Scylla spp., Scylla olivacea, Scylla serrata
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Stock Status Overview
|Northern Territory||Arafura-West Mud Crab Fishery||AWMCF||Sustainable||Catch, effort, catch rate|
|Northern Territory||Western Gulf of Carpentaria Mud Crab Fishery||WGOCMCF||Transitional-depleting||Catch, effort, catch rate, fishing mortality|
- Arafura-West Mud Crab Fishery (NT)
- Western Gulf of Carpentaria Mud Crab Fishery (NT)
Two species of Mud Crabs are found in Australian waters: ‘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 ‘Mud Crab’ in the Northern Territory and Queensland are well documented1–6 but, with some exceptions7–9, 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 presented here refers to the ‘Mud Crab’ (S serrata), unless otherwise indicated.
Female ‘Mud Crabs’ in northern Australia migrate up to 95 km offshore to release their eggs3, which average around 4.5 million per individual10. Coupled with a planktonic larval stage that can last for several weeks11, this facilitates significant gene flow between areas.
Genetic evidence suggests that there are at least two biological stocks of ‘Mud Crabs’ in Australian waters: one to the west and another to the south-east of the Torres Strait12, referred to as the Northern Australian and East Coast biological stocks, respectively.
Both previous national stock status reports on Mud Crabs provided overall assessments for these two biological stocks. However, there have been significant changes in the relative performance of the various fisheries operating across these stocks since 2014. These changes, combined with different management arrangements for each of the four jurisdictions that harvest Mud Crabs, and (in some cases) the need for more information on local population dynamics, has resulted in this status report providing different status determinations for Mud Crabs at the level of fishery management units.
The management units within the range of the northern Australian ‘Mud Crab’ biological stock include: the Kimberley Developing Mud Crab Fishery (Western Australia), the Arafura-West Mud Crab Fishery (Northern Territory), the Western Gulf of Carpentaria Mud Crab Fishery (Northern Territory) and the Gulf of Carpentaria (Queensland) Mud Crab Fishery management unit. The point of separation for the two management units in the Northern Territory is Cape Grey (13°00’S latitude, 136°39’ E longitude).
The management units within the range of the East Coast ‘Mud Crab’ biological stock include the East Coast (Queensland) Mud Crab Fishery management unit and the Estuary General Fishery (New South Wales).
Here, assessment of stock status is presented at the level of the above management units.
Arafura-West Mud Crab Fishery
The Arafura-West Mud Crab Fishery (Northern Territory) (AWMCF) encompasses the city of Darwin and the non-commercial harvest of Mud Crabs close to this population centre is substantial. The only simultaneous 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 time13,17 (using a regional weight multiplier of 0.80 kg per crab13). A more recent, non-Indigenous, resident only angler survey confirms the ongoing significance of the recreational harvest18 in this region. 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 minimum size limits for Mud Crabs (Scylla spp.) in the Northern Territory are 140 mm carapace width for males and 150 mm carapace width for females. These limits ensure that at least 50 per cent of male crabs and around 98 per cent of female crabs reach sexual maturity before harvest6.
Commercial catches by the AWMCF averaged 124 t for the decade spanning 2005–14 (range 106–149 t). The catch in 2015 equates to 85 per cent of this long-term average. Catch rates from 2005–14 were more variable, ranging from 0.3 kg per pot-lift–0.7 kg per pot-lift (average 0.5 kg per pot-lift), with the relatively low catch rate experienced in 2015 (0.3 kg per pot-lift) being a function of increased fisher competition in a few key areas following effort displacement from the Western Gulf of Carpentaria Mud Crab Fishery.
Relatively stable catches over the past decade, combined with protective management measures (including minimum size limits), 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 current19 (that can facilitate long distance dispersal of larvae) indicate that the biomass of ‘Mud Crabs’ within the AWMCF is unlikely to be recruitment overfished and that current fishing effort is unlikely to cause this management unit to become recruitment overfished.
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 oceanography of the Gulf of Carpentaria differs from that of adjacent water bodies in northern Australia, such as the Arafura and Timor seas to the west and the Coral Sea to the east19,20. Features including rotating currents (gyres) and cross-gulf up-welling and down-welling systems may restrict the dispersal of larval Mud Crabs in the Gulf of Carpentaria compared to other sections of the northern Australian coastline, resulting in Mud Crab in the Gulf being more dependent on recruitment from locally produced larvae, and so susceptible to recruitment decline at low local stock sizes.
The Western Gulf of Carpentaria Mud Crab Fishery (Northern Territory) (WGOCMCF) has accounted for more than 70 per cent of the commercial ‘Mud Crab’ harvest in the Northern Territory over the past 20 years. The harvest by resident recreational fishers, visiting recreational fishers and Indigenous fishers within this management unit in 2000–01 was relatively low, at around 10 per cent of the overall take13,17 (using a regional weight multiplier of 0.80 kg per crab13).
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 ‘Mud Crab’ by resident anglers within the WGOCMCF is less than five per cent of the overall species harvest by this group across the Northern Territory18. It also showed that the ‘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 ‘Mud Crabs’ by visiting recreational fishers, resident recreational fishers and Indigenous fishers within this management unit means that the assessment presented here is primarily based on data from commercial logbooks.
Catch rates (as an index of recruitment and abundance) of ‘Mud Crabs’ are positively correlated with environmental variables, with wet season rainfall showing the strongest correlation at lower latitudes21,22. Monsoonal rainfall across many catchments emptying into the Gulf of Carpentaria has been lower than average for much of the past 5 years, coinciding with a period of predominantly neutral or negative values of the Southern Oscillation Index that began in early 2012. This has been associated with a decline in ‘Mud Crab’ availability and catch rates.
A size-age-sex stock synthesis model applied to pooled catch and effort data (to December 2010) for the entire Northern Territory Mud Crab Fishery showed no indication of reduced average recruitment at lower levels of spawning stock size observed over that period23. The inclusion of more recent data to December 2015 has shown an emerging trend of lower stock size and subsequent low recruitment over the past few years. This pattern suggests that the stock is approaching an overfished state.
A delay-difference modelling approach, using data for the WGOCMCF (to December 2015), was recently developed to assess the recruitment overfishing risk in this management unit24. Model runs using an assumed catchability (q) of 0.25–2.0 x 10-3 indicated that overfishing (defined as F/FMSY >1) is not currently occurring, but may have occurred between 2009 and 2012. However, if changes in fishing practices have resulted in q exceeding 2.0 x 10-3, there is an increased risk that overfishing is occurring at present while unfavourable environmental conditions are constraining recruitment. It therefore appears that intermittent overfishing of the WGOCMCF has occurred in recent years and that overfishing could be occurring at present. The above evidence indicates that the current level of fishing pressure may cause the stock to become recruitment overfished.
On the basis of the evidence provided above, the Western Gulf of Carpentaria Mud Crab Fishery (Northern Territory) management unit is classified as a transitional–depleting stock.
‘Mud Crab’ biology2,6,8,33,34
|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|
Distribution of reported commercial catch of ‘Mud Crab’
|Traps and Pots|
|Hand Line, Hand Reel or Powered Reels|
|Coastal, Estuary and River Set Nets|
|Traps and Pots|
|Hand Line, Hand Reel or Powered Reels|
|Coastal, Estuary and River Set Nets|
|Traps and Pots|
|Protection of berried females|
|Protection of soft-shelled crabs|
|Protection of berried females|
|42 in MCF|
- Mud Crab Fishery (NT)
|Commercial||105.15t in AWMCF, 80.58t in WGOCMCF|
|Indigenous||69 t (2000–01)|
|Recreational||24 t (2009–10)|
- Arafura-West Mud Crab Fishery (NT)
- Western Gulf of Carpentaria Mud Crab Fishery (NT)
a 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.
b New South Wales – Indigenous (management methods) 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 themselves. Aboriginal cultural fishing authority - the authority that Indigenous persons can apply to take catches outside the recreational limits under the Fisheries Management Act 1994 (NSW), Section 37 (1)(c1), Aboriginal cultural fishing authority.
c 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.
d 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 crab13) is now considered unrealistically high given that the average weight of harvested Mud Crabs in Western Australia was recently estimated at 0.65 kg15.
Commercial catch of Mud Crab - note confidential catch not shown
Effects of fishing on the marine environment
- Entanglement of turtles in polyethylene mesh traps is a problem in eastern Queensland and New South Wales35,36. 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 fishing37,38.
- Discard rates of undersized Mud Crabs can be as high as 70 per cent of the total catch in some areas34. Bycatch of small fishes (particularly Yellowfin Bream) is also of concern on the east coast27. Research by the Northern Territory Department of Primary Industry and Resources, 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 traps31,39,40. The former organisation has also developed inexpensive escape vents to fit rectangular wire mesh traps, with around 25 per cent of licensees in the Northern Territory using them on a voluntary basis. Some management agencies are currently considering the mandatory use of escape vents in Mud Crab traps.
- ‘Mud Crabs’ sometimes lose limbs when caught in or removed from traps; the injury rate is related to the style of trap used27. Although limb loss appears to have little impact on the short-term survivorship of ‘Mud Crabs’27, repeated limb damage may potentially compromise their growth and/or reproductive success (based on observations of other crab species41,42). More than 20 per cent of ‘Mud Crabs’ caught in monofilament tangle nets lose one or more limbs during capture and/or removal from the net26. This gear also poses a significant ghost-fishing risk if lost, and is prohibited in most jurisdictions.
Environmental effects on MUD CRABS
- Commercial catch rates generally show positive correlations with environmental factors such as rainfall and sea surface temperature, depending on location21. Catch rates are more strongly linked to sea surface temperatures at higher latitudes and rainfall at lower latitudes.
- Juvenile ‘Mud Crabs’ prefer to settle on seagrass rather than mud or sand43 and also utilise mangrove forests7. Therefore, any significant reduction in these habitat types (through human or natural disturbances, including cyclones) could affect recruitment success.
- Mud Crabs may potentially benefit from moderate climate change in some areas44. 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.
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