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
|Western Australia||Kimberley Crab Managed Fishery||Sustainable||Catch, effort, catch rate|
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).
Kimberley Crab Managed Fishery
The Kimberley Crab Managed Fishery (Western Australia) (KCMF) operates in a remote part of Western Australia and commercial harvests are a mixture of Giant Mud Crab and Orange Mud Crab in relatively small quantities. Estimates of the harvest of Mud Crabs by recreational and Indigenous fishers in the Kimberley zone of the North Coast bioregion of Western Australia (the section of the tropical Western Australian coastline east of 120°00’ east) indicate that the non-commercial harvest is around two-thirds of the commercial catch. The catch is calculated from the estimated number of crabs kept, multiplied by a regional average weight of 0.67 kg per crab (across both species) [Henry and Lyle 2003, Ryan et al. 2019], a method employed in this assessment of individual management units.
Western Australia 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 not available for either species within this state, studies of their reproductive biology in other places (the Northern Territory and Malaysia, respectively) suggest that the two size limits allow a large proportion of each species to reach sexual maturity before harvest. In the case of Giant Mud Crab, approximately 50 per cent of males and 98 per cent of females attain sexually maturity at a size of 150 mm CW [Knuckey 1999]. Reproductive development of the Orange Mud Crab begins at a much smaller size, with around 98 per cent of both sexes 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 around 300 to 30 000 pot-lifts. Confidentiality provisions preclude the disclosure of exact catch figures for 2019 as they are based on data from less than three operators but 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 fishery recorded a standardised catch rate of 0.92 kg/traplift in 2017 that was significantly above the (draft) harvest strategy threshold [Johnston et al. 2020]. The 2018 catch rate of 0.5 kg/traplift represented a 46% decrease from 2017, but was still above the threshold. However, the catch rate in 2019 (0.33 kg/traplift) fell below the threshold, but remained above the limit. Consequently, catch and effort in this fishery will be closely monitored in the near future. Note that minimal fishing occurred in 2019 which may have adversely impacted the catch rate. The relatively small catch by commercial and recreational fishers, the wide distribution of the species throughout the region, and minimum legal sizes set well above size at first maturity indicate that the crab stock in this management unit is sustainable. The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired.
There are no estimates of the fishing mortality rate in the KCMF. However, the small and sporadic commercial catch by the fishery is considered to have little impact on the resource, given the harvest rates. The above evidence 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 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]
|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|
|Coastal, Estuary and River Set Nets|
|Hook and Line|
|Coastal, Estuary and River Set Nets|
|Rod and reel|
|Protection of berried females|
|Protection of berried females|
|Charter||< 1 t|
|Indigenous||6 t (2000–01)|
|Recreational||3 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. .
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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