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

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]. Due to the geographic separation between the major fishing grounds for Blue Swimmer Crab in New South Wales and Queensland, they are managed as two separate biological stocks. 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).

North-Eastern Australia

The Queensland Blue Swimmer Crab fishery primarily operates in southern Queensland. Prior to 1998, the majority of fishing was conducted inshore, in and around Moreton Bay. In 1998 commercial pot fishers began exploiting Blue Swimmer Crab populations further offshore, in areas that were previously lightly fished. Fishing in offshore waters peaked in 2003, when the offshore harvest contributed approximately 70 per cent to total harvest. By 2015, the offshore harvest had decreased and returned to levels slightly higher than those pre-expansion. This rise and subsequent fall of harvest in the offshore area may indicate a decline in fishable biomass for the offshore area and the biological stock as a whole [Johnston et al. 2018]. However, proposed management reforms and subsequent over-reporting within the fishery prior to the investment warning released in 2003 likely reduced the reporting reliability of commercial catch records around this time [QDAF 2019]. 

Queensland assessed the north-eastern Australian Blue Swimmer Crab stock in 2020 (including data up until the 2018–19 financial year) using an integrated stock assessment model. The model estimated exploitable biomass to be at around 33 per cent relative to unfished levels [Lovett et al. 2020]. Under the current management arrangements (i.e. minimum legal size, no-take females), maximum sustainable yield (MSY) is estimated to be around 722 t [Lovett et al. 2020]. Average combined commercial and recreational harvest from the most recent five years was approximately 394 t (336 and 28 t respectively), well below estimated MSY [Lovett et al. 2020].

Catch rates have been declining since 2003 with the 2018–19 season having recorded the lowest catch rate over the last two decades (17 kg/day). While catch rates in recent years have been low, historical records indicate periods where catch rates have been close to this figure (18 to 22 kg/day between the 1991–92 and 1997–98 fishing seasons). Declining catch rates may indicate a reduction in the level of biomass for the Queensland east coast biological stock. Fishery independent surveys show that recruitment abundance has generally been stable through time, with three distinct peaks in 2008, 2014, and 2017 [Bessell-Browne et. al. 2020], although recruitment target reference points are yet to be established by Fisheries Queensland. The most recent stock assessment [Lovett et al. 2020] estimates that spawning biomass displays similar trends to the exploitable biomass, and in 2018–19 was estimated to be around 60 per cent of 1988–89 levels. This stock is not considered to be recruitment impaired.

Active commercial pot fishing licences and fishing effort (in days fished) decreased between 2003 and 2019 by approximately 70 and 74 per cent respectively [QFISH 2020]. Long term trends in total catch and effort are directly proportional to the expansion and subsequent contraction of fishing in offshore areas [Sumpton et al. 2015], with overall fishing pressure on the Blue Swimmer Crab stock showing a decline.

Spatial closures within the Moreton Bay, Great Sandy Strait and Great Barrier Reef Marine Parks provide some additional protection of the Blue Swimmer Crab biomass from fishing mortality [Johnstone et al. 2018]. Management arrangements in Queensland prohibit the take of female crabs, and a minimum legal size of 115 millimeters carapace width ensures that a high proportion of male Blue Swimmer Crabs have an opportunity to mate before recruitment into the fishery [Johnston et al. 2018]. This level of fishing mortality is unlikely to cause the stock to become recruitment impaired.

On the basis of the evidence provided above, the North-Eastern Australia (Queensland) biological stock is classified as a sustainable 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

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

New South Wales – Recreational (Catch)  Recreational catch estimate of 26.7 t is based on (i) an estimated recreational catch of 50 637 Blue Swimmer Crabs by NSW resident recreational anglers in 2013–14 [West et al. 2015]; and (ii) an assumed mean weight of kept Blue Swimmer Crabs of 0.530 kg/crab. This remains the most reliable estimate of annual recreational catch because the 2017-18 survey estimate of 14.2 t  estimated using a mean weight of 0.225 kg/ crab [Murphy et al. 2020] applies only to 1-3 year recreational licence holders.

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

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