Table 1: Stock status determination for Giant Mud Crab
Jurisdiction |
Northern Territory, Queensland, Western Australia |
New South Wales, Queensland |
Stock |
Northern Australian
(MCF [NT], MCF [QLD], MCF [WA]) |
East coast
(EGF, MCF [QLD]) |
Stock status |
|
|
Sustainable |
Undefined |
Indicators |
Catch, effort and CPUE, size frequency and sex ratio of harvest (from monthly processor monitoring in Northern Territory only) |
Catch, effort and CPUE |
CPUE = catch per unit effort; EGF = Estuary General Fishery (New South Wales); MCF [NT] = Mud Crab Fishery (Northern Territory); MCF [QLD] = Mud Crab Fishery (Queensland); MCF [WA] = Mud Crab Fishery (Western Australia)
Two species of Mud Crab are found in Australian waters: the Giant Mud Crab (Scylla serrata) and the Orange Mud Crab (S. olivacea). The former constitutes the majority (>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 catch composition in the Western Australian Mud Crab fishery is uncertain.
The life history and fisheries biology of Giant Mud Crab in the Northern Territory and Queensland are well documented1–5 but, with the exception of Butcher6, comparatively little information is available on these aspects from Western Australia or New South Wales. 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.
Ovigerous female Giant Mud Crabs migrate up to 95 km offshore to release their eggs3, which can number up to 10.8 million7. These features, coupled with a planktonic larval stage that can last for several weeks (depending on water temperature and salinity)8 give this species significant capacity for dispersal.
A recent study on Giant Mud Crabs from around the Indo–West Pacific region9 found two distinct genetic clades: a widespread clade comprising three separate geographic clusters (west Indian Ocean; Red Sea – South China Sea; and west Pacific, including the eastern seaboard of Australia), and an endemic north-west Australia clade, extending from Western Australia to the tip of Cape York. In this chapter, the first clade is referred to as the 'east coast' biological stock, and the second as the
'northern Australian' biological stock.
The lack of gene flow between the two clades found in Australia is probably due to the constricted westward flow of waters from the Coral Sea imposed by the Torres Strait, which limits the passage of 'west Pacific larvae' into the Arafura Sea and beyond; and the southward flow of the East Australian Current.
Northern Australian biological stock
The limited infrastructure in northern Australia means that commercial crabbing activities are restricted to a few key areas serviced by roads and boat ramps; large areas of Mud Crab habitat receive little or no commercial crabbing effort. Catch by recreational and Indigenous fishers accounts for around 20 per cent of the overall catch of the northern Australian biological stock (as indicated by Henry & Lyle10, and associated commercial catch data). The relatively small take by these sectors has not been incorporated into this assessment. Each jurisdiction assesses that part of the biological stock that occurs in its waters. The status presented here for the entire biological stock has been established using evidence from all three jurisdictions.
The Northern Territory Mud Crab fishery accounts for approximately 70 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 corresponding commercial catch-and-effort data (to December 2010; Carl Walters, unpublished) suggests that monthly fishing mortality ranges from benign rates of 0.05 for females and 0.15 for males during periods of low vulnerability (the timing of which differs between sexes) to very high rates of 2.85 for males and 3.25 for females during periods of peak vulnerability. The monthly values, when averaged over the course of a year, yield annual fishing mortality rates of 1.23 and 1.17 for males and females, respectively. These figures approximate the estimate of natural mortality for Giant Mud Crabs of 1.05. The general rule in fisheries population dynamics is that fishing mortality should usually be kept at or below this value to avoid overfishing and prevent the biological stock from becoming overfished.
The stock synthesis model also produced estimates of annual recruits (in year t +1) and female vulnerable abundance in January one year earlier (year t), over the past 25 years (i.e. since the beginning of the fishery). 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 of overfishing. The Northern Territory component of the northern Australian biological stock has not been recruitment overfished.
The western part of the Queensland Mud Crab fishery accounts for almost all of the remaining 30 per cent of the total commercial harvest of the northern Australian Giant Mud Crab biological stock. The male-only harvest policy in this state (see below, under 'Main features and statistics for Mud Crab fisheries in Australia in 2010') means that the biomass of female Giant Mud Crabs
is largely unaffected by the fishery, although a small level of handling and post-release mortality of females is likely11.
A stock assessment of the western part of the Queensland Mud Crab fishery for 1998 to 2008 (which incorporated a 5 per cent increase in fishing efficiency each year12) suggested that the fishing mortality rate for males in 2008, when the catch was 178 tonnes (t), was around 0.6. Since 2010 catch in the fishery was almost the same (177 t), it is reasonable to assume that the fishing mortality rate in 2010 was the same as (or similar to) that in 2008. This level of fishing mortality is unlikely to cause the western Queensland component of the northern Australian biological stock to become recruitment overfished.
The Western Australian Mud Crab fishery 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 catch in this state is a result of the logistical difficulties associated with operating in, and transporting product from, the far north of Western Australia, rather than low abundances of Giant Mud Crabs. A minimum legal size of 15 cm carapace width (applied to both sexes of Giant Mud Crab) ensures that at least 90 per cent of harvested crabs will have reached sexual maturity before capture (based on data from the Northern Territory5). This ensures that a large proportion of mature crabs have zero fishing mortality.
The biology of Giant Mud Crab (high fecundity and scope for larval dispersal), 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 biological stock is classified as a sustainable stock.
East coast biological stock
This cross-jurisdictional biological stock spans the east coast of Australia from the tip of Cape York (Queensland) to the southern limit of New South Wales. With the exception of far north Queensland (i.e. north of Cairns), roads and fishing infrastructure on the eastern seaboard are well developed and do not restrict access to the Giant Mud Crab resource. Catch by recreational and Indigenous fishers accounts for around 40 per cent of the overall catch of the east coast biological stock (as indicated by Henry & Lyle10, and associated commercial catch data).
The most recent estimate of the fishing mortality rate for male Giant Mud Crabs in eastern Queensland (based on commercial data to 2008) is around 1.512. Although this level of fishing mortality is unlikely to cause this part of the biological stock to become recruitment overfished, thereis con siderable uncertainty about the reliability of commercial catch-and-effort data for eastern Queensland12. Coupled with limited data on the comparatively large non-commercial take and on the size at maturity of Giant Mud Crab harvested in New South Wales, this means that it is not possible
to confidently determine the status of the east coast Giant Mud Crab biological stock. Hence, the biological stock is classified as an undefined stock.
Table 2: Giant Mud Crab biology1,5,6,9,13–14
Longevity and maximum size |
3–4 years; 23 cm CW, but rarely exceeds 20 cm CW in most areas |
Maturity (50%) |
Varies by sex and location but generally 12–15 cm CW |
CW = carapace width
Figure 1: Distribution of reported commercial catch of Giant Mud Crab in Australian waters, 2010
- The commercial catch of Giant Mud Crab is predominantly taken using Mud Crab traps. Recreational fishers may harvest by hand, or use traps, spears, crab hooks, dillies or lift nets, depending on location.
- A range of input and output controls are in place across jurisdictions:
- Input controls include limited entry, spatial closures and gear restrictions (such as restrictions on the number of traps, size of traps, mesh size and number of entry funnels).
- Output controls include minimum legal sizes, male-only harvesting (Queensland only) and bag limits (for recreational fishers).
- Limits on the number of commercial Mud Crab licences and endorsements associated with a given vessel (or business) differ between jurisdictions. For example, commercial fishers in the Northern Territory can run several licences per vessel (but usually no more than two), whereas those in Queensland can run only one. Arrangements in New South Wales are different again;
a fishing business with a Mud Crab trapping endorsement is permitted to use up to 10 traps at any one time, regardless of whether the business owns one or several endorsements. In view of these differences, commercial Mud Crab fishing on a national scale is best described by the number of effective licences or fishing businesses, rather than the number of vessels involved.
- In 2010, all 49 licensees in the Northern Territory accessed the fishery (i.e. zero latency). Of the
437 licences issued in Queensland that year, 375 accessed the fishery (i.e. ~14 per cent latency), with 325 fishing in eastern Queensland and 59 fishing in western Queensland (some licensees operated in both areas). Latency in the Mud Crab trapping component of the New South Wales Estuary General Fishery in 2010 was comparatively high, with roughly half of the 217 endorsed fishing businesses accounting for 95 per cent of the gross value of production. Of the six Mud Crab licences issued in Western Australia in 2010, only three accessed the fishery.
- Total commercial catch of Giant Mud Crab across Australia in 2010 was approximately
1694 t, comprising 105 t in New South Wales, 395 t in the Northern Territory, 1015 t in eastern Queensland, 177 t in western Queensland and less than 2 t in Western Australia. Recreational and Indigenous catch was estimated to be approximately 850 t in 2000–0110; a significant proportion of the recreational and Indigenous catch in Western Australia would have been Orange Mud Crab.

Figure 2: Commercial catch of northern Australian and east coast stocks of Giant Mud Crab, 2000–10 (calendar year)
Note: Catch totals for the northern Australian biological stock do not include the mixed species catch in Western Australia. Catch totals for the east coast biological stock are from calendar-year data for Queensland and financial-year data for New South Wales. New South Wales catch is for the financial year starting with the year shown; e.g. 2010–11 data are plotted against 2010.
Commercial catches of the northern Australian biological stock exceeded 1100 t in 2000 and
2001 but then declined, and have stabilised at around 400–700 t per year for the past five years. This change was driven by variable catches in the Northern Territory; while catches in western Queensland were relatively stable (135–175 t). The record catches in 2000 and 2001 are thought to be due to high recruitment during favourable environmental conditions (i.e. high wet-season rainfall) in the Northern Territory. The commercial catch in western Queensland has been higher than average in recent years (2008–10), possibly due to increases in effort, a greater abundance of legal-sized males and/or an increase in their catchability. The decline in catch in the Northern
Territory is probably due to a decrease in wet-season rainfall and the introduction (in 2001) of the
'commercially unsuitable crab' rule, which prohibits the retention of recently moulted 'soft' crabs by commercial fishers in the Northern Territory.
Commercial catches of the east coast biological stock have been more consistent, ranging between 900 and 1100 t since 2005. In 2010, catch increased by 16 per cent over the catch in 2009, suggesting a greater abundance of legal-sized males and/or an increase in the catchability of individuals. In this case, almost all of the interannual variability was driven by catches in eastern
Queensland, since the New South Wales catch was relatively small.
- Entanglement of turtles in polyethylene mesh traps is a problem in eastern Queensland15. To address this, Fisheries Queensland has released a guide to responsible crabbing, which outlines several gear modifications aimed at reducing turtle interactions and preventing trap loss and subsequent ghost fishing16.
- Discard rates of undersized Mud Crabs can be as high as 70 per cent of the total catch in some areas14. Bycatch of small fishes (particularly Yellowfin Bream) is also of concern on the east coast (P Butcher, pers. comm. 2012). The Northern Territory Department of Primary Industry and Fisheries, and Fisheries New South Wales have both evaluated the effectiveness of escape vents in reducing the retention of undersized Mud Crabs and small teleost bycatch in Mud Crab traps. The results will be available in late 2012. The Northern Territory department has also developed escape vents that can be fitted to wire mesh traps. These are currently being distributed to fishers in Queensland and the Northern Territory for evaluation.
- Limb loss of crabs caught in monofilament tangle nets (hoop nets) is a problem in New South Wales (P Butcher, pers. comm. 2012). To address this, a study by Fisheries New South Wales has compared limb loss of crabs caught in a variety of gear types (including tangle nets) and has recommended that the use of tangle nets be discontinued11.
- Commercial catch rates generally show positive correlations with environmental factors such as rainfall or sea surface temperature, depending on location17. Catch rates are more strongly linked to sea surface temperature at higher latitudes and rainfall at lower latitudes.
- Juvenile Giant Mud Crabs prefer to settle on seagrass rather than on mud or sand18. Hence, the availability of this habitat type may affect recruitment success of this species.
- Mud Crabs could potentially benefit from moderate climate change in some areas. Increased water temperatures at higher latitudes might increase growth rates and reproductive activity, while greater rainfall in the tropics might increase primary and secondary productivity, thereby providing more food for juvenile crabs.
a Department of Primary Industry and Fisheries, Northern Territory
b Department of Agriculture, Fisheries and Forestry, Queensland