MUD CRABS (2018)
Scylla spp., Scylla olivacea, Scylla serrata
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Two species of Mud Crabs are found in Australian waters: Giant Mud Crab and Orange Mud Crab. The stocks in WA, NT and QLD are sustainable and NSW stock is undefined.
Stock Status Overview
|New South Wales||Estuary General Fishery||EGF||Undefined||Catch|
- Estuary General Fishery (NSW)
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 [Alberts-Hubatsch H 2015, Heasman MP 1980, Hill BJ 1994, Hill et al. 1982, Hyland et al. 1984, Knuckey IA 1999] but, with some exceptions [Alberts-Hubatsch et al. 2014, Butcher PA 2004, Butcher et al. 2003], 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 BJ 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 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); Arafura-West Mud Crab Fishery (Northern Territory), Western Gulf of Carpentaria Mud Crab Fishery (Northern Territory); Gulf of Carpentaria (Queensland), East Coast (Queensland); and the Estuary General Fishery (New South Wales).
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 10 per cent of the overall Giant Mud Crab harvest in this state [West et al. 2016] (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.
The catch by the EGF increased 70 per cent between the 2010–11 and 2014–15 financial years (from 111 t to 189 t, respectively), and the catch for the 2016–17 financial year was 181 t. Catch in the EGF is (as of 1 December 2017) is controlled through an Interim Total Commercial Access Level (ITCAL) of 205 t, with catch allocations based on current shareholdings. The length composition of commercial landings for this species have been stable since monitoring commenced in 2009 [Stewart et al. 2015, NSW DPI unpublished data]. Issues within the EGF pertaining to the use of excess gear (above the allowable pot limit) means that it is not appropriate to infer the status of the stock from catch rate data. There are no estimates of the biomass within, or the fishing mortality rate exerted by, the EGF and so there is insufficient information to confidently classify the status of this stock.
On the basis of the evidence provided above, the Estuary General Fishery (New South Wales) management unit is classified as an undefined stock.
Giant Mud Crab biology [Butcher et al. 2003, Grubert and Lee 2013, Heasman MP 1980, Jebreen et al. 2008, Knuckey IA 1999]
|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|
|New South Wales|
|Traps and Pots|
|Hook and Line|
|Traps and Pots|
|Gillnets and entanglement nets|
|Hook and Line|
|Traps and Pots|
|Gillnets and entanglement nets|
|Method||New South Wales|
|Protection of berried females|
|Section 37 (1d)(3)(9), Aboriginal cultural fishing authority|
|Protection of berried females|
|New South Wales|
|Commercial||161.15t in EGF|
|Recreational||21 t (2013–14)|
- Estuary General Fishery (NSW)
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) Under the Fisheries Act 1994 (Qld), Indigenous fishers in Queensland are entitled to use prescribed traditional and non-commercial fishing apparatus in waters open to fishing. Size and possession limits, and seasonal closures do not apply to Indigenous fishers. Further exemptions to fishery regulations may be applied for through permits.
New South Wales – Indigenous (management methods) (a) Aboriginal Cultural Fishing Interim Access Arrangement—allows an Indigenous fisher in New South Wales to take in excess of a recreational bag limit in certain circumstances; for example, if they are doing so to provide fish to other community members who cannot harvest for themselves; (b) The Aboriginal cultural fishing authority is the authority that Indigenous persons can apply to take catches outside the recreational limits under the Fisheries Management Act 1994 (NSW), Section 37 (1d)(3)(9), Aboriginal cultural fishing authority; and (c) In cases where the Native Title Act 1993 (Cth) applies fishing activity can be undertaken by the person holding native title in line with S.211 of that Act, which provides for fishing activities for the purpose of satisfying their personal, domestic or non-commercial communal needs. In managing the resource where native title has been formally recognised, the native title holders are engaged with to ensure their native title rights are respected and inform management of the State's fisheries resources.
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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