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Australia has ten stocks of Blue Swimmer Crab across WA, Qld, NSW and SA. Eight of those stocks are sustainable with one stock in WA classified as depleted and one stock In South Eastern Australia classified as depleting.

Blue Swimmer Crab is distributed in Australia from the south coast of Western Australia, north to the Northern Territory, across Queensland, down the east coast and to the New South Wales–Victoria border. They are also found in the warmer waters of the South Australian gulfs [Kailola et al. 1993].

In Western Australia, Blue Swimmer Crab is fished in numerous fisheries across five regions. The stock delineation between these regions is unknown [Chaplin et al. 2001; Chaplin et al. 2008]. Stock structure on the east coast of Australia is uncertain, involving overlapping stocks or a semi-continuous stock [Chaplin et al. 2001]. Using a high-resolution oceanographic model coupled with a Lagrangian particle tracking framework to simulate larval dispersal, Hewitt et al. 2022 suggest populations of Blue Swimmer Crab in NSW and Queensland appear to constitute demographically separate stocks, supporting the current assessment and management at the state level. In South Australia, research has identified three separate biological stocks of Blue Swimmer Crab—in Spencer Gulf, Gulf St Vincent and on the coastline west of the Eyre Peninsula [Bryars and Adams 1999; Dixon and Hooper 2011].

Here, assessment of stock status is presented at the biological stock level—North-Eastern Australia (Queensland), South-Eastern Australia (New South Wales), Spencer Gulf, Gulf St Vincent and West coast (South Australia), and at the management unit level—Shark Bay, Cockburn Sound, Peel-Harvey Estuary, Western Australian North Coast and Western Australian South-West Coast (Western Australia).

South Eastern Australia

Blue Swimmer Crabs occur in coastal and estuarine waters along the length of the New South Wales coastline. New South Wales Blue Swimmer Crab populations are at the southern end of the species' distribution along the east coast. A legal minimum size (LMS) of 65 mm carapace length (equivalent to 140 mm carapace width) is enforced for recreational and commercial fishers. 

The most recent estimate of the recreational harvest of Blue Swimmer Crabs in NSW was approximately 42,200 crabs weighing approximately 20 t during 2019–20 [Murphy et al. 2022]. The annual recreational harvest of Blue Swimmer Crabs in New South Wales was previously estimated to lie between 150 and 310 t based on the results of the National Recreational and Indigenous Fishing Survey [Henry and Lyle 2003].

The primary indicators for biomass and fishing mortality are commercial catch and standardised catch rates from the commercial fishery (2009–22) and catch rates from fishery-independent surveys in Wallis Lake (2018–22).  Commercial catches of this species tended to fluctuate around a long-term average of about 144 t over the period 2000–01 to 2016–17. However, following the implementation of quota management (December 2017) and the increase in LMS (60 to 65 mm CL) reported commercial landings in 2017–2018 and 2018–19 declined to 104 t and 84 t, respectively [Johnson 2023]. State-wide estuary catch during the most recent complete quota year (July 2021 to June 2022) was 47 t. Standardised commercial catch rates (in mean CPUE kg-day^-1) sharply declined under revised management arrangements in 2017–18. From 2017–18 standardised catch rates for fishing trapping in Wallis Lake (60% of average landings) and other main estuaries (15% of average landings) have declined. Catch rates in Wallis Lake and other main estuaries in 2021–22 were 54% and 25% lower than 2017-–18, respectively [Johnson 2023]. In 2020–21, average catch rate of undersized crabs from fishery-independent surveys (3.4 crabs per trap^-1) was 65% lower than 2019–20. Similarly, catch rates of legal sized crabs from fishery independent surveys declined for the fourth consecutive year with catch rates in 2021–22 (0.94 crabs per trap ^-1), 45% lower than 2018–19 (1.73 crab per trap^-1) [Johnson 2023]. 

The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired. However, for the period 2017–18 to 2021–22 the biomass declined, but the stock is not yet considered to be recruitment impaired.

Catch-MSY model-assisted catch-only assessment [Martell and Froese 2013] was fitted to commercial catch from 1984–85 to 2021–22 using the 'simpleSA' package in R [Haddon et al. 2018]. Modelling suggests that the current biomass of Blue Swimmer Crab in NSW waters is depleted to 31% relative to unfished levels with a 95% confidence interval of 6%–57% [Johnson 2023]. The assessment estimated maximum sustainable yield (MSY) to be around 149 t. Average combined commercial and recreational harvest over the last five years was approximately 100 t (90 and 12 t respectively), with estimated landings in 2014–15 (239 t) equivalent to the upper bound of estimated MSY (238 t).

The increased LMS is greater than the onset of sexual maturity protecting a large proportion of egg producing females from harvesting [Johnson et al. 2010; Johnson 2023]. The size-structure of observed catches from the Wallis Lake trap fishery (2018-20) indicate that 60% of the total catch of female and berried Blue Swimmer Crabs were below the current LMS [Johnson 2023]. The magnitude of winter harvest in Wallis Lake, that disproportionally impacts large mated pre-spawning females resulting in declining summer harvest in the following year, has declined in recent years [Schilling et al. 2022]. Additionally, the implementation of a daily possession limit for all ocean fisheries (25 kg) has reduced fishing pressure on the offshore spawning stock, resulting in a decline in harvest rate. However, if the level of fishing mortality permitted under existing management arrangements (i.e., TAC 225 t) is combined with recreational catches estimated at 10–20% of total harvest, Catch-MSY model outputs include some trajectories that deplete biomass to less than 20% of unfished levels within five years [Johnson 2023].

NSW estuaries are subject to periodic flooding events which have the potential to limit catches during large-scale floods, the most recent occurring in 2018 and 2022.The impact of factors other than changed population dynamics, including changed fishing practices, locations, catch reporting and catchability need to be investigated further. Nevertheless, under the rapidly declining biomass and relatively low levels of recruitment in recent years, the above evidence indicates that the current level of fishing mortality is likely to cause the stock to become recruitment impaired.

On the basis of the evidence provided above, the South Eastern Australia biological stock is classified as a depleting stock.

Blue Swimmer Crab Biology [de Lestang et al. 2003a,b; Sumpton et al. 2003]

Blue Swimmer Crab Spatial Distribution

Western Australia – Recreational (Catch). Boat-based recreational catch in 2017–18 [Ryan et al. 2019]. Does not include scoop netting and other methods of recreational fishing.

Queensland – Indigenous (Management Methods). for more information see: https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing

Queensland – Commercial (Catch). Queensland commercial and charter data have been sourced from the commercial fisheries logbook program. Further information available through the Queensland Fisheries Summary Report https://www.daf.qld.gov.au/business-priorities/fisheries/monitoring-research/data/queensland-fisheries-summary-report

Queensland – Commercial (Management Methods). Harvest strategies available at: https://www.daf.qld.gov.au/business-priorities/fisheries/sustainable/harvest-strategy

Queensland – Recreational Fishing (Catch). Data are based at the whole of Queensland level and derived from statewide recreational fishing surveys. Where possible, estimates have been converted to weight (tonnes) using best known conversion multipliers. Conversion factors may display regional or temporal variability. In the absence of an adequate conversion factor, data presented as number of fish.

New South Wales – Indigenous (Management Methods). (https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing)

New South Wales – Recreational (Catch) . Recreational catch estimate of 20 t t is based on (i) an estimated recreational catch of 42,200 Blue Swimmer Crabs by NSW resident recreational anglers in 2019–20 [Murphy et al. 2021]; and (ii) an assumed mean weight of kept Blue Swimmer Crabs of 0.225 kg/crab.

Commercial catch of Blue Swimmer Crab - note confidential catch not shown.

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