Blue Swimmer Crab (2020)
Date Published: June 2021
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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.
Stock Status Overview
|New South Wales||South Eastern Australia||Sustainable||
Catch, standardised catch rates, estimated biomass
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).
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 60 and 65 mm carapace length (equivalent to 130 and 140 mm carapace width, CW) is enforced for recreational and commercial fishers, respectively. Female crabs close to the LMS are sexually mature, and are capable of producing one–three batches of eggs within a season [Johnson et al. 2010].
The most recent estimate of the recreational harvest of Blue Swimmer Crabs in NSW was approximately 63 000 crabs at around 14 t during 2017–18 [Murphy et al. 2020]. This estimate was based on a survey of Recreational Fishing Licence (RFL) households. RFL households were comprised of at least one member who possessed a long-term (1 and 3 years duration) fishing licence and included other fishers resident within their households.
A similar survey of RFL households was done in 2013–14 and provides a comparison with data from the 2017–18 survey. The recreational catch of Blue Swimmer Crabs in NSW in 2013–14 was approximately 50 600 crabs estimated to weigh 27 t [Murphy et al. 2020]. 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] and surveys undertaken by New South Wales Department of Primary Industries.
The primary indicators for biomass and fishing mortality are commercial catch and standardised commercial catch rate. 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 and the increase in LMS, reported commercial landings in 2017–2018 and 2018–19 declined to 104 t and 79 t, respectively [Johnson 2020]. This decline is attributed to the management changes rather than a decline in relative abundance. Four estuaries account for 85 per cent of commercial Blue Swimmer Crab landings in New South Wales, the most important being Wallis Lake (45.3 t in 2018–19). Catch during the most recent complete quota year (July 2018 to June 2019) was 79 t, indicating that the newly implemented total allowable commercial catch (225 t) failed to operate as an effective fishery control and could be adjusted in future to control catches if desired. Standardised commercial catch rates (in mean CPUE kg-day-1) are likely to be the most reliable index of relative abundance for Blue Swimmer Crabs. For recent data analysed as mean daily catch rates (available from 2009–10 to 2018–19), catch rates in Wallis Lake have declined by more than 50% in the two most recent years [Johnson 2020]. The impact of recent management changes (i.e. quota management and MLL increase) on catch-rates has not been quantified but are expected to have driven this decline. While catch rates in Wallis Lake in recent years have declined below the recent peak (2014–2016), they are similar to historic levels calculated for a period with consistent management arrangements (1997–2009). In contrast, since 2011, catches rates in the other main estuaries (landings > 1 t/ year) have fluctuated around the long-term average [Johnson 2020].
Catch-MSY model-assisted catch-only assessment [Martell and Froese 2013] was fitted to commercial catch from 1984–85 to 2018–19 using the 'simpleSA' package in R [Haddon et al. 2018]. Results of modified Catch-MSY modelling suggest that the current biomass of Blue Swimmer Crab in NSW waters is depleted to 32% relative to unfished levels with a 95% confidence interval of 6%–57% [Johnson 2020]. The assessment estimated maximum sustainable yield (MSY) to be around 155 t. Average combined commercial and recreational harvest over the last six years was approximately 170 t (151 and 19 t respectively), with estimated landings in 2014–15 (239 t) equivalent to the upper bound of estimated MSY (238 t). The increased LMS for the commercial sector potentially protected 55% of egg producing females from harvesting [Johnson et al. 2010]. Additionally, the implementation of daily possession limit for all ocean fisheries (25 kg) has reduced fishing pressure on the spawning stock, resulting in a decline in harvest rate over the last two years. Nominal effort levels (in the number of fisher days) over the past eight years have remained steady (~4000 per year) and, are well below historical levels (> 8 000 per year). This reduction in fishing effort in combination with stable size compositions in landings [Johnson 2020] indicates that fishing mortality is constrained in New South Wales waters to sustainable levels. The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired. However, if the level of fishing mortality permitted under existing management arrangements (i.e. TAC 225 t) is combined with recreational catches estimated at 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, 2020]. The modelling approaches used in the current assessment are relatively simplistic; therefore, results should be interpreted with caution. There is high uncertainty in the estimates of biomass depletion, harvest rate and MSY derived from catch data using Schaefer production model-assisted Catch-MSY analysis.
On the basis of the evidence provided above, the South Eastern Australia biological stock is classified as a sustainable stock.
Blue Swimmer Crab Biology [de Lestang et al. 2003a,b, Sumpton et al. 2003]
|Species||Longevity / Maximum Size||Maturity (50 per cent)|
|Blue Swimmer Crab||3–4 years, ~ 200 mm CW||Varies among locations, 6–14 months, 86–110 mm CW|
Blue Swimmer Crab Spatial Distribution
|New South Wales|
|Traps and Pots|
|Hook and Line|
|Traps and Pots|
|Method||New South Wales|
|Protection of egg-bearing females|
|Total allowable catch|
|Customary fishing management arrangements|
|General recreational licence or fishing boat licence (not species specific)|
|Protection of egg-bearing females|
|New South Wales|
|Recreational||14 t (2017–18)|
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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