The maximum sustainable yield for low–trophic level (forage) species, such as Australian Sardine, is typically achieved at depletion levels of approximately 60 per cent, equivalent to 40 per cent of unfished biomass5. However, harvest strategies for fisheries for these species also need to consider potential impacts on biodiversity and ecosystem health6. Biomass levels above 75 per cent of the unfished level have been identified as a global average for achieving a balance between protecting ecosystem function and biodiversity and providing for food production and economic development of low–trophic level species5. Australian ecosystems are considerably less sensitive to harvesting of low–trophic level species than other systems worldwide5.
Low–trophic level species often undergo large fluctuations in abundance over a range of spatial and temporal scales. For example, two mass mortality events in the 1990s each killed up to 70 per cent of the adult population of Australian Sardine2,7–8. Hence, performance indicators that relate to levels of depletion of unfished biomass are unsuitable for the management of Commonwealth and state fisheries for Australian Sardine9–10.
Spawning biomass of Australian Sardine can be estimated using the daily egg production method (DEPM)11–13. Exploitation rate (i.e. catch/spawning biomass) is a suitable performance indicator for Australian Sardine. Indicators of ecosystem health monitored and modelled in relation to the Sardine Fishery (South Australia), combined with other information on the low sensitivity of Australian ecosystems to harvesting of low–trophic level species, suggests that 30 per cent is a conservative limit reference point for defining overfishing that takes into account the species’ ecological importance (see Figure 4)5– 6,9–10.
The information available to assess biological stock status and the frequency of formal assessments vary among jurisdictions, largely in response to recent catch levels. Catch-and-effort data are monitored annually in all jurisdictions. Estimates of spawning biomass have been obtained using the DEPM for each of the four biological stocks, and population modelling has been undertaken for the southern Australian biological stock and the two Western Australian biological stocks.
Eastern Australian biological stock
Estimates obtained in 1997, 1998 and 2004, using the DEPM, suggest that the spawning biomass of Australian Sardine off eastern Australia is at least 25 000–30 000 tonnes (t)14–16. The biological stock is not considered to be recruitment overfished. Recent catches of approximately 3000–5000 t equate to exploitation rates of less than 25 per cent17. This level of fishing mortality is below the limit reference point and unlikely to cause the biological stock to become recruitment overfished.
On the basis of the evidence provided above, the eastern Australian Sardine biological stock is classified as a sustainable stock.
Southern Australian biological stock
Assessment of the southern Australian Sardine biological stock has involved annual and, more recently, biennial DEPM surveys; in non-survey years, it has involved population modelling based on spawning biomass (estimated through the DEPM), and catch-per-unit-effort and catch-at-age data12–13,18. Estimates of spawning biomass obtained using the DEPM are shown in Figure 3. The current spawning biomass is approximately 200 000 t13, which is above the lower limit reference point of 150 000 t identified in the Sardine Fishery (South Australia) harvest strategy (Figure 3). The exploitation rate (i.e. catch/spawning biomass) is also below the upper limit reference point of 30 per cent (Figure 4). The biological stock is not considered to be recruitment overfished. Recent annual catches of approximately 34 000 t equate to an exploitation rate of approximately 17 per cent. This level of fishing mortality is below the limit reference point and unlikely to cause the biological stock to become recruitment overfished (Figure 4).
On the basis of the evidence provided above, the southern Australian Sardine biological stock is classified as a sustainable stock.
Western Australian west coast biological stock
Population modelling, based on spawning biomass estimates (from the DEPM), and catch-at-age and catch data19 suggest that exploitation rates since the late 2000s for the Western Australian west coast biological stock are less than 10 per cent (2328 t from an estimated spawning biomass of approximately 25 000 t)20. The biological stock is not considered to be recruitment overfished. This level of fishing mortality is below the limit reference point and is unlikely to cause the biological stock to become recruitment overfished.
On the basis of the evidence provided above, the Western Australian west coast biological stock of Australian Sardine is classified as a sustainable stock.
Western Australian south coast biological stock
Population modelling based on spawning biomass estimates (from the DEPM), and catch-at-age and catch data19 suggest that exploitation rates since the late 2000s for the Western Australian south coast stock are around 3 per cent (<3000 t from an estimated spawning biomass of approximately 97 000 t)20. The stock is not considered to be recruitment overfished. This level of fishing mortality is below the limit reference point and unlikely to cause the biological stock to become recruitment overfished.
On the basis of the evidence provided above, the Western Australian south coast biological stock of Australian Sardine is classified as a sustainable stock.
Small-scale fisheries for Australian Sardine have operated in southern Australia since the 1800s. National catches were below 1000 t until the 1970s, when several purse-seine fisheries were established in south-west Western Australia. The Western Australian catch increased steadily, reaching about 9000 t in 1989. In 1991, a purse-seine fishery was established in South Australia to provide food for farmed tuna off Port Lincoln.
In 1995 and 1998, two mass mortality events reduced the Australian biomass of Australian Sardine by up to 70 per cent. Total catches off Western Australia have remained below 3000 t since 1999. The South Australian fishery recovered relatively quickly from the mortality events8,13, and catches increased from around 3500 t in 1998 to around 21 000 t in 2002–03, stabilising at 30 000–34 000 t in recent years. Off the east coast, Australian Sardine catches have exceeded 1000 t per year since 2002–03; they reached about 5000 t in 2008–09, before declining to around 3000 t in 2009–10 and 2010–11.