MUD CRABS (2020)
Scylla spp., Scylla serrata, Scylla olivacea
Date Published: June 2021
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Two species of Mud Crabs are found in Australian waters: Giant Mud Crab and Orange Mud Crab. All six stocks are sustainable.
Stock Status Overview
Catch, effort, catch rate, biomass, fishing mortality
|Queensland||Gulf of Carpentaria||Sustainable||
Catch, effort, catch rate, biomass, fishing mortality
Two species of Mud Crabs are found in Australian waters: Giant 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 Giant Mud Crab in the Northern Territory and Queensland are well documented [Heasman 1980, Hill et al. 1984, Hill 1994, Knuckey 1999, Alberts-Hubatsch 2015] but, with some exceptions [Butcher et al. 2003, Butcher 2004, Alberts-Hubatsch et al. 2014], corresponding information from Western Australia and New South Wales is scarce. There are no published accounts on the biology of Orange Mud Crab in Australian waters. Hence, all catch, and biological information presented here refers to the Giant Mud Crab (S. serrata), unless otherwise stated.
Genetic evidence suggests that there are at least two biological stocks of Giant Mud Crab in Australian waters: one to the west and another to the south east of the Torres Strait [Gopurenko and Hughes 2002], referred to as the Northern Australian and East Coast biological stocks, respectively.
Female Giant Mud Crab in northern Australia migrate up to 95 km offshore to release their eggs [Hill 1994], which average around 4.5 million per individual [Mann et al. 1999]. Coupled with a planktonic larval stage that can last for several weeks [Nurdiani and Zeng 2007], this reproductive strategy may facilitate significant gene flow between areas (depending on local oceanography). However, there have been significant changes in the relative performance of some fisheries operating across these stocks since 2014, suggesting that, despite larval connectivity, there are different exploitation rates on components of the adult stock in different areas. These changes, combined with different management arrangements for each of the four jurisdictions that harvest Giant Mud Crab, and (in some cases) the need for more information on local population dynamics, and fine-scale stock structure, have resulted in this status report providing status determinations for Giant Mud Crab at the level of fishery management units: Kimberley Developing Mud Crab Fishery (Western Australia); Arnhem-West Mud Crab Fishery (Northern Territory), Western Gulf of Carpentaria (Northern Territory); Gulf of Carpentaria (Queensland), East Coast (Queensland); and the Estuary General Fishery (New South Wales).
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 [Henry and Lyle 2003]). A recent stock assessment utilised recreational catch estimates of around 331 t based on 2013 survey data (Webley et al. 2015; Northrop et al. 2019), but noted the need for more up-to-date estimates. Note that this catch may include some harvest by Indigenous fishers (survey respondents were not asked about their heritage). These estimates confirm the ongoing significance of the non-commercial harvest, at around 27 per cent of the overall take [Northrop et al. 2019]. These recent annual catch estimates for recreational and Indigenous fishers are taken into consideration for stock status alongside data from commercial logbooks.
The male-only harvest policy in Queensland means that fishing mortality on female Giant Mud Crab is minimal. However, female crabs may be handled and released many times during their life and so some incidental damage and discard mortality is probable [Grubert and Lee 2013]. Nonetheless, this harvest policy maximises the number of females that survive to spawn. Protection of some sexually mature male crabs in Queensland is afforded by a minimum size limit of 150 mm CW (above the size at first maturity in this state [Heasman 1980]), recreational possession and boat limits, and restrictions on commercial licence numbers and fishing effort.
The East Coast (Queensland) MCF management unit accounts for approximately 86.6 per cent of the commercial harvest of the east coast Giant Mud Crab biological stock. The commercial catch in 2019 (687 tonnes (t)) was 32 per cent below the average catch for the previous 10 years (1 008 t). The nominal catch rate in 2019 was 23 kg per fishing day, 15 per cent below the 10-year average (27 kg per fishing day; range 23─33 kg per fishing day). The most recent stock assessment [Northrop et al. 2019] considered the use of catch rates to calculate biomass but inaccuracies in logbook effort data (i.e. pot lifts) meant that there were technical issues with this approach. Instead, a simplified catch-MSY model was adopted. This stock assessment indicated that east coast mud crabs are likely to be 62% of unfished levels (range: 37–69 per cent) with commercial catch estimated to be 685 t in the 2017─18 financial year (catch takes into consideration 30% over-reporting).
Several no-take zones (applying to all marine organisms) along the east coast of Queensland provide additional protection to Giant Mud Crab and result in higher crab densities and larger mean sizes within the protected areas, as well as spillover of crabs into adjacent fished areas [Pillans et all. 2005, Alberts-Hubatsch 2015]. 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 Giant Mud Crab spawning biomass across the entire East Coast (Queensland) MCF management unit is unlikely to be great when considering the suite of protective measures already in place. All the above evidence indicates that the biomass of the management unit is unlikely to be depleted and that recruitment is unlikely to be impaired.
The most recent estimate of fishing mortality in the East Coast (Queensland) MCF management unit (based on commercial data to 2008) was around 1.5 [Brown 2010], 24 per cent above the estimate of natural mortality for S. serrata (1.2—derived from crabs in the Northern Territory [Knuckey 1999]). 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 others [Grubert and Lee 2013]. Annual fishing effort in 2019 (around 30 000 fishing days) was 9 per cent above the 2008 figure (33 000 fishing days) and the 2019 catch rate (23 kg per fishing day) was 8 per cent below the 2008 value. The effect of the increase in effort since 2008 on the fishing mortality rate of male crabs is not known. However, the proportional change in fishing effort and catch rate is minor when compared to the variation seen in equivalent data for the Northern Territory. The above evidence indicates that the current level of fishing mortality is unlikely to cause the stock to become recruitment impaired.
On the basis of the evidence provided above, the East Coast (Queensland) Mud Crab Fishery management is classified as a sustainable stock.
Gulf of Carpentaria
The commercial sector takes 88 per cent of the Giant Mud Crab catch in the Gulf of Carpentaria (Queensland) Mud Crab Fishery (MCF) management unit [Northrop et al. 2019] and so the status determination for this management unit is primarily based on data from commercial logbooks.
Female Giant Mud Crab cannot be retained in Queensland and the minimum legal size for males (150 mm CW ) ensures that 50 per cent of males attain sexual maturity before harvest (based on male size at maturity estimates from the WGOCMCF, [Knuckey 1999]). Although female Giant Mud Crab are not retained by the Gulf of Carpentaria (Queensland) MCF management unit, they are probably impacted by it to some degree, given that they may be handled and released many times. Indirect evidence of potential post-release mortality in Queensland comes from an analysis of female size frequency distributions inside and outside parts of the Moreton Bay Marine Park [Grubert and Lee 2013], with larger females being more prevalent in the catch in areas closed to fishing for 12 years compared to those where fishing is still allowed, even though female harvest is prohibited in both areas. Nonetheless, the male-only harvest policy maximises the number of female crabs that survive to spawn the next generation.
Catches in the Gulf of Carpentaria (Queensland) MCF management unit have historically been the most stable of any commercial Scylla fishery in Australia, averaging 160 t (range 100–199 t) between 2008 and 2018 [QFISH 2020]. As occurred in other areas, the catch and catch rate for this fishery in 2016 (100 t and 25 kg per fishing day, respectively) were the lowest in a decade, following several years of poor monsoon rainfall in the eastern Gulf of Carpentaria. By contrast, the catch in 2018 (157 t) was the highest in five years and the catch rate (39 kg per fishing day) was 23 per cent greater than the 10 year average (30 kg per fishing day). While catch in 2019 declined again, the catch rate (34 kg per fishing day) remained higher than this average, showing that the Giant Mud Crab population within this management unit rebuilds rapidly and is resilient to the above environmental impacts. The above evidence indicates that the biomass of the management unit is unlikely to be recruitment depleted and that recruitment is unlikely to be impaired.
A recent stock assessment estimated biomass to be around 61% of unfished levels (range 31─72%) and recommended a total allowable commercial catch for the region to be set at 97 t; slightly less than what was harvested by the commercial sector in the 2017─18 financial year. While this fishery still operates under a non-quota based management regime, historical commercial catch levels and the smaller impact of the recreational sector will mean that Gulf of Carpentaria stocks are likely to maintain sustainable biomass levels.
There has been a history of comparatively light exploitation of male crabs (as indicated by the relatively low fishing mortality rate), female Giant Mud Crab are completely protected, and the species exhibits rapid growth and high fecundity [Knuckey 1999, Mann et al. 1999]. The above evidence indicates that the current level of fishing mortality is unlikely to cause this management unit to become recruitment impaired.
On the basis of the evidence provided above, the Gulf of Carpentaria (Queensland) Mud Crab Fishery management unit is classified as a sustainable stock.
Giant Mud Crab biology [Heasman 1980, Knuckey 1999, Butcher et al. 2003, Jebreen et al. 2008, Grubert and Lee 2013]
|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 Trap|
|Traps and Pots|
|Traps and Pots|
|Indigenous||13 t (2000–01)|
|Recreational||331 t in East Coast MCF (2013–14), 15 t in Gulf of Carpentaria MCF (2013–14)|
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 crab) is now considered unrealistically high given that the average weight of harvested Mud Crabs in Western Australia was recently estimated at 0.65 kg.
Northern Territory — Charter (management methods) In the Northern Territory, charter operators are regulated through the same management methods as the recreational sector but are subject to additional limits on license and passenger numbers.
Northern Territory – Indigenous (management methods) The Fisheries Act 1988 (NT), specifies that “…without derogating from any other law in force in the Territory, nothing in a provision of this Act or an instrument of a judicial or administrative character made under it limits the right of Aboriginals who have traditionally used the resources of an area of land or water in a traditional manner from continuing to use those resources in that area in that manner”.
Queensland – Indigenous (management methods) for more information see https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing
New South Wales – Indigenous (management methods)https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing
New South Wales – Recreational (Catch) Murphy et al. .
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.
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