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MUD CRABS

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 Fisheries, Western Australia)
  • Megan Leslie (Department of Agriculture and Fisheries, Queensland)

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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, fishing mortality
MCF
Mud Crab Fishery (QLD)
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Stock Structure

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.

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Stock Status

Gulf of Carpentaria

The commercial sector harvests the majority (90 per cent) of the ‘Mud Crab’ resource in the Gulf of Carpentaria (Queensland) Mud Crab Fishery (MCF) management unit25 (using a regional weight multiplier of 1.00 kg per crab13) and so the status determination for this management unit is primarily based on data from commercial logbooks.

Catches in the Gulf of Carpentaria (Queensland) MCF management unit have historically been the most stable of any commercial Mud Crab fishery in Australia, averaging 170 t (range 136–199 t) between 2005 and 2014. However, the catch (126 t) and catch rate (26 kg per fishing day) in this fishery in 2015 was the lowest and second lowest in a decade, respectively, coincident with several years of poor monsoonal rainfall in the eastern Gulf of Carpentaria. Nevertheless, the catch rate in 2015 was 92 per cent of the average catch rate across the previous 10 years (29 kg per fishing day).

Female Mud Crabs cannot be retained in Queensland and the minimum legal size for male crabs (150 mm carapace width) ensures that roughly 50 per cent of males attain sexual maturity before harvest (based on male size at maturity estimates from the Western Gulf of Carpentaria MCF6). The ‘Mud Crab’ population within this management unit is therefore expected to recover relatively quickly when favourable environmental conditions return. The above evidence indicates the biomass of the management unit is unlikely to be recruitment overfished.

A growth-type groups model applied to catch and effort data (spanning 1998–2008) from the Gulf of Carpentaria (Queensland) MCF management unit (which incorporated an assumed five per cent increase in fishing efficiency each year) estimated that the annual fishing mortality rate for male ‘Mud Crabs’ in 2008 was around 0.626, 50 per cent below the estimate of annual natural mortality for crabs caught in the adjacent Northern Territory (1.2)6. Nominal fishing effort in the Gulf of Carpentaria (Queensland) MCF management unit has decreased 16 per cent since 2008, so it is unlikely that the fishing mortality rate has increased significantly after 2008.

Although female Mud Crabs are not retained by the Gulf of Carpentaria (Queensland) MCF management unit, they are most likely impacted by it to some degree, given that they may be handled and released many times. This can potentially result in handling damage27, post-release mortality, or non-lethal effects such as reduced reproductive performance (based on observations of captive Rock Lobsters28). Nonetheless, the male-only harvest policy maximises 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 ‘Mud Crabs’, in conjunction with the rapid growth and high fecundity of this species6,10, 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 Gulf of Carpentaria (Queensland) Mud Crab Fishery management unit is classified as a sustainable stock.

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)13. More recent surveys, which may include some harvest by Indigenous fishers (as 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 take25,29. However, a 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 East Coast (Queensland) MCF management unit accounts for approximately 85 per cent of the commercial harvest of the east coast ‘Mud Crab’ biological stock. The catch in 2015 (1056 t) was six per cent above the average catch across the previous 10 years (995 t). Nominal catch rates by this fishery averaged 28 kg per fishing day for the decade concluding in 2014 (range 24–33 kg per fishing day). The nominal catch rate in 2015 (28 kg per fishing day) was the same as the 10-year average value given above. Although standardised catch rates have not been calculated for recent years, comparisons of nominal and standardised catch rates in earlier years show good agreement.

The male-only harvest policy in Queensland means that fishing mortality on female ‘Mud Crabs’ is zero. However, female crabs may be handled and released many times during their life and so some incidental damage and discard mortality (as described above) is probable. Nonetheless, this harvest policy maximises the number of females that contribute to the next generation. Protection of some sexually mature male crabs in Queensland is afforded by a minimum size limit (150 mm carapace width; above size at first maturity in this state2) recreational possession limits, and restrictions on commercial licence numbers and fishing effort.

A number of “no take” zones (applying to all marine organisms) along the east coast of Queensland provide additional protection to ‘Mud Crabs’ (particularly males) and result in higher crab densities and larger mean sizes (within the protected area), as well as spill over of crabs into adjacent fished areas1,30. 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 ‘Mud Crab’ spawning biomass across the entire East Coast (Queensland) MCF management unit is unlikely to be great when considering the other protective measures that are in place.

The most recent estimate of fishing mortality in the East Coast (Queensland) MCF management unit (based on commercial data to 2008) was around 1.526, 24 per cent above the estimate of natural mortality for S. serrata (1.26; derived from crabs in the Northern Territory). 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 others31. Annual fishing effort in 2015 (around 38 000 fishing days) was 15 per cent above the 2008 figure and the 2015 catch rate (28 kg per fishing day) was 11 per cent above the 2008 value. The effect of the modest increase in effort since 2008 on the fishing mortality rate of male crabs is not known.

Recruitment of Mud Crabs in Australia appears to be driven by environmental variables such as rainfall and water temperature, with the impact on catches evident 12–18 months after an environmental perturbation21,22. The catch and catch rate of ‘Mud Crabs’ in the East Coast (Queensland) MCF management unit in 2015 were amongst the highest on record, even though rainfall during the two preceding calendar years was average or below average across much of the eastern seaboard of Queensland. The current high apparent productivity, combined with the protection all females and many mature males, indicates that the East Coast (Queensland) MCF management unit is unlikely to be recruitment overfished and that the current level of fishing pressure is unlikely to cause the stock to become recruitment overfished.

On the basis of the evidence provided above, the East Coast (Queensland) Mud Crab Fishery management is classified as a sustainable stock.

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Biology

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

‘Mud Crab’ biology2,6,8,33,34

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Distributions

Distribution of reported commercial catch of ‘Mud Crab’

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Tables

Fishing methods
Queensland
Commercial
Mud Crab Trap
Indigenous
Spearfishing
Hand collection
Hand Line, Hand Reel or Powered Reels
Cast Net
Coastal, Estuary and River Set Nets
Dip Net
Beach Seine
Pots and Traps
Recreational
Spearfishing
Hand collection
Hand Line, Hand Reel or Powered Reels
Cast Net
Coastal, Estuary and River Set Nets
Dip Net
Beach Seine
Pots and Traps
Management methods
Method Queensland
Commercial
Effort limits
Gear restrictions
Limited entry
Male-only harvest
Protection of berried females
Size limit
Spatial closures
Spatial zoning
Indigenous
Gear restrictions
Spatial closures
Recreational
Gear restrictions
Male-only harvest
Possession limit
Protection of berried females
Size limit
Spatial closures
Spatial zoning
Active vessels
Queensland
318, 39 in MCF
MCF
Mud Crab Fishery (QLD)
Catch
Queensland
Commercial 1.18Kt 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)

Indigenousa,b,d  Active vesselsc

a 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 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 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 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.

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Catch Chart

Commercial catch of ‘Mud Crab’

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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.
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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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References

  1. 1 Alberts-Hubatsch, H 2015, Movement patterns and habitat use of the exploited swimming crab Scylla serrata (Forskål, 1775), PhD thesis, University Bremen, Germany.
  2. 2 Heasman, MP 1980, Aspects of the general biology and fishery of the Mud Crab Scylla serrata (Forskål), in Moreton Bay, Queensland, PhD thesis, University of Queensland, Brisbane.
  3. 3 Hill, BJ 1994, Offshore spawning by the portunid crab Scylla serrata (Crustacea Decapoda), Marine Biology, 120: 379–384.
  4. 4 Hill, BJ, Williams, MJ and Dutton, P 1982, Distribution of juvenile, subadult and adult Scylla serrata on tidal flats in Australia, Marine Biology, 69: 117–120.
  5. 5 Hyland, SJ, Hill, BJ and Lee, CP 1984, Movement within and between different habitats by the portunid crab Scylla serrata, Marine Biology, 80: 57–61.
  6. 6 Knuckey, IA 1999, Mud Crab (Scylla serrata) population dynamics in the Northern Territory, Australia and their relationship to the commercial fishery, PhD thesis, Northern Territory University, Darwin.
  7. 7 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.
  8. 8 Butcher, PA 2004, Mud Crab (Scylla serrata) and marine park management in estuaries of the Solitary Islands Marine Park, New South Wales, PhD thesis, University of New England, Armidale.
  9. 9 Butcher, PA, Boulton, AJ and Smith, SDA 2003, Mud Crab (Scylla serrata: Portunidae) populations as indicators of the effectiveness of estuarine marine protected areas, In JP Beumer, A Grant and DC Smith (ed.s), Aquatic protected areas: what works best and how do we know? Proceedings of the world congress on aquatic protected areas, Australian Society for Fish Biology, Cairns, Queensland, 421–427.
  10. 10 Mann, D, Asakawa, T and Blackshaw, A 1999, Performance of mud crab Scylla serrata broodstock held at Bribie Island Aquaculture Research Centre, In CP Keenan and A Blackshaw (ed.s), Mud Crab aquaculture and biology, Proceedings of an international scientific forum held in Darwin, Australia, 101–105, ACIAR Proceedings No. 78.
  11. 11 Nurdiani, R and Zeng, CS 2007, Effects of temperature and salinity on the survival and development of Mud Crab, Scylla serrata (Forskål), larvae, Aquaculture Research, 38: 1529–1538.
  12. 12 Gopurenko, D and Hughes JM 2002, Regional patterns of genetic structure among Australian populations of the mud crab Scylla serrata (Crustacea: Decapoda): evidence from mitochondrial DNA, Marine and Freshwater Research, 53: 849–857.
  13. 13 Henry, GW and Lyle, JM (ed.s) 2003, The national recreational and Indigenous fishing survey, Fisheries Research Development Corporation project 99/158, Australian Government Department of Agriculture, Fisheries and Forestry, Canberra.
  14. 14 Ryan, KL, Hall, NG, Lai, EK, Smallwood, CB, Taylor, SM and Wise, BS 2015, State-wide survey of boat-based recreational fishing in Western Australia 2013/14, fisheries research report no. 268, Department of Fisheries, Western Australia.
  15. 15 Johnston, D, Evans, R, Marsh, C, Blay, N and Wallis, D 2015, North Coast Crab Fishery Status Report, In WJ Fletcher and K Santoro (ed.s), Status reports of the fisheries and aquatic resources of Western Australia 2014/15: the state of the fisheries, Department of Fisheries, Western Australia, Perth, pp 220–228.
  16. 16 Ikhwanuddin, M, Azmiea, G, Juariah, HM, Zakaria, MZ and Ambak, MA 2011, Biological information and population features of mud crab, genus Scylla from mangrove areas of Sarawak, Malaysia, Fisheries Research, 108: 299–306.
  17. 17 Coleman, APM 2004, The national recreational fishing survey: the Northern Territory, Fishery report 72, Northern Territory Department of Business, Industry and Resource Development, Darwin.
  18. 18 West, LD, Lyle, JM, Matthews, SR and Stark, KE 2012, A survey of recreational fishing in the Northern Territory, 2009–10, Fishery report 109, Northern Territory Government Department of Resources, Darwin.
  19. 19 Schiller, A 2011, Ocean circulation on the North Australian Shelf, Continental Shelf Research, 31: 1087–1095.
  20. 20 Condie, S 2011, Modeling seasonal circulation, upwelling and tidal mixing in the Arafura and Timor Seas, Continental Shelf Research, 31: 1427–1436.
  21. 21 Meynecke, JO, Grubert, MA and Gillson, J 2012, Giant mud crab (Scylla serrata) catches and climate drivers in Australia—a large scale comparison, Marine and Freshwater Research, 63: 84–94.
  22. 22 Meynecke, JO, Grubert, MA, Arthur, JM, Boston, R, Lee, SY 2012, The influence of the La Niña-El Niño cycle on giant mud crab (Scylla serrata) catches in Northern Australia, Estuarine, Coastal and Shelf Science, 100: 93-101.
  23. 23 Grubert, MA, Saunders, TM, Martin, JM, Lee, HS and Walters, CJ 2013, Stock assessments of selected Northern Territory fishes, Fishery report 110, Northern Territory Department of Primary Industry and Fisheries, Darwin.
  24. 24 Walters, CJ 2016, Delay difference model for the Western Gulf of Carpentaria Mud Crab Fishery.
  25. 25 Taylor, S, Webley, J and McInnes, K 2012, 2010 statewide recreational fishing survey, Queensland Department of Agriculture, Fisheries and Forestry, Brisbane.
  26. 26 Brown, IW 2010, Taking female Mud Crabs (Scylla serrata): assessment of risks and benefits, FRDC final report 2009/031, Queensland Government Department of Employment, Economic Development and Innovation, Deception Bay.
  27. 27 Butcher, PA, Leland, JC, Broadhurst, MK, Paterson, BD and Mayer, DG 2012, Giant Mud Crab (Scylla serrata): relative efficiencies of common traps and impacts to discards, ICES Journal of Marine Science, 69: 1511–1522.
  28. 28 Smith, GG and Ritar, AJ, 2005, Effect of physical disturbance on reproductive performance in the spiny lobster, Jasus edwardsii, New Zealand Journal of Marine and Freshwater Research, 39: 317–324.
  29. 29 Webley, J, McInnes, K, Teixeira, D, Lawson, A and Quinn, R 2015, Statewide Recreational Fishing Survey 2013-14, State of Queensland, Department of Agriculture and Fisheries.
  30. 30 Pillans, S, Pillans, RD, Johnstone, RW, Kraft, PG, Haywood, MDE and Possingham, HP 2005, Effects of marine reserve protection on the Mud Crab Scylla serrata in a sex-based fishery in subtropical Australia, Marine Ecology Progress Series, 295: 201–213.
  31. 31 Grubert, MA and Lee, HS 2013, Improving gear selectivity in Australian Mud Crab fisheries, Fishery report 112, Northern Territory Government Department of Primary Industry and Fisheries, Darwin.
  32. 32 West, LD, Stark, KE, Murphy, JJ, Lyle, JM and Ochwada-Doyle, FA 2016, Survey of recreational fishing in New South Wales and the ACT, 2013/14, Fisheries final report series 149, New South Wales Government Department of Primary Industries, Sydney.
  33. 33 Jebreen, E, Helmke, S, Lunow, C, Bullock, C, Gribble, N, Whybird, O and Coles, R 2008, Fisheries long term monitoring program—Mud Crab (Scylla serrata) report: 2000–2002, PR08-3498, Queensland Government Department of Primary Industries and Fisheries, Brisbane.
  34. 34 Ward, TM, Schmarr, DW and McGarvey, R 2008, Northern Territory Mud Crab Fishery: 2007 stock assessment, report to the Northern Territory Department of Primary Industry, Fisheries and Mines, South Australian Research and Development Institute research report series no. 244, SARDI, West Beach.
  35. 35 Fisheries Research and Development Corporation 2010, Library shares bycatch innovations, FISH, 18(4): 24–25, FRDC, Canberra.
  36. 36 Price, S 2014, Two more turtles die in controversial crab traps, Port Stephens Examiner, 18 February.
     
  37. 37 New South Wales Department of Primary Industries 2014, Crab traps—preventing turtle drownings, NSW DPI, Sydney.
  38. 38 Queensland Department of Employment, Economic Development and Innovation 2010, Responsible crabbing, DEEDI, Brisbane.
  39. 39 Broadhurst, MK, Butcher, PA and Cullis, BR 2014, Effects of mesh size and escape gaps on discarding in an Australian Giant Mud Crab (Scylla serrata) trap fishery, PLoS ONE, 9(9): e106414.
  40. 40 Rotherham, D, Johnson, DD, Macbeth, WG and Gray, CA 2013, Escape gaps as a management strategy for reducing bycatch in net-covered traps for the Giant Mud Crab Scylla serrata, North American Journal of Fisheries Management, 33: 307–317.
  41. 41 Norman, CP and Jones, MB 1991, Limb loss and its effect on handedness and growth in the Velvet Swimming Crab Necora puber (Brachyura: Portunidae), Journal of Natural History, 25: 639–645.
  42. 42 Smith, LD 1992, The impact of limb autotomy on mate competition in Blue Crabs Callinectes sapidus Rathbun, Oecologia, 89: 494–501.
  43. 43 Webley, JAC, Connolly, RM and Young, RA 2009, Habitat selectivity of megalopae and juvenile Mud Crabs (Scylla serrata): implications for recruitment mechanism, Marine Biology, 156: 891–899.
  44. 44 Welch, DJ, Saunders, T, Robins, J, Harry, A, Johnson, J, Maynard, J, Saunders, R, Pecl, G, Sawynok, B and Tobin, A 2014, Implications of climate change on fisheries resources of northern Australia, Part 1: Vulnerability assessment and adaptation options, FRDC and James Cook University, Canberra.

Archived reports

Click the links below to view reports from other years for this fish.