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MUD CRABS (2020)

Scylla spp., Scylla serrata, Scylla olivacea

  • Thor Saunders (Department of Industry, Tourism and Trade, Northern Territory)
  • Daniel Johnson (Department of Primary Industries, New South Wales)
  • Danielle Johnston (Department of Primary Industries and Regional Development, Western Australia)
  • Lisa Walton (Department of Agriculture and Fisheries, Queensland)

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Summary

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

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Stock Status Overview

Stock status determination
Jurisdiction Stock Stock status Indicators
New South Wales Estuary General Fishery Sustainable

Catch, catch rate, biomass, fishing mortality

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Stock Structure

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 per cent 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. 1984, Hill 1994, 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); Arnhem-West 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).

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Stock Status

Estuary General Fishery

The Estuary General Fishery (New South Wales) (EGF) accounts for approximately 17 per cent of the commercial harvest from the East Coast Giant Mud Crab biological stock, with the annual catch composition by sex being very close to 1:1 (49 per cent female, 51 per cent male). A recent survey of recreational fishing in New South Wales (which may include some harvest by Indigenous fishers) indicated that the non-commercial take accounts for around 20 per cent of the overall Giant Mud Crab harvest in this state [Murphy et al. 2020] (using a regional weight multiplier estimated at 0.70 kg per crab).

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. 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 (CPUE).  Standardised catch rates were predicted from generalised linear models (GLM). For recent data analysed as mean daily catch rates (available from 2009–10 to 2018–19), catch rates (zones combined) have remained stable and were above the 10-year average from 2013–14 to 2018–19 [Johnson 2020]. 

Catch-MSY model-assisted catch-only assessment [Martell and Froese 2013] was fitted to commercial catch from 1978–79 to 2018–19 using the 'simpleSA' package in R [Haddon et al. 2018]. Results of modified Catch-MSY modelling suggest that the current biomass of Giant Mud Crab in NSW waters is depleted to 53 per cent of the estimated maximum biomass with a 95 per cent confidence interval (CI) of 27–80 per cent [Johnson 2020]. Five-year stock projections at catch equal to the current total allowable commercial catch (~206 t) with recreational catches estimated at 20 per cent of total harvest, indicate that biomass is predicted to slowly decline [Johnson 2020].

In 2020, a Schaefer surplus production model (BSM) was fitted to commercial catch (1978–79 to 2018–19) and CPUE (1984–85 to 2018–19) using CMSY+ and BSM [Froese et al. 2019].  Based on BSM analysis, biomass in the last year (1 650 t, CI: 1 240–1 990 t) is estimated to be greater than Bmsy (1 390 t, CI: 910–2 130 t). Estimates of fishing mortality (F) and exploitation rate (F/Fmsy) in 2018–19 were 0.09 (CI: 0.07–0.12) and 0.95 (CI: 0.64–1.64), respectively [Johnson 2020]. All the above evidence indicates that the biomass of the management unit is unlikely to be depleted and that recruitment is unlikely to be impaired. The above evidence also indicates that the current level of fishing mortality is unlikely to cause the stock to become recruitment impaired. 

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

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Biology

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

Biology
Species Longevity / Maximum Size Maturity (50 per cent)
MUD CRABS 3–4 years, 230 mm CW, but rarely exceeds 200 mm CW in most areas Varies by sex and location but generally 120–150 mm CW 
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Distributions

Distribution of reported commercial catch of Mud Crabs
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Tables

Fishing methods
New South Wales
Commercial
Mesh Net
Various
Traps and Pots
Indigenous
Hand collection
Hook and Line
Dip Net
Traps and Pots
Hoop Net
Recreational
Hand collection
Hook and Line
Dip Net
Traps and Pots
Hoop Net
Charter
Traps and Pots
Management methods
Method New South Wales
Charter
Gear restrictions
Possession limit
Size limits
Spatial closures
Commercial
Catch limits
Effort limits
Gear restrictions
Limited entry
Protection of berried females
Size limit
Spatial closures
Spatial zoning
Indigenous
Customary fishing management arrangements
Recreational
Gear restrictions
Possession limit
Protection of berried females
Size limit
Spatial closures
Spatial zoning
Catch
New South Wales
Commercial 119.38t
Indigenous Unknown
Recreational 37.9 t (2017–18)

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 “…without derogating from any other law in force in the Territory, nothing in a provision of this Act or an instrument of a judicial or administrative character made under it limits the right of Aboriginals who have traditionally used the resources of an area of land or water in a traditional manner from continuing 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

New South Wales – Indigenous (management methods)https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing

New South Wales – Recreational (Catch) Murphy et al. [2020].

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.

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Catch Chart

Commercial catch of Mud Crabs - note confidential catch not shown
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References

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  2. Alberts-Hubatsch, H, Lee SY, Diele, K, Wolff, M and Nordhaus, I 2014, Microhabitat use of early benthic stage mud crabs, Scylla serrata (Forskål, 1775), in eastern Australia. Journal of Crustacean Research, 34: 604–610.
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