Scylla spp., Scylla olivacea, Scylla serrata
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Stock Status Overview
|Western Australia||Kimberley Developing Mud Crab Fishery||KDMCF||Sustainable||Catch, effort, catch rate|
- Kimberley Developing Mud Crab Fishery (WA)
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.
Kimberley Developing Mud Crab Fishery
The Kimberley Developing Mud Crab Fishery (Western Australia) (KDMCF) operates in a remote part of Western Australia and harvests a mixture of ‘Mud Crab’ and Orange Mud Crab in relatively small quantities. Commercial fishing activity in this region over the past decade has been sporadic, with annual effort (as pot-lifts) ranging over two orders of magnitude. Confidentiality provisions preclude the disclosure of exact catch and effort figures for 2015 as they are based on data from less than three operators. Nevertheless, to provide some perspective, annual catches by the KDMCF have yet to exceed 20 tonnes (t) and annual effort has not reached 30 000 pot-lifts. The average catch rate for the period 2005–14 was 0.7 kg per pot-lift (range 0.5–1.1 kg per pot-lift), which is considered high when benchmarked against the equivalent figure for the Arafura-West Mud Crab Fishery in the Northern Territory (0.5 kg per pot-lift).
Estimates of the Mud Crab harvest 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 (based on survey estimates of the number of crabs kept13,14 multiplied by a regional average weight of 0.65 kg per crab15, a method employed in assessments of individual management units).
Western Australia is the only Australian jurisdiction to impose separate minimum size limits for ‘Mud Crab’ (150 mm carapace width) and Orange Mud Crab (120 mm carapace width). Although size at maturity estimates are not available for either species from this state, reproductive studies from other areas (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 ‘Mud Crabs’, approximately 50 per cent of males and 98 per cent of females are sexually mature at 150 mm carapace width6. Reproductive development of Orange Mud Crabs begins at much smaller size, with around 98 per cent of both sexes mature at 120 mm carapace width16.
There are no estimates of biomass or fishing mortality rate in the KDMCF. However, the modest and sporadic commercial catch by the fishery is considered to have little impact on the resource at the current harvest rate (bearing in mind that the harvest consists of a mixture of two species). The catch rate in 2015 (0.6 kg per pot-lift) was just below the 10-year average of 0.7 kg per pot-lift. The above evidence indicates that the biomass of this stock is unlikely to be overfished.
A history of comparatively light exploitation, high catch rates and minimum size limits that allow a significant proportion of populations to reach sexual maturity before harvest, indicate that the current level of fishing pressure is unlikely to cause this management unit to become recruitment overfished.
On the basis of the evidence provided above, the Kimberley Developing Mud Crab Fishery (Western Australia) management unit is classified as a sustainable 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’
|Hand Line, Hand Reel or Powered Reels|
|Coastal, Estuary and River Set Nets|
|Hand Line, Hand Reel or Powered Reels|
|Coastal, Estuary and River Set Nets|
|Protection of berried females|
|Protection of berried females|
|Indigenous||6 t (2000–01)|
|Recreational||4 t (2013–14)|
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.
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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