MUD CRABS (2020)
Scylla spp., Scylla serrata, Scylla olivacea
Date Published: June 2021
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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.
Stock Status Overview
|Northern Territory||Arnhem-west Northern Territory Management Unit||Sustainable||Catch, effort, catch rate|
|Northern Territory||Western Gulf of Carpentaria||Sustainable||Catch, effort, catch rate, biomass, fishing mortality|
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).
Arnhem-west Northern Territory Management Unit
The Arnhem-West Northern Territory (AWNT) management unit encompasses all NT waters outside of the Gulf of Carpentaria. This unit includes the Darwin region that supports a substantial non-commercial harvest of Mud Crabs. The estimates of the harvest by recreational and Indigenous fishers within the AWNT indicate that their combined take accounted for around 40 per cent of the overall harvest within this management unit during 2000 - 2001 [Henry and Lyle 2003, Coleman 2004]. A more recent, non-Indigenous, resident-only angler survey confirmed the significance of the recreational harvest in this region (23 per cent) [West et al. 2012]. However, a lack of annual catch estimates for recreational and Indigenous fishers means that the assessment presented here is primarily based on data from commercial logbooks.
The most recent stock assessment based on the model used in Grubert et al.  estimated the biomass in 2019 as a proportion of the biomass at Maximum Sustainable Yield (B2019/BMSY) was 1.23, indicating that the stock is currently above the target reference level. The above evidence indicates that the biomass of the stock is unlikely to be depleted and that recruitment is unlikely to be impaired.
The same stock assessment estimated that fishing mortality in 2019 as a proportion of that to achieve MSY (F2019/FMSY) was 0.8 indicating that overfishing is not occurring. Consequently, this level of fishing mortality is unlikely to cause the stock to become recruitment impaired.
On the basis of the evidence provided above, the Western Northern Territory management unit is classified as a sustainable stock.
Western Gulf of Carpentaria
The Western Gulf of Carpentaria management unit (WGOC) has accounted for more than 70 per cent of the commercial harvest of Mud Crabs in the Northern Territory over the past 20 years. There are no records indicating the presence of Orange Mud Crab within this management unit [Keenan et al. 1998, Atlas of Living Australia 2021] so the catch of Mud Crabs in this region is assumed to consist entirely of Giant Mud Crab. A survey of Northern Territory anglers in 2009–2010 estimated that the harvest of Giant Mud Crab by resident anglers within the WGOC is less than five per cent of the overall harvest of Scylla spp. by this sector across the Northern Territory [West et al. 2012]. However, the sporadic collection of recreational harvest data for this species within this management unit means that the assessment presented here is based on data from commercial logbooks.
In addition to fisheries impacts, the GOC has a unique environment that has had a recent period of drought Duke et al.  and a series of extreme ocean warming events [Benthuysen et al. 2018]. The culmination of these factors probably contributed significantly to the commercial catch in 2016 declining to a low level (51 t) [NTG 2018]. However, this historical low catch was followed by a substantial increase in 2017 (185 t). This increase is attributed to above average rainfall during the 2016–17 monsoon season, recovery of juvenile nursery areas and a return to average sea surface temperatures that year [COA BOM 2017, IMOS 2018] highlighting the resilience of this species.
The most recent stock assessment based on the model used in Grubert et al.  estimated the biomass in 2019 as a proportion of the biomass at Maximum Sustainable Yield (B2019/BMSY) was 0.6. While this value would be low for many species, the biomass in this management unit has increased substantially from the low levels in 2016 (B2016/BMSY = 0.32) demonstrating that Mud Crab stocks can rebuild very quickly under favourable environmental conditions. The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired.
Moreover, the same stock assessment estimated that fishing mortality in 2019 as a proportion of that to achieve MSY (F2019/FMSY) was 0.73 indicating that overfishing is not occurring. Consequently, this level of fishing mortality is unlikely to cause the stock to become recruitment impaired.
On the basis of the evidence provided above, the Western Gulf of Carpentaria 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]
|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|
|Hook and Line|
|Traps and Pots|
|Hook and Line|
|Traps and Pots|
|Traps and Pots|
|Protection of berried females|
|Protection of soft-shelled crabs|
|Protection of berried females|
|Indigenous||69 t (2000–01)|
|Recreational||29 t (2015)|
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. .
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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