Table 1: Stock status determination for Giant Crab
Jurisdiction |
South Australia, Tasmania, Victoria, Western Australia |
Stock |
Southern Australian
(NZGCF, SZGCF, SCC, GCF [TAS], GCF [VIC], WCDSCF) |
Stock status |
|
Sustainable |
Indicators |
Percentage of egg production relative to unfished level, proportion of spawning stock protected by minimum size limits |
GCF [TAS] = Giant Crab Fishery (Tasmania); GCF [VIC] = Giant Crab Fishery (Victoria); NZGCF = Northern Zone Giant Crab Fishery (South Australia); SCC = South Coast Crustacean Fishery (Western Australia); SZGCF = Southern Zone Giant Crab Fishery (South Australia); WCDSCF = West Coast Deep Sea Crustacean Fishery (Western Australia)
Giant crab, from Western Australia to Tasmania, is considered a single biological stock because the species occurs in a continuous distribution across this range. The larval duration is around 50 days, with larval release occurring along the edge of the continental shelf. The shelf is a high-current area, facilitating dispersal. Oceanographic modelling has indicated that Giant Crab dispersal occurs over large spatial scales1–3.
Southern Australian biological stock
This cross-jurisdictional biological stock has components in Tasmania, Western Australia, South Australia and Victoria. Each jurisdiction assesses that part of the biological stock that occurs in its waters. The status presented here for the entire biological stock has been established using evidence from all four jurisdictions.
For the Tasmanian part of the biological stock (where most of the commercial catch is taken), a length-based model has been developed to estimate annual levels of biomass and egg production; the model is based on data that include catch-and-effort data from commercial fisheries4. The assessment estimated that egg production is stable at 19 per cent of unfished levels5, which is conservative relative to benchmarks in similar crustacean fisheries6. Hence, this part of the biological stock is not considered to be recruitment overfished.
The total allowable commercial catch (TACC) for the Tasmanian part of the biological stock has been reduced from 104 tonnes (t) in 2003–04 to 47 t in 2012–13, with the objective of increasing abundance and catch rates. In the 2010–11 quota year (March to February), only 90 per cent of the TACC was taken. Commercial catch has been below the TACC since at least 2005–06. This level of fishing mortality is unlikely to cause this part of the biological stock to become recruitment overfished.
For the remainder of the biological stock, commercial catch is relatively low (a total of 35 t in 2010–11) and comes from across the continental shelf break, from Albany (Western Australia) to eastern Victoria. Across this broad region, commercial catch rates provide an indication of the status of the legal-sized portion of the biological stock (but not the total biological stock); the commercial catch rate is stable in South Australia7 but falling in Victoria8. Management of fishing mortality in this broader area is through both a TACC and use of legal minimum lengths to protect the abundance of mature undersize crabs. Male and female Giant Crabs reach maturity below the 15 cm size limit applied across this region9. The legal minimum lengths aim to ensure that egg production remains at least 40 per cent of the unfished levels10. This is a more conservative measure than applied in the Tasmanian fishery because of the data constraints.
The non-Tasmanian part of the biological stock is not considered to be recruitment overfished, and current levels of fishing mortality are unlikely to cause this part of the biological stock to become recruitment overfished.
On the basis of the evidence provided above, the biological stock is classified as a sustainable stock.
Table 2: Giant Crab biology1–3,10
Longevity and maximum size |
30+ years; >20 cm CL; ~10 kg |
Maturity (50%) |
12.5–14 cm CL, depending on region |
CL = carapace length
Figure 1: Distribution of reported commercial catch of Giant Crab in Australian waters, 2010
- Most Giant Crab is taken by targeted fishing using steel-framed traps. The fishery is also associated with the South Australian Southern Rock Lobster Fishery, with Giant Crabs taken as bycatch by vessels targeting lobsters. Most catch is taken in water deeper than the main lobster fishery, along the shelf break, between 150 and 300 m deep.
- The biological stock is managed by South Australia, Tasmania, Victoria and Western Australia, using a range of input and output controls:
- Input controls include limited entry, and spatial and temporal closures.
- Output controls include size limits and TACCs, with individual transferable quota units.
- The number of vessels reporting commercial catch across jurisdictions in 2010 is not known, because some jurisdictions do not include vessels recording low catches (<2 t) in their logbooks.
- The commercial catch of Giant Crab in 2010 was 92.3 t, comprising 18.8 t in South Australia, 53.7 t in Tasmania, 11.3 t in Victoria and 7 t in Western Australia. Giant Crabs are also taken occasionally by trawl in the trawl sectors of the Southern and Eastern Scalefish and Shark Fishery (Commonwealth), with 1.5 t taken in 2010. The recreational and Indigenous catch of Giant Crab is thought to be negligible.
Figure 2: Commercial catch of Giant Crab in Australian waters, 2000–10 (calendar year)
Commercial catches increased rapidly in the mid-1990s, as a result of the development of high-value export markets. Management controls have subsequently reduced the commercial catch to much lower levels. Catches have been stable over the past decade for most of the biological stock. However, there has been a steady decline in the Tasmanian component of the catch, through reduction in the TACC, reflecting both the need to prevent stock decline and a desire to increase economic yield11. The TACC in the Tasmanian fishery was reduced from 104 t in 2000 to 47 t in 2010–11 to attempt to maintain catch rates at economically viable levels5.
The TACC is seldom fully caught in the Tasmanian Giant Crab fishery due to the structure of the fishery. Most vessels capable of targeting Giant Crab are owned by fishers who mainly target rocklobster but have a small amount of Giant Crab quota. These fishers often prefer to leave their crab quota uncaught so that they can target rocklobster.
- Research sampling of bycatch from Giant Crab traps has sampled more than 3000 traps. This research concluded that the fishery is of low risk to other species due to the small amount of trapping effort. As well, the majority of the bycatch consists of species that do not have swim bladders and are returned to the sea unharmed (Draftboard Shark and hermit crabs)5.
- No interactions with protected species have been reported by observers or fishers targeting Giant Crabs. This result would be expected, given that Giant Crabs are targeted in deep water, away from coastal areas frequented by animals that could become entangled, such as juvenile seals and cormorants5.
- The Giant Crab fishery is based mainly on habitat found along the edge of the continental shelf; this bryzoan turf habitat is formed from encrusting filter-feeding organisms growing on sandy and mud sediments3. The risk that gear could have an impact on this habitat is considered to be low because gear is not dragged, and the fishing footprint is insignificant relative to the size of the habitat area3.
- Recruitment is not distributed evenly; some areas appear to have much higher levels of juvenile abundance than others. This is not a function of habitat but appears to be related to larval drift and thus current movement3. Changes in ocean currents through climate change or upwelling events would be expected to affect recruitment.
a Institute for Marine and Antarctic Studies, Tasmania
b South Australian Research and Development Institute
c Department of Primary Industries, Victoria