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MUD CRABS (2018)

Scylla spp., Scylla olivacea, Scylla serrata

  • Mark Grubert (Department of Primary Industry and Resources, Northern Territory)
  • Daniel Johnson (Department of Primary Industries, New South Wales)
  • Danielle Johnston (Department of Primary Industries and Regional Development, Western Australia)
  • Sue Helmke (Department of Agriculture and Fisheries, Queensland)

You are currently viewing a report filtered by jurisdiction. View the full report.

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Summary

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.

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Stock Status Overview

Stock status determination
Jurisdiction Stock Fisheries Stock status Indicators
Northern Territory Arafura-West Mud Crab Fishery AWMCF Sustainable Catch, effort, catch rate
Northern Territory Western Gulf of Carpentaria Mud Crab Fishery WGOCMCF Sustainable Catch, effort, catch rate, biomass, fishing mortality
AWMCF
Arafura-West Mud Crab Fishery (NT)
WGOCMCF
Western Gulf of Carpentaria Mud Crab Fishery (NT)
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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 [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).

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Stock Status

Arafura-West Mud Crab Fishery

The Arafura-West Mud Crab Fishery (Northern Territory) (AWMCF) encompasses the city of Darwin. The non-commercial harvest of Mud Crabs close to this population centre is substantial. The only concurrent estimates of the harvest by visiting recreational fishers, resident recreational fishers and Indigenous fishers within the AWMCF (derived from surveys in 2000–01) indicate that their combined take accounted for around 40 per cent of the overall harvest within this management unit at that time [Coleman, 2004, Henry and Lyle, 2003] (using a regional weight multiplier of 0.80 kg per crab; Henry and Lyle, 2003). A more recent, non-Indigenous, resident only angler survey confirms the significance of the recreational harvest in this region [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.

Commercial catch rates during the period 2007–16 have been variable, ranging from 0.2 kg per pot-lift–0.7 kg per pot-lift (average 0.5 kg per pot-lift). Both the catch and catch rate for this fishery in 2016 were the lowest in more than a decade, with a similar pattern seen in the WGOCMCF that year. However, the catch across the AWMCF increased in 2017, to 92 t (28 per cent above the 2016 catch) and the catch rate increased to 0.4 kg per pot-lift (double the 2016 catch rate). The catch rate in 2017 was 30 per cent above the upper-most trigger reference point in the harvest strategy for this fishery [NTG 2017].

The performance of the AWMCF in 2016 is considered to have been affected by ocean warming events [Benthuysen et al. 2018] and a concurrent influx of fishers from the Western Gulf of Carpentaria Mud Crab Fishery (where fishing conditions were even more strongly affected) that increased fishing pressure in a few key areas, temporarily reducing catch rates. The rebound in the fishery in 2017, which saw the return of more typical environmental and operational conditions, illustrates the resilience of the stock to a range of perturbations. The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired.

A range of protective management measures (contained within the management plan and harvest strategy for the fishery), long sections of sparsely populated coastline subject to little or no crab fishing (particularly in Arnhem Land), and a strong, westward flowing long-shore wet season current (that can facilitate long distance dispersal of larvae [Schiller A 2011]) lessen the impact of moderate fishing pressure in a few discrete areas within the AWMCF. Commercial minimum size limits for both species of Scylla found in the Northern Territory are the same; 140 mm CW  for males and 150 mm CW  for females. In the case of the Giant Mud Crab, these limits ensure that more than 50 per cent of male crabs and over 98 per cent of female crabs reach sexual maturity before harvest [Knuckey IA 1999]. Although the size at maturity of Orange Mud Crab within the AWMCF is not known, contemporary market monitoring data indicate that less than 1 per cent of individuals harvested by licensees are female [Grubert MA, unpublished data]. 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 Arafura-West Mud Crab Fishery (Northern Territory) management unit is classified as a sustainable stock.

Western Gulf of Carpentaria Mud Crab Fishery

The Western Gulf of Carpentaria Mud Crab Fishery (Northern Territory) (WGOCMCF) 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, The Atlas of Living Australia 2018] and so the catch of Mud Crabs in this region is assumed to consist entirely of Giant Mud Crab. The harvest of this species by resident recreational fishers, visiting recreational fishers and Indigenous fishers within the WGOCMCF in 2000–01 was estimated to be relatively low, at around 10 per cent of the overall take within this management unit [Coleman APM 2004, Henry and Lyle 2003,] (using a regional weight multiplier of 0.80 kg per crab [Henry and Lyle 2003]).

A more recent (2009–10) survey of Northern Territory anglers (which also collected information on visiting fisher activity at three popular fishing sites) confirmed that the harvest of Giant Mud Crab by resident anglers within the WGOCMCF is less than five per cent of the overall harvest of Scylla spp. by this sector across the Northern Territory [West et al. 2012]. It also showed that the Giant Mud Crab harvest by interstate fishers visiting King Ash Bay (on the McArthur River) was almost eight times greater than that of resident recreational fishers at this site. The lack of current estimates of the overall harvest of Giant Mud Crab by visiting recreational fishers, resident recreational fishers and Indigenous fishers within this management unit means that the assessment presented here is based on data from commercial logbooks.

The oceanography of the southern Gulf of Carpentaria, and topography of the adjacent coast, differs markedly from that of other areas in northern Australia. Wind is a significant driver of seasonal fluctuations in sea level in this semi-enclosed water body, and its impact on biological processes in this region can be profound [Wolanski 1993]. Large areas of low-lying salt pans (behind the mangrove fringe) can be inundated during temporary rises in sea level and flood events, providing extensive nursery areas for juvenile Mud Crabs.

By contrast, prolonged declines in sea level may compromise survival of mangroves, which form a key habitat for juvenile and adult Mud Crabs [Alberts-Hubatsch 2015]. Duke et al. [2017] cites a temporary drop in sea level as one of several potential factors that caused a widespread mangrove die-back in the Gulf of Carpentaria in 2015─16. Other possible causes of natural mortality of Mud Crabs at that time include a prolonged period of drought [Duke et al. 2017] and a series of extreme ocean warming events in the area [Benthuysen et al. 2018].

The commercial catch in 2016 was 51 t, a historically low level [NTG 2018]. The combination of reduced coastal productivity during a protracted drought, loss of mangrove foliage and associated mortality of juvenile Giant Mud Crabs (through increased desiccation and/or predation), and extreme water temperatures (avoided by adult crabs so that they were temporarily unavailable to the fishery) are considered to be likely causes of the historically low catch and catch rate in the WGOCMCF in 2016. The subsequent substantial increase in catch in 2017 (to 185 t) 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].

Estimates of spawning biomass within the WGOCMCF at the end of 2017 (Bt) as a proportion of spawning biomass at Maximum Sustainable Yield (BMSY) derived by Grubert et al. [in press], ranged from 1.04 to 1.07, indicating that the stock is currently above the target reference level. Estimated female Spawning Stock Biomass (FSSB) at the end of the most recent fishing year (which is used as a performance indicator in the harvest strategy for the fishery [NTG 2017]) was 61 t; roughly twice the previous five year average and 87 per cent of the harvest strategy target of 70 t. 2015 was also the first time in over five years when recruitment was above average. The above evidence indicates that the biomass of the management unit is unlikely to be recruitment depleted and that recruitment is unlikely to be impaired.

The analysis by Grubert et al. [in press] estimates that overfishing (defined as F/FMSY > 1) is not currently occurring under any modelled catchability scenario. The WGOCMCF has been operating under a formal harvest strategy for two years, with the strategy supported by a year-round market monitoring program and fishery observer program targeting the months of April and May (to validate catch rate estimates derived from logbook data). Commercial minimum size limits of 140 mm CW  for males and 150 mm CW  for females ensure that at least 50 per cent of male and around 98 per cent of female Giant Mud Crabs reach sexual maturity before harvest [Knuckey IA 1999]. 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 Mud Crab Fishery (Northern Territory) management unit is classified as a sustainable stock.

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Biology

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

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
Northern Territory
Commercial
Crab Trap
Indigenous
Spearfishing
Hand collection
Hook and Line
Cast Net
Coastal, Estuary and River Set Nets
Dip Net
Beach Seine
Traps and Pots
Recreational
Spearfishing
Hand collection
Hook and Line
Cast Net
Coastal, Estuary and River Set Nets
Dip Net
Beach Seine
Traps and Pots
Management methods
Method Northern Territory
Commercial
Effort limits
Gear restrictions
Limited entry
Protection of berried females
Protection of soft-shelled crabs
Size limit
Spatial closures
Spatial zoning
Recreational
Gear restrictions
Possession limit
Protection of berried females
Size limit
Spatial closures
Spatial zoning
Vessel limits
Active vessels
Northern Territory
35 in MCF
MCF
Mud Crab Fishery (NT)
Catch
Northern Territory
Commercial 91.89t in AWMCF, 183.66t in WGOCMCF
Charter 1.22t in FTO,
Indigenous 69 t (2000–01)
Recreational 24 t (2009–10)
AWMCF
Arafura-West Mud Crab Fishery (NT)
WGOCMCF
Western Gulf of Carpentaria Mud Crab Fishery (NT)

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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Catch Chart

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

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