Mud Crab Scylla serrata, S. olivacea

Mark Gruberta, Megan Leslieb and Daniel Bucherc


Mud Crab
Giant Mud Crab (Scylla serrata)

Table 1: Stock status determination for Giant Mud Crab

Jurisdiction

Western Australia, Northern Territory, Queensland

Queensland, New South Wales

Stock

Northern Australian

(GOCMCF [Qld], KDMCF [WA], MCF [NT])

East coast

(ECMCF [Qld], EGF [NSW])

Stock status

Sustainable

Sustainable

Indicators

Catch, effort, CPUE

Catch, effort, CPUE


CPUE = catch per unit effort; ECMCF [Qld] = East Coast Mud Crab Fishery (Queensland); EGF [NSW] = Estuary General Fishery (New South Wales); GOCMCF [Qld] = Gulf of Carpentaria Mud Crab Fishery (Queensland); KDMCF [WA] = Kimberley Developing Mud Crab Fishery (Western Australia); MCF [NT] = Mud Crab Fishery (Northern Territory)


Stock Structure

Two species of Mud Crab 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 Mud Crab catch 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 documented1–5 but, with some exceptions6,7, corresponding information from Western Australia and New South Wales is scarce. There are no published accounts of the biology of Orange Mud Crab in Australian waters. Hence, all catch and biological information in this chapter refers to S. serrata, unless otherwise indicated.

Egg-bearing female Giant Mud Crabs migrate up to 95 km offshore to release their offspring2, which can number up to 10.8 million per individual8. These features, coupled with a planktonic larval stage that can last for several weeks9, give this species significant capacity for dispersal.

A recent study on Giant Mud Crabs from around the Indo–West Pacific region10 revealed two genetically distinct stocks in Australian waters: a ‘northern Australian’ biological stock extending from south-west Western Australia across the tropics to the tip of Cape York (Queensland), and an ‘east coast’ biological stock stretching from the tip of Cape York to southern New South Wales.

The lack of gene flow between the two Australian stocks is most likely due to the narrow and turbulent Torres Strait (which prevents larvae from the Coral Sea entering the Arafura Sea and vice versa), and the southward flow of the East Australian Current.


Stock Status

Northern Australian biological stock

The northern Australian biological stock spans three jurisdictions (Western Australia, the Northern Territory and Queensland), with each applying a different set of management controls. Size limits for Giant Mud Crabs vary across the north, but all ensure that at least 40 per cent of male crabs and 95 per cent of female crabs reach sexual maturity before commercial harvest; female Mud Crabs cannot be taken in Queensland. The status presented here for the northern Australian biological stock has been established using evidence from all three jurisdictions.

Mud Crabs are an important traditional food source for Indigenous Australians and are highly prized by recreational fishers. The only concurrent estimates of the catch by both sectors (which are now more than a decade old) suggest that their combined take accounts for around 20 per cent of the overall harvest of the northern Australian biological stock (based on Henry & Lyle11, and commercial catch data from 2001). More recent angler surveys confirm the significance of the recreational harvest12–14. However, a lack of regular catch estimates for recreational and Indigenous fishers means that the stock status presented here is based on data from commercial logbooks and departmental monitoring programs.

The Mud Crab Fishery (Northern Territory) accounts for around 55 per cent of the total commercial harvest of the northern Australian Giant Mud Crab biological stock. A size–age–sex monthly stock synthesis model applied to commercial catch, and fishing effort data (to December 2010)15 suggest that fishing mortality since 2001 has ranged from benign rates of 0.05/month for females and 0.15/month for males during periods of low vulnerability (the timing of which differs between sexes) to very high rates of 2.85/month for males and 3.25/month for females during periods of peak vulnerability. The monthly values, when averaged over a year, yield annual fishing mortality rates of 1.22 and 1.17 for males and females, respectively. These values approximate the estimate of annual natural mortality for Giant Mud Crabs of 1.21 (derived in 1999)5. A general rule in fisheries population dynamics is that fishing mortality should be constrained such that it does not exceed natural mortality (as is the case here), to avoid overfishing and prevent the stock from becoming overfished16. The above evidence indicates that the current level of fishing pressure is unlikely to cause this part of the stock to become recruitment overfished.

The stock synthesis model also produced estimates of annual recruits, and adult female abundance in January one year earlier, during the period 1984 to 2010. These estimates, when plotted against each other, showed no indication of reduced average recruitment at low spawning stock sizes—that is, there was no evidence that this part of the stock has been recruitment overfished. The spread of estimates also suggested that large numbers of female Giant Mud Crabs do not necessarily guarantee strong recruitment the following year and, conversely, that a small spawning stock can still result in good recruitment. Environmental factors such as wet seasons with lower than average rainfall appear to have a negative impact on Mud Crab recruitment and subsequent catch rates17. Poor rainfall in the western Gulf of Carpentaria—the primary fishing ground for the Northern Territory Mud Crab Fishery—during the 2012–13 wet season (and preceding wet seasons) is considered the main reason for the suppressed catch across that area in 2013.

The Gulf of Carpentaria Mud Crab Fishery (Queensland) accounts for almost all of the remaining 45 per cent of the total commercial harvest of the northern Australian Giant Mud Crab biological stock. Annual catches by this fishery are the most stable of any commercial Mud Crab fishery in Australia, and typically vary by less than 15 per cent from one year to the next. An assessment of the Gulf of Carpentaria Mud Crab Fishery using data from 1998 to 2008 (which incorporated a 5 per cent increase in fishing efficiency each year) suggested that the fishing mortality rate for Giant Mud Crab males in 2008 was around 0.618, 50 per cent below the estimate of natural mortality (1.215).

The size-frequency distribution of the catch can often provide an indicator of the relative impacts of fishing. Where fishing effort is moderate to high, there may be a noticeable decline in the relative frequency of legal-sized animals (known as knife-edge selection). No such decline is evident in the size distribution of male Mud Crabs from the eastern Gulf of Carpentaria19, supporting the argument that fishing mortality in this part of the stock is comparatively low. This suggests that the current level of fishing pressure is unlikely to cause the western Queensland component of the stock to become recruitment overfished.

Although female Mud Crabs are not harvested by the Gulf of Carpentaria Mud Crab Fishery, they are most likely impacted by it, given that they may be handled and released many times. This can potentially result in handling damage20, post-release mortality, or nonlethal effects such as reduced reproductive performance (based on observations of captive rocklobsters21). These impacts are considered an acceptable trade-off to maximise the number of female crabs that contribute to the next generation.

A history of comparatively light exploitation of male crabs only (as indicated by the relatively low fishing mortality rate) and the complete protection of female Giant Mud Crabs, in conjunction with the rapid growth and high fecundity of this species5,8, suggests that the Queensland part of the stock is unlikely to be recruitment overfished.

The Kimberley Developing Mud Crab Fishery (Western Australia) accounts for less than 0.5 per cent of the total commercial harvest of the northern Australian Giant Mud Crab biological stock. The small commercial take in Western Australia is a result of the logistical difficulties associated with operating in, and transporting product from, the far north of that state, rather than any shortage of Mud Crabs. There are no estimates of biomass or fishing mortality rate in the Kimberley Developing Mud Crab Fishery. However, given the large area of preferred Mud Crab habitat in the Kimberley region (mangroves and mudflats), the modest and sporadic commercial catch by the fishery (which rarely exceeds 5 t per year of S. serrata and S. olivacea combined) is considered to have little impact on the resource at the current extraction rate.

The biology of Giant Mud Crab (in particular, high fecundity and dispersive capacity), combined with conservative catch controls, large unfished areas (which may buffer the effects of fishing), and low to moderate levels of fishing mortality (for one or both sexes), means that the biomass of this biological stock is unlikely to be recruitment overfished and that current catch levels are unlikely to cause the biological stock to become recruitment overfished.

On the basis of the evidence provided above, the entire biological stock is classified as a sustainable stock.

East coast biological stock

The east coast biological stock extends from the tip of Cape York to the southern limit of New South Wales. The only concurrent estimates of the recreational and Indigenous harvest of this resource (which are now more than a decade old) suggest that the combined take by these sectors equates to around 40 per cent of the overall catch of the east coast biological stock (based on Henry & Lyle11, and commercial catch data from 2001). A more recent survey13 confirms the significance of the recreational harvest, which constitutes more than 25 per cent of the overall total. However, a lack of regular catch estimates for recreational and Indigenous fishers means that the stock status presented here is based on data from commercial logbooks and departmental monitoring programs.

The East Coast Mud Crab Fishery (Queensland) accounts for around 90 per cent of the commercial harvest of the east coast biological stock. The most recent estimate of fishing mortality in this fishery (based on commercial data to 2008) was around 1.518, 24 per cent above the estimate of natural mortality for S. serrata (1.215). Knife-edge selection in the size distribution of male crabs is evident in some parts of the fishery (particularly in more southern areas) but not others19, suggesting that fishing effort (by all sectors) is not evenly distributed along the coast, and that overall fishing mortality is moderate.

Although female Mud Crabs are not harvested by the East Coast Mud Crab Fishery (Queensland), they are likely to be impacted by it, given they may be handled and released many times. Some incidental damage and mortality of female crabs (as described above) is probable, but this is considered an acceptable trade-off to maximise the number of females that contribute to the next generation.

The Estuary General Fishery (New South Wales) can harvest both sexes of Giant Mud Crab and accounts for the remaining 10 per cent of the commercial take of the east coast biological stock. The fishing mortality rate of Giant Mud Crabs in New South Wales is not known.

The offshore spawning behaviour of female Mud Crabs2, combined with the southward flow of the East Australian Current, suggests that recruitment success at higher latitudes may be influenced by spawning success (and larval survivorship) at lower latitudes. Hence, adult Giant Mud Crabs protected by the network of marine reserves along the coast of Queensland and northern New South Wales probably supply recruits to fished areas south of the reserves.

Giant Mud Crab density is significantly higher inside than outside marine reserves6,22, and the mean size of crabs is also greater in some instances19,22. Thus the reproductive output of Mud Crabs within reserves is predicted to exceed that of crabs exposed to fishing pressure22. Considering the moderate fishing mortality rate along the eastern seaboard and the protection afforded to large, fecund crabs inside marine reserves, the current level of fishing pressure is unlikely to cause the east coast Giant Mud Crab biological stock to become recruitment overfished.

There is no clear stock–recruitment relationship for Mud Crabs in the Northern Territory15, and the same may also be true in Queensland and New South Wales. Recruitment in all jurisdictions appears to be driven by environmental variables such as rainfall and water temperature17. Recent annual catches and catch rates in eastern Queensland are among the highest ever recorded and follow a series of high-rainfall years. Although other factors can also influence these catch statistics, the current high apparent productivity suggests that the east coast Giant Mud Crab biological stock is unlikely to be recruitment overfished.

On the basis of the evidence presented above, the entire biological stock is classified as a sustainable stock.


Table 2: Giant Mud Crab biology1,5,6,23–24

Longevity and maximum size

3–4 years; 230 mm CW, but rarely exceeds 200 mm CW in most areas

Maturity (50%)

Varies by sex and location but generally 120–150 mm CW


CW = carapace width


Figure 1: Distribution of reported commercial catch of Giant Mud Crab in Australian waters, 2013 (calendar year)
Figure 1: Distribution of reported commercial catch of Giant Mud Crab in Australian waters, 2013 (calendar year)



Table 3: Main features and statistics for Giant Mud Crab fisheries in Australia, 2013 (calendar year)

Jurisdiction

Western Australia

Northern Territory

Queensland

New South Wales

Fishing methods

Commercial

Trap

Gillnet

Recreational

Trap

Rod and line

Dilly/hoop net/drop net

Inverted dilly/tangle net

Scoop net

Cast net

Beach seine net

Hand collection

Spearfishing

Indigenousa,b

Trap

Rod and line

Dilly/hoop net/drop net

Inverted dilly/tangle net

Scoop net

Cast net

Beach seine net

Hand collection

Spearfishing

Management methods

Commercial

Limited entry

Spatial closures

Spatial zoning

Size limits

Effort limits

Gear restrictions

Male-only harvest

Protection of berried females

Hard shell–only harvest

Recreational

Gear restrictions

Spatial closures

Spatial zoning

Bag limits

Possession limits

Vessel limits

Size limits

Male-only harvest

Protection of berried females

Indigenousa,b

Gear restrictions

Spatial closures

Active licences or businessesc

3 in KDMCF*c

 

49 in MCF*c

 

330 in ECMCF*c

62 in GOCMCF*c

207 in EGF†c

 

Catch

Commercial

7 t in KDMCF

227 t in MCF

1174 t in ECMCF

174 t in GOCMCF

88 t in EGF

Recreational

8 t (2011–12)

66 t (2000–01)

360 t in ECMCF (2010)

7 t in GOCMCF (2010)

30 t (2000–01)

Indigenous

12 t (2000–01)

69 t (2000–01)

13 t (2000–01)

Unknown

Markets

Domestic

Export


ECMCF = East Coast Mud Crab Fishery (Queensland); EGF = Estuary General Fishery (New South Wales); GOCMCF = Gulf of Carpentaria Mud Crab Fishery (Queensland); KDMCF = Kimberley Developing Mud Crab Fishery (Western Australia); MCF = Mud Crab Fishery (Northern Territory)

a In Queensland, under the Fisheries Act 1994 (Qld), Indigenous fishers in Queensland are able to use prescribed traditional and noncommercial 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.

bSubject to the defence that applies under section 211. of the Native Title Act 1993 (Cth), and the exemption from a requirement to hold a recreational fishing licence, the non-commercial take by indigenous fishers is covered by the same arrangements as that for recreational fishing.

c Active licences (*) or businesses () are used 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.


Figure 2: Commercial catch of Giant Mud Crab in Australian waters, 2000 to 2013 (calendar years)
Figure 2: Commercial catch of Giant Mud Crab in Australian waters, 2000 to 2013 (calendar years)



Effects of fishing on the marine environment
  • Entanglement of turtles in polyethylene mesh traps is a problem in eastern Queensland and New South Wales25,26. To address this, both jurisdictions have released guides to responsible crabbing, which outline gear modifications and alternative fishing strategies to reduce turtle interactions, prevent trap loss and minimise ghost fishing27,28.
  • Discard rates of undersized Mud Crabs can be as high as 70 per cent of the total catch in some areas24. Bycatch of small fishes (particularly Yellowfin Bream) is also of concern on the east coast20. Recent research by the Northern Territory Department of Primary Industry and Fisheries, and the New South Wales Department of Primary Industries has demonstrated the value of escape vents in reducing the retention of undersized Mud Crabs and small teleost bycatch in a variety of Mud Crab traps19,29,30. The former organisation has also developed inexpensive escape vents to fit rectangular wire mesh traps, with around 25 per cent of licensees in the Mud Crab Fishery (Northern Territory) using them on a voluntary basis. Some management agencies are currently considering the mandatory use of escape vents in Mud Crab traps.
  • Giant Mud Crabs sometimes lose limbs when caught in, or removed from, traps; the injury rate is related to the style of trap used20. Although limb loss appears to have little impact on the short-term survivorship of Giant Mud Crabs20, repeated limb damage may potentially compromise their growth and/or reproductive success (based on observations of other crab species31,32). More than 20 per cent of Giant Mud Crabs caught in monofilament tangle nets lose one or more limbs during capture and/or removal from the net20. This gear also poses a significant ghost-fishing risk if lost, and is prohibited in most jurisdictions.

    Environmental effects on Mud Crab
    • Commercial catch rates generally show positive correlations with environmental factors such as rainfall and sea surface temperature, depending on location17. Catch rates are more strongly linked to sea surface temperatures at higher latitudes and rainfall at lower latitudes.
    • Juvenile Giant Mud Crabs prefer to settle on seagrass rather than mud or sand33. Therefore, any significant reduction in seagrass area (through human or natural disturbances, including cyclones) could affect recruitment success.
    • Mud Crabs may potentially benefit from moderate climate change in some areas34. Increased water temperatures at higher latitudes might increase growth rates and reproductive activity. Greater rainfall in the tropics might increase primary and secondary productivity, thereby providing more food for juvenile crabs. Any such benefits will, of course, only occur within the physiological tolerances of the particular developmental stage affected.


    a Department of Primary Industry and Fisheries, Northern Territory
    b Department of Agriculture, Fisheries and Forestry, Queensland
    c Marine Ecology Research Centre, Southern Cross University, New South Wales