stock to become recruitment overfished⁷ .

On the basis of the evidence provided above, the entire biological stock is classified as a sustainable stock.

Eastern Australian biological stock

Available information indicates that there is little catch of Gummy Shark (less than 50 t per year) from the eastern Australian biological stock ¹². Hence, insufficient information is available to confidently classify the status of this stock.

On the basis of the evidence provided above, the biological stock is classified as an undefined stock.