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Two species of Mud Crabs are found in Australian waters: Giant Mud Crab and Orange Mud Crab. All six stocks are sustainable.

Two species of Mud Crabs are found in Australian waters: Giant Mud Crab (Scylla serrata) and Orange Mud Crab (S. olivacea). The former constitutes more than 99% of the commercial catch of Mud Crabs in the Northern Territory and Queensland, and the entire commercial catch in New South Wales. The species composition in the Kimberley Developing Mud Crab Fishery (Western Australia) is uncertain but is known to vary considerably between locations.

The life history and biology of Giant Mud Crab in the Northern Territory and Queensland are well documented [Heasman 1980; Hill et al. 1982; Hill 1994; Hyland et al. 1984; Knuckey 1999; Alberts-Hubatsch 2015] but, with some exceptions [Butcher et al. 2003; Butcher 2004; Alberts-Hubatsch et al. 2014], corresponding information from Western Australia and New South Wales is scarce. There are no published accounts on the biology of Orange Mud Crab in Australian waters. Hence, all catch, and biological information presented here refers to the Giant Mud Crab (S. serrata), unless otherwise stated.

Genetic evidence suggests that there are at least two biological stocks of Giant Mud Crab in Australian waters: one to the west and another to the south-east of the Torres Strait [Gopurenko and Hughes 2002], referred to as the Northern Australian and East Coast biological stocks, respectively.

Female Giant Mud Crab in northern Australia migrate up to 95 km offshore to release their eggs [Hill 1994], which average around 4.5 million per individual [Mann et al. 1999]. Coupled with a planktonic larval stage that can last for several weeks [Nurdiani and Zeng 2007], this reproductive strategy may facilitate significant gene flow between areas (depending on local oceanography). However, there have been significant changes in the relative performance of some fisheries operating across these stocks since 2014, suggesting that, despite larval connectivity, there are different exploitation rates on components of the adult stock in different areas. These changes, combined with different management arrangements for each of the four jurisdictions that harvest Giant Mud Crab, and (in some cases) the need for more information on local population dynamics, and fine-scale stock structure, have resulted in this status report providing status determinations for Giant Mud Crab at the level of fishery management units: Kimberley Developing Mud Crab Fishery (Western Australia); North West Northern Territory Mud Crab Fishery (Northern Territory), Western Gulf of Carpentaria (Northern Territory); Gulf of Carpentaria (Queensland), East Coast (Queensland); and the Estuary General Fishery (New South Wales).

Estuary General Fishery

Part of the Giant Mud Crab population in New South Wales is protected through a minimum size limit (85 mm CL) although the effectiveness of this measure is uncertain because the size at maturity of S. serrata in this jurisdiction is not known. That is, whilst studies on the reproductive biology of S. serrata from different catchments in northern Australia have reported regional differences in size at sexual maturity [Knuckey, 1999], the life history of S. serrata in New South Wales may differ from populations elsewhere as this jurisdiction represents the southern limit of the species’ typical distribution on the eastern seaboard.

Several “no take” zones (applying to all marine organisms) along the New South Wales coast afford some protection to Giant Mud Crab and result in higher crab densities in the closed areas, size class distributions biased towards larger crabs, and spillover of crabs into adjacent fished areas [Butcher et al. 2003; Butcher et al. 2014]. However, these spatial closures are relatively small and fragmented, and their cumulative benefit on a fishery-wide scale has not been quantified.

Historically, the primary indicators for biomass and fishing mortality are commercial catch and standardised commercial catch rate.  Standardised catch rates were predicted from generalised linear models (GLM). The GLM statistical modelling provided an estimate of mean catch rates that were corrected for a variety of variables that bias raw data. Explanatory model terms considered different catch rates between fishing years, months, management zones, individual fisher operations and their transformed fishing effort. For recent data analysed as mean daily catch rates (available from 2009–10 to 2021–22), catch rates (zones combined) remained stable and above average from 2013–14 to 2017–18, although declining from 2018–19 to 2020–21 [Johnson 2023]. For 2021–22, standardised catch rates from Regions 1 (adjacent to the Queensland border) and 2 (Clarence River) were greater than or equal to the long-term average, while catch rates from regions 3 and 4 located further south were less than the long-term average. Standardised catch rates for the fishery in 2021–22 were greater than 2020–21 [Johnson 2023]. 

Results of Catch-MSY modelling [Haddon et al., 2018] using NSW commercial catch data from 1978–79 to 2021–22 suggest that the current biomass of Giant Mud Crab in NSW waters is 34% of the unfished biomass with a 95% confidence interval (CI) of 11%–58% [Johnson 2023]. Five-year stock projections at catch equal to the current total allowable commercial catch (206 t) with recreational catches estimated at 10–20% of total harvest, indicate that biomass is predicted to slowly decline. However, stock projections at 5-year average harvest indicate that biomass is predicted to remain stable [Johnson 2023]. Notwithstanding the former predicted declining trend at the highest catch scenario, but assuming the latter catch scenario of the 5-year average harvest rate, the above evidence indicates that the biomass of this stock is currently unlikely to be depleted and that recruitment is unlikely to be impaired. 

In 2023, a Schaefer surplus production model (BSM) was fitted to commercial catch (1978–79 to 2021–22) and CPUE (1984–85 to 2021–22) using CMSY+ and BSM [Froese et al. 2019]. The main advantage of BSM compared to other implementations of surplus production models is its focus on informative priors and the acceptance of short and incomplete (= fragmented) CPUE data [See Froese et al. 2017 for full description]. Based on BSM analysis, mean biomass in the last year (1,270 t, CI: 1,010–1,510 t) is estimated to be lower than Bmsy (1,450 t), but greater than the lower 95% confidence interval of Bmsy (CI: 944 t). Estimates of fishing mortality (F) and exploitation rate (F/Fmsy) in 2021–22 were 0.07 (CI: 0.06–0.09) and 0.85 (CI: 0.57–1.48), respectively [Johnson 2023]. There is high uncertainty in the estimates of biomass depletion, harvest rate and MSY derived from catch data using Schaefer production model-assisted analysis, therefore results should be interpreted cautiously. The above evidence indicates that the current level of fishing mortality is unlikely to cause the stock to become recruitment impaired.

Particle dispersal modelling indicates a relatively high level of inter-jurisdictional connectivity for Giant Mud Crab, predominantly between Queensland and northern NSW [Hewitt et al. 2022]. The male-only harvest policy in Queensland likely provides a degree of stability in spawning biomass, which may support higher levels of recruitment for northern NSW estuaries that are well connected with spawning in Queensland waters [Hewitt et al. 2022; Taylor et al. 2023].

On the basis of the evidence provided above, the Estuary General Fishery (New South Wales) management unit is classified as a sustainable stock.

Giant Mud Crab biology [Heasman 1980; Knuckey 1999; Butcher et al. 2003; Jebreen et al. 2008; Grubert and Lee 2013]

Distribution of reported commercial catch of Mud Crabs

Western Australia – Indigenous (Catch). The estimate of the Indigenous harvest tonnage of Mud Crabs in Western Australia has been revised down as the weight multiplier previously used to calculate this value (1.34 kg per crab) is now considered unrealistically high given that the average weight of harvested Mud Crabs in Western Australia was recently estimated at 0.65 kg.

Northern Territory — Charter (Management Methods). In the Northern Territory, charter operators are regulated through the same management methods as the recreational sector but are subject to additional limits on license and passenger numbers.

Northern Territory – Indigenous (Management Methods). The Fisheries Act 1988 (NT), specifies that: “Unless expressly provided otherwise, nothing in this Act derogates or limits the right of Aboriginal people who have traditionally used the resources of an area of land or water in a traditional manner to continue to use those resources in that area in that manner.”

Queensland – Indigenous (Management Methods). For more information see https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing

Queensland – Recreational Fishing (Catch). Data are based at the whole of Queensland level and derived from statewide recreational fishing surveys. Where possible, estimates have been converted to weight (tonnes) using best known conversion multipliers. Conversion factors may display regional or temporal variability. In the absence of an adequate conversion factor, data presented as number of fish.  

Queensland – Commercial (Catch). Queensland commercial and charter data have been sourced from the commercial fisheries logbook program. Further information available through the Queensland Fisheries Summary Report https://www.daf.qld.gov.au/business-priorities/fisheries/monitoring-research/data/queensland-fisheries-summary-report  

Queensland – Commercial (Management Methods). Harvest strategies available at: https://www.daf.qld.gov.au/business-priorities/fisheries/sustainable/harvest-strategy   

New South Wales – Indigenous (Management Methods). for more information please see https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing

New South Wales – Recreational (Catch). For more information please see Murphy et al. [2022].

Active Vessels. The number of active exemption holders (for Western Australia), licences (for the Northern Territory and Queensland) or businesses (for New South Wales) are shown here because the number of active vessels is not an appropriate measure of effort in Australian Mud Crab fisheries. Licensing arrangements also vary significantly between jurisdictions.

Commercial catch of Mud Crabs - note confidential catch not shown

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