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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).

Kimberley Developing Mud Crab Fishery

The Kimberley Crab Managed Fishery (KCMF) targets the Giant Mud Crab (Scylla serrata) and the Orange Mud Crab (S. olivacea) in the remote northwest of Western Australia (WA). Estimates of the harvest of Mud Crabs by recreational fishers in the Kimberley zone of the North Coast Bioregion (120° Longitude east to the Northern Territory border) suggests the recreational harvest is similar to the commercial catch. WA is the only Australian jurisdiction to impose separate minimum size limits for Giant Mud Crab (150 mm CW) and Orange Mud Crab (120 mm CW). Although size at maturity estimates are currently unavailable for either species within this state, these metrics will be developed as part of a three-year FRDC project (2022-138 - Developing a Kimberley Aboriginal Mud Crab Fishery) due for completion in 2026. However, studies of their reproductive biology in other jurisdictions suggest that the two size limits allow a large proportion of each species to reach sexual maturity before harvest. Approximately 50% of male and 98% of female Giant Mud Crabs attain sexually maturity at 150 mm CW in the Northern Territory [Knuckey, 1999], while in Malaysian waters around 98% of both sexes of Orange Mud Crab mature at 120 mm CW [Ikhwanuddin et al. 2011].  

Commercial fishing activity in the KCMF over the past decade has been sporadic, with annual effort ranging widely from approximately 300– 30,000 traplifts. While confidentiality provisions preclude the disclosure of catch figures for 2022 as they are based on data from less than three commercial operators, historic annual catches by the KCMF have yet to exceed 20 t. The annual standardised catch rate from the KCMF provides an index of abundance that can be used to assess this fishery’s performance. The 2022 catch rate of 0.28 kg/traplift^-1 fell marginally below the limit reference point (0.29kg/traplift^-1) of the preliminary harvest strategy [Johnston et al. 2020], initiating a review of the fishery. The low catch rate was considered to be due to the whole 2022 catch being landed by a new operator in the fishery who was acclimatising to fishing in the remote Kimberley region. Nevertheless, catch and effort in this fishery will be closely monitored in the near future.

Recreational catch for this individual management unit is calculated by multiplying the estimated number of retained crabs by regional average weights per crab [S. serrata - 1.061 kg, S. olivacea - 0.603 kg; Ryan et al. 2022]. The 2020–21 integrated survey of recreational boat-based fishing estimated the total state-wide recreational boat-based catch of Mud Crab to be 3 t (95% CI 1.0–4.9 t). Approximately 91% of this catch (S. serrata – 1.80 t, S. olivacea - 1.16 t) was taken in the North Coast Bioregion [Ryan et al. 2022].

The relatively small catch by commercial and recreational fishers and wide distribution of the species throughout the region suggest the biomass of this stock is unlikely to be depleted nor recruitment to be impaired.

On the basis of the evidence provided above, the Kimberley Crab Managed Fishery (Western Australia) 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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