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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)

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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
Queensland East Coast MCF Sustainable Catch, effort, catch rate, fishing mortality
Queensland Gulf of Carpentaria MCF Sustainable Catch, effort, catch rate, biomass, fishing mortality
MCF
Mud Crab Fishery (QLD)
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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

East Coast

The only simultaneous estimates of the recreational and Indigenous harvest within the East Coast (Queensland) Mud Crab Fishery (MCF) management unit are now more than a decade old but indicate that the combined take by these sectors once exceeded 50 per cent of the total catch within this management unit (using a regional weight multiplier of 1.00 kg per crab [Henry and Lyle 2003]). More recent surveys, which may include some harvest by Indigenous fishers (survey respondents were not asked about their heritage), confirm the ongoing significance of the non-commercial harvest, at around 24 per cent of the overall take [Taylor et al. 2012, Webley et al. 2015]. The lack of annual catch estimates for recreational and Indigenous fishers means that the stock status presented here is primarily based on data from commercial logbooks.

The male-only harvest policy in Queensland means that fishing mortality on female Giant Mud Crab is minimal. However, female crabs may be handled and released many times during their life and so some incidental damage and discard mortality is probable (see the above-mentioned study in Moreton Bay). Nonetheless, this harvest policy maximises the number of females that survive to spawn. Protection of some sexually mature male crabs in Queensland is afforded by a minimum size limit of 150 mm CW  (above the size at first maturity in this state [Heasman MP 1980], recreational possession limits and restrictions on commercial licence numbers and fishing effort.

The East Coast (Queensland) MCF management unit accounts for approximately 83 per cent of the commercial harvest of the east coast Giant Mud Crab biological stock. The commercial catch in 2017 (862 t) was 16 per cent below the average catch for the previous 10 years (1 030 t) [QDAF 2018]. The nominal catch rate in 2017 was 23 kg per fishing day, 18 per cent below the 10 year average (28 kg per fishing day; range 23─33 kg per fishing day). Although standardised catch rates have not been calculated for recent years, comparisons of nominal and standardised catch rates in earlier years showed good agreement.

Several no take zones (applying to all marine organisms) along the east coast of Queensland provide additional protection to Giant Mud Crab and result in higher crab densities and larger mean sizes within the protected areas, as well as spillover of crabs into adjacent fished areas [Alberts-Hubatsch H 2015, Pillans et al. 2005]. However, the benefit of these closures, over and above the single-sex harvest policy (and the male size limit) on a fishery-wide scale has not been quantified. While the localised benefits of spatial closures can be significant, their cumulative benefit on Giant Mud Crab spawning biomass across the entire East Coast (Queensland) MCF management unit is unlikely to be great when considering the suite of protective measures already in place. 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 most recent estimate of fishing mortality in the East Coast (Queensland) MCF management unit (based on commercial data to 2008) was around 1.5 [Brown IW 2010], 24 per cent above the estimate of natural mortality for S. serrata (1.2 - derived from crabs in the Northern Territory [Knuckey IA 1999]). There are indications from the size distribution of male crabs that fishing mortality is not evenly spread across eastern Queensland, with localised depletion in some areas and low to moderate fishing mortality in others [Grubert and Lee 2013]. Annual fishing effort in 2017 (around 37 000 fishing days) was 12 per cent above the 2008 figure (33 000 fishing days) and the 2017 catch rate (23 kg per fishing day) was 7 per cent below the 2008 value [QDAF 2018]. The effect of the increase in effort since 2008 on the fishing mortality rate of male crabs is not known. However, the proportional change in fishing effort and catch rate is minor when compared to the variation seen in equivalent data for the Northern Territory. 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 East Coast (Queensland) Mud Crab Fishery management is classified as a sustainable stock.

Gulf of Carpentaria

The commercial sector takes 90 per cent of the Giant Mud Crab catch in the Gulf of Carpentaria (Queensland) Mud Crab Fishery (MCF) management unit [Taylor et al. 2012] (using a regional weight multiplier of 1.00 kg per crab [Henry and Lyle 2003]) and so the status determination for this management unit is primarily based on data from commercial logbooks.

Female Giant Mud Crab cannot be retained in Queensland and the minimum legal size for males (150 mm CW ) ensures that 50 per cent of males attain sexual maturity before harvest (based on male size at maturity estimates from the WGOCMCF, [Knuckey IA 1999]).

Although female Giant Mud Crab are not retained by the Gulf of Carpentaria (Queensland) MCF management unit, they are probably impacted by it to some degree, given that they may be handled and released many times. Indirect evidence of potential post-release mortality in Queensland comes from an analysis of female size frequency distributions inside and outside parts of the Moreton Bay Marine Park [Grubert and Lee 2013], with larger females being more prevalent in the catch in areas closed to fishing for 12 years compared to those where fishing is still allowed, even though female harvest is prohibited in both areas. Nonetheless, the male-only harvest policy maximises the number of female crabs that survive to spawn the next generation.

Catches in the Gulf of Carpentaria (Queensland) MCF management unit have historically been the most stable of any commercial Scylla fishery in Australia, averaging 161 t (range 100–199 t) between 2005 and 2016 [QDAF 2018]. As occurred in other areas, the catch and catch rate for this fishery in 2016 (100 t and 25 kg per fishing day, respectively) were the lowest in a decade, following several years of poor monsoon rainfall in the eastern Gulf of Carpentaria. By contrast, the catch in 2017 (130 t) was the highest in three years and the catch rate (35 kg per fishing day) was 25 per cent greater than the 10 year average (28 kg per fishing day); showing that the Giant Mud Crab population within this management unit is rebuilding rapidly, and is resilient to the above environmental impacts. 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.

Brown IW [2010] applied a high-resolution age-sex structured assessment model (that divides each age cohort into ‘growth-type groups’ - Walters [2007]) to catch and effort data from the Gulf of Carpent​-aria (Queensland) MCF management unit for the period 1998–2008. This analysis, which also incorporated an assumed five per cent increase in fishing efficiency each year, estimated that the annual fishing mortality rate for male Giant Mud Crab in 2008 was around 0.6, 50 per cent below the estimate of annual natural mortality for crabs caught in the adjacent Northern Territory (1.2 [Knuckey IA 1999]). Fishing effort in this management unit has decreased 16 per cent since 2008.

There has been a history of comparatively light exploitation of male crabs (as indicated by the relatively low fishing mortality rate), female Giant Mud Crab are completely protected, and the species exhibits rapid growth and high fecundity [Knuckey IA 1999, Mann et al. 1999]. The above evidence indicates that the current level of fishing mortality is unlikely to cause this management unit to become recruitment impaired.

On the basis of the evidence provided above, the Gulf of Carpentaria (Queensland) Mud Crab Fishery 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
Queensland
Commercial
Net
Traps and Pots
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 Queensland
Charter
Gear restrictions
Male-only harvest
Possession limit
Size limit
Spatial closures
Spatial zoning
Commercial
Effort limits
Gear restrictions
Limited entry
Male-only harvest
Size limit
Spatial closures
Spatial zoning
Recreational
Gear restrictions
Male-only harvest
Possession limit
Size limit
Spatial closures
Spatial zoning
Active vessels
Queensland
295, 33 in MCF
MCF
Mud Crab Fishery (QLD)
Catch
Queensland
Commercial 987.98t in MCF
Indigenous 13 t (2000–01)
Recreational 332 t in East Coast MCF (2013–14), 7 t in Gulf of Carpentaria MCF (2010–11)
MCF
Mud Crab Fishery (QLD)

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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  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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Archived reports

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