I & I NSW Wild Fisheries research Program Giant Mud ( serrata)

Exploitation Status Undefined

Significant commercial and recreational fisheries occur in NSW which is at the southernmost extent of the ’ range. Local biological information has not been analysed in detail.

Scientific name common name comment

Scylla serrata giant mud crab Also known as crab.

Scylla serrata Image © Bernard Yau

Background in WA. Giant mud are usually found in The giant mud crab (Scylla serrata), along with sheltered waters, particularly estuaries, tidal most species of commercially harvested crabs flats and mangrove areas, and can tolerate ° in Australia, belongs to the family . wide variations in temperature (12-35 C) and Species in this family are usually referred to salinity (2-50‰). However, it has been found as swimmer or paddle crabs, and are easily that their activity and feeding slows greatly in ° recognized by their broadly flattened back legs temperatures less than 20 C. During the day, with paddle-like last segments. The giant mud giant mud crabs may live intertidally in burrows crab or mangrove crab has a smooth carapace but most bury in the mud at subtidal levels. (back), is deep green in colour, and differs from They forage at night for food, feeding mainly on other swimmer crabs by having very robust molluscs, and worms. claws and 9 similarly-sized spines on each Giant mud crabs grow in a step-wise fashion side of its carapace. It is the largest species of through a succession of moults until they swimmer crab and can reach 28 cm in carapace reach maturity after 18-24 months. The moult width and 3 kg in weight but is more commonly that marks the transition from a juvenile to a 15-20 cm in width and 0.5-1.0 kg. mature state is the final or terminal moult for Giant mud crabs are widely distributed through most male and almost all female crabs. Most the Indo-Pacific region and around the Indian giant mud crabs do not grow further after this Ocean to east Africa. In Australia, they are found terminal moult, and death is thought to occur from the Bega River in southern NSW north to at a maximum of 4 years. However, it has been the Gulf of Carpentaria and around to Broome found that about 3% of females do moult (and

status of fisheries resources in nsw, 2008/09 giant mud Crab | p 147 wild fisheries research program

Additional Notes breed) a second time. At maturity, the claws of male giant mud crabs increase in proportion • NSW is at the southern extent of the to body size, comprising up to 40% of the species’ range and our fishery is likely to be body weight, making them prized for the meat dependent upon recruitment from further content. north. Female giant mud crabs carry between 2 and • NSW commercial landings and catch rates 5 million eggs and are thought to spawn only were stable until 2007/08, but there was a at sea. The young crabs hatch as zoea larvae significant decline in 2008/09 - the reasons which require marine conditions to develop for this decline are under investigation. The properly. After a series of moults the zoea recreational catch is significant, but not well larvae metamorphose into ‘megalopa’ (the first estimated. stage resembling the adult crab form) which • A joint research program is underway to move inshore and moult into juvenile crabs. study the association between environmental The main fishery for giant mud crabs in factors and mud crab landings, following Australia is in Queensland where 800-1000 t significant declines in landings in the are harvested annually. In NSW, yearly landings northern state’s fisheries. are around 100-120 t. The commercial fishery is • Monitoring of the size composition of seasonal with the greatest harvest occurring in commercial catches from the Clarence River summer, peaking in February and March. Giant and Wallis Lakes is continuing - no trends are mud crabs are sold locally and interstate, with apparent since 2003/04. only a small percentage exported overseas. • There is a minimum legal length of 8.5 cm Recreational catches of giant mud crabs are carapace length and a recreational bag limit also significant, and in some areas outweigh of 5 giant mud crabs. the commercial catches. Crab pots are the primary means of capturing giant mud crabs by both commercial and recreational fishers. Catch Recreational fishers also use hoop nets or dillies. Recreational fishers are limited to using Recreational Catch of Giant Mud Crab one crab trap, and/or five hoop nets (‘dillies’ or The annual recreational harvest of giant mud ‘witches hats’) to target a bag limit of 5 crabs. crab in NSW is likely to lie between 30 and 60 t. Female giant mud crabs are protected in This estimate is based upon the results of the Queensland, NT, and WA, but in NSW only offsite National Recreational and Indigenous females with eggs are protected although these Fishing Survey (Henry and Lyle, 2003) and are rarely caught by commercial or recreational onsite surveys undertaken by I & I NSW. fishers. Minimum size and maximum bag limits vary between states. The minimum legal size for giant mud crabs (male or female) in NSW is Historical Landings of Giant Mud Crab 8.5 cm carapace length. 150 100 Landings (t) 50 0

78/79 83/84 88/89 93/94 98/99 03/04 08/09 Financial Year

Commercial landings (including available historical records) of giant mud crab for NSW from 1978/79 to 2008/09 for all fishing methods.

p 148 | giant mud Crab status of fisheries resources in nsw, 2008/09

Landings by Commercial Fishery of Giant Mud Crab Length Frequency of Giant Mud Crab

Estuary General (Primary Species) * Estuary Trawl (Byproduct Species) 2003/04−2004/05

0.20 n = 1227 0.15 150 0.10 Proportion 100 Landings (t) 0.05 50 0.00

5 10 15 20 0

97/98 99/00 01/02 03/04 05/06 07/08

Financial Year 2005/06−2006/07

0.20 n = 3238 Reported landings of giant mud crab by NSW commercial fisheries from 1997/98. * Fisheries which contribute less

than 2.5% of the landings are excluded for clarity and 0.15 privacy. 0.10 Proportion 0.05 Catch Per Unit Effort Information of Giant Mud Crab Harvested by Crab Potting in NSW 0.00 5 10 15 20 1.0

2007/08−2008/09 0.8

0.20 n = 3752 0.6 0.15 0.4 0.10 Proportion Relative Catch Rate 0.05 0.2 0.00 0.0

98/99 00/01 02/03 04/05 06/07 08/09 5 10 15 20 Financial Year CL (cm) Catch rates of giant mud crab harvested using crab The length distribution of giant mud crabs landed by potting for NSW. Two indicators are provided: (1) median NSW commercial fishers is comprised mainly of crabs catch rate (lower solid line); and (2) 90th percentile of between 9 and 14 cm carapace length (CL), and has been the catch rate (upper dashed line). Note that catch rates relatively stable in recent years. The minimum legal are not a robust indicator of abundance in many cases. length of mud crabs in NSW is currently 8.5 cm CL. Caution should be applied when interpreting these results.

giant Mud Crab | p 149 wild fisheries research program

Further Reading Allan, G. and D. Fielder, Eds. (2003). Mud Resources. Canberra, Australia, Bureau and Southeast Asia. of Resource Sciences, Department of Primary Working Paper no. 54. Canberra, Australia, Industries and Energy, and the Fisheries Research Australian Centre for International Agricultural and Development Corporation. Research. Queensland Fisheries. (2010). Stock status of Handley, A.J., Editor (2010). Mud Crab Fishery, Queensland’s fisheries resources 2009-10. pp 65-75. In, Fishery Status Reports 2009. Darwin, Queensland, Department of Employment, Economic Northern Territory Government Department of Development and Innovation: 65 pp. Resources. Williams, M.J. and B.J. Hill (1982). Factors influencing Butcher, P. (2004). Mud crabs, Scylla serrata, and pot catches and population estimates of the marine park management in estuaries of the Portunid crab Scylla serrata. Marine Biology. 71 (2): Solitary Islands Marine Park, NSW. National Marine 187-192. Science Centre. Armidale, NSW, University of New Please visit the CSIRO website, England. PhD Thesis. http://www.marine.csiro.au/caab/ and search for the Gray, C.A., D.D. Johnson, D.J. Young and M.K. species code (CAAB) 28 911008, common name or Broadhurst (2003). Bycatch assessment of the scientific name to find further information. Estuarine Commercial Gill Net Fishery in NSW. Final Report to Fisheries Research and Development Corporation. Project 2000/172. Hay, T. and A. Souter (2005). Mud Crab Status Report 2004, Northern Territory Government, Department of Primary Industries, Fisheries and Mines: 11. Heasman, M. (1980). Aspects of the general biology and fishery of the mud crabScylla serrata “Forskal” in Moreton Bay, Queensland, University of Queensland. PhD Thesis. Henry, G.W. and J.M. Lyle (2003). The National Recreational and Indigenous Fishing Survey. Final Report to the Fisheries Research & Development Corporation and the Fisheries Action Program Project FRDC 1999/158. NSW Fisheries Final Report Series No. 48. 188 pp. Cronulla, NSW Fisheries. Jones, D.S. and G.J. Morgan (1994). A Field Guide to Crustaceans of Australian Waters. Sydney, Reed. Kailola, P.J., M.J. Williams, P.C. Stewart, R.E. Reichelt, A. McNee and C. Grieve, Eds. (1993). Australian

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Disclaimer: The information contained in this publication is based on knowledge and understanding at the time of writing (April 2010). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of Industry and Investment NSW or the user’s independent adviser. p 150 | giant mud Crab