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Blue Swimmer Crab (Portunus armatus) Resource in the West Coast Bioregion, Western Australia Part 2: Warnbro Sound, Comet Bay, Mandurah to Bunbury, Leschenault Estuary, Geographe Bay and Hardy Inlet

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Blue swimmer crabs (Portunus armatus) are found all along the coastline of Western Australia in a range of estuarine, coastal and continental shelf (<50 m) environments. This document describes and assesses the blue swimmer crab (Portunus armatus) and all commercial and recreational fishing activities of the small blue swimmer crab fishery (Warnbro Sound (WS), Mandurah to Bunbury (MB), Comet Bay (CB), Leschenault Estuary (LE ), Geographe Bay (GB) and the Hardy Inlet (HI) that impact this resource, in the West Coast Bioregion of Western Australia (WCB).

Figure  2.1.  An  outline  of  the  risk-based  planning  cycle  used  for  determining  Departmental  priorities and activities
Figure 2.1. An outline of the risk-based planning cycle used for determining Departmental priorities and activities

Fisheries/Sectors Capturing Resource

For detailed biological information of the blue swimmer crab (Figure 5.1) in south-western Australia, refer to Johnston et al., (2020b) Blue Swimmer Crab (Portunus armatus) Resource in the West Coast Bioregion, Western Australia. The Department is now implementing the Minister's decisions and is also working with Recfishwest, WAFIC and the Southern Seafood Producers Association to consider other possible changes to the management of the South West blue crab resource.

Commercial Fishery

  • Warnbro Sound Crab Managed Fishery
  • Mandurah to Bunbury Developing Crab Fishery
  • Leschenault Estuary
  • Geographe Bay
  • Hardy Inlet

A management review focused on increasing the protection of the southwestern blue swimmer crab breeding stock at a resource level (DPIRD, 2018). Most of the waters covered by Area 3 of the WCEMF are suitable habitat for the blue swimmer crab.

However, subsequent catches have remained low (~5 t; Figure 6.1; Figure 6.2).
However, subsequent catches have remained low (~5 t; Figure 6.1; Figure 6.2).

Recreational Fishery

  • History of Development
  • Current Fishing Activities
  • Fishing Methods and Gear
  • Susceptibility

Fishing for blue swimmer crabs is one of the most popular pastimes for locals and tourists in Great Britain (Borg and Campbell, 2003). Spring (September-November) was the most popular season for crabbing in Great Britain, accounting for 54% of boat effort and 52% of shore effort. The most popular crabbing area was near the Port Geographe Marina and from Busselton Pier eastwards to the Wonnerup Estuary.

Recreational fisheries for blue swimmer crabs are conducted in the waters of SCE, Cockburn Sound (CS), WS, PHE, LE and GB where they dominate the inshore recreational catch.

Customary Fishing

The area overlap using scoop nets in the other West Coast stock beyond PHE is relatively low (~ <10%) due to the small area available for land-based scoop nets. However, due to the boat-based nature of gillnets, the areal overlap is greater (~10-30%). The moderation of recreational fishing methods in all stocks is high, as the methods used specifically target blue swimming crabs.

Post-catch mortality is generally low, as recreational anglers must return undersized females with berries to the water within 5 minutes of landing.

Illegal, Unreported or Unregulated Fishing

For the purpose of the productivity analysis (PSA, section 9.3.6), the blue swimming crab assemblages in the WCB coast have been divided into two main stocks; one includes CS and SCE, while the other includes WS, PHE, the coastal waters of CB and MB, LE, GB and HI. Although undersize or female crabs are often caught by recreational drop net fishermen, divers and seines actively avoid them. 309 with the stable compliance effort, suggests that the level of non-compliance in this fishery has decreased, these data are likely to be strongly influenced by changes in the abundance and availability of crabs between years (Johnston et al., 2015b - Addendum 2019 ) .

The introduction of an extension of the season closure to November 2020 will reduce catches of undersized crabs, which are currently common in the fishery.

Management System

Harvest Strategy

Warnbro Sound Crab Managed Fishery

The primary performance indicator for the WSCMF harvesting strategy is the standardized catch rate (CPUE in kg/traplift) during the fishing season, using the reference period (Table 7.1). The reference period covers the period during which the previous operator fished, during which the target (range between the threshold and the highest historical catch rate), the threshold (lowest historical catch rate) and the limit (30% below the lowest historical catch rate) were set. Performance indicators and harvesting strategies will be reviewed when commercial fishing ceases.

Mandurah to Bunbury Developing Crab Fishery

HI blue swimming crab stock assessment will continue to be based on reported commercial catch and effort data. Improving confidence in the management of the blue swimming crab (Portunus armatus) in Shark Bay. Protection of the breeding population of the blue swimming crab resource in the Southwest - A review of management schemes.

Biology of the blue crab Portunus pelagicus in the Leschenault Estuary and Koombana Bay in southwestern Australia.

Area 3 of the West Coast Estuarine Managed Fishery (The Hardy Inlet)

External Influences

Environmental Factors

A risk assessment of WA's most important commercial and recreational finfish and invertebrate species showed that climate change has a major impact on some exploited stocks (Caputi et al., 2015). This occurs mainly through changes in the frequency and intensity of ENSO events, decadal variability in the Leeuwin Current (LC), increase in water temperature and salinity, and change in frequency and intensity of storms and tropical cyclones affecting the state (Caputi et al., 2015). In 2010/11, a very strong LC led to unusually warm ocean temperatures in coastal waters of southwestern WA (Pearce et al., 2011).

Many of the biological processes of crabs are highly influenced by environmental variables, indicating a high sensitivity to climate change.

Introduced Pest Species

The effects of climate change are likely to differ between blue swimmer crab fisheries in WA, based on the large latitudinal range between crab fisheries.

Market Influences

Range of Information

Dependent monitoring of commercial catch and effort trends, calculation of catch rates and fishing area. Dependent monitoring of recreational catch and effort trends with iSurveys on the boat and surveys of boat and mowers on shore. Independent catch rates provide an index of spawning stock abundance as well as information on the sex ratio and reproductive rate of females.

Monitoring

  • Commercial Catch and Effort
  • Recreational/Charter Catch and Effort
  • Fishery-Dependent Monitoring
  • Fishery-Independent Monitoring
  • Environmental Monitoring
  • Other Information

Daily research trap logs were introduced as part of the exemption requirements in the MBDCF to obtain comprehensive (finer resolution) daily catch and effort data from commercial trap fishermen (Figure 8.1). For each line of crab traps set, fishermen must record a latitude and longitude or block reference, the number of traps in the line, depth, trap soaking time, and an estimate of total catch in either kilograms or baskets of crabs. A logbook program for recreational fishers (Figure 8.2) started in 2013 as part of an external project funded by the Recreational Fishing Initiatives Fund (RFIF) and continues for the Swan-Canning Estuary (SCE), LE and GB, to provide estimates of recreational activities crab catch rates and size composition (Harris et al., 2017).

Research hourglass traps are used (Figure 8.5), as they are most efficient at sampling larger crabs (Johnston et al., 2020b).

Figure 8.1 Blue Swimmer Crab Daily Research log sheet completed by crab trap fishers in the  Mandurah to Bunbury Developing Crab Fishery
Figure 8.1 Blue Swimmer Crab Daily Research log sheet completed by crab trap fishers in the Mandurah to Bunbury Developing Crab Fishery

Assessment Principles

Assessment Overview

Peer Review of Assessment

Analyses and Assessments

Data Used in Assessment

Catch and Effort Trends

Very low catches (< 1.5 t) of blue swimming crabs have been reported from HI since 2005, primarily in the summer/autumn period (November to April) as a by-product of the gillnet fishery. Following recommendations from a management review of the south-west WA blue swimmer crab resource (DIPRD, 2018), the removal of commercial fishing effort from WS is to be completed by 2020 via a voluntary fisheries adaptation scheme. Following recommendations from a management review of the south-west WA blue swimmer crab resource (DPIRD, 2018), the removal of all commercial fishing effort from the MBDCF is to be completed by 2020 via a voluntary fisheries adaptation scheme.

In 2015, an experimental trap exception was issued to catch blue swimmer crabs in HI.

Table 9.1.  Annual Landings (t) of blue swimmer crabs reported by the minor commercial crab  fisheries in the West Coast Bioregion during 2019, compared to the five-year mean  and total catch for each fishery from 2014-18 (calendar year)
Table 9.1. Annual Landings (t) of blue swimmer crabs reported by the minor commercial crab fisheries in the West Coast Bioregion during 2019, compared to the five-year mean and total catch for each fishery from 2014-18 (calendar year)

Catch Distribution Trends

309 Figure 9.15 Initial trap line locations in Comet Bay (Mandurah to Bunbury Zone 1. Crab Fishery Development) reported in Daily Research Logs between 2015-19. Commercial trapping for blue swimming crabs in MBDCF Area 2 is concentrated in nearshore waters (<1 km offshore) between White Hill and Binningup (Figure 9.19). Catch rates of male and female crabs without berries were evenly distributed across fishing grounds, while females with berries were more abundant in southern waters near the shoal bank.

Catch rates of male and female berryless crabs were reasonably consistent across all commercially fished areas in the UK, while berry-bearing females were typically more abundant in northern coastal fishing waters.

Figure 9.10 Start locations of trap lines sampled during commercial monitoring surveys in the  Warnbro Sound Crab Managed Fishery between 2007/08-2018/19 (left), and during  2018/19 (right)
Figure 9.10 Start locations of trap lines sampled during commercial monitoring surveys in the Warnbro Sound Crab Managed Fishery between 2007/08-2018/19 (left), and during 2018/19 (right)

Fishery-Dependent Catch Rate Analyses

Commercial fishing resumed in December 2015, with the 2015/16 annual standardized catch rate of 1.0 kg/traplift again above the harvest strategy threshold. After a gradual increase in the standardized annual catch rate for CB from 0.3 kg/grap in 1998 to 0.8 kg/grap in 2004, subsequent catch rates have been relatively stable (0.5–0.9 kg/grap; Figure 9.26). The 2019 catch rate of 0.9 kg/traplift represents the highest annual standardized catch rate recorded for this fishery.

After a gradual increase in the annual standardized catch rate of bluefin for CB from 0.3 kg/trap lift in 1998 to 0.8 kg/trap lift in 2004, catch rates were relatively stable.

Figure  9.25  Primary  performance  indicator,  annual  standardised  commercial  catch  rate  (kg/traplift), of blue swimmer crabs by fishing season in the Warnbro Sound Crab  Managed Fishery, with 95% confidence limits, relative to the associated referen
Figure 9.25 Primary performance indicator, annual standardised commercial catch rate (kg/traplift), of blue swimmer crabs by fishing season in the Warnbro Sound Crab Managed Fishery, with 95% confidence limits, relative to the associated referen

Fishery-Independent Data Analyses

However, egg production in LE was relatively consistent between years (14-22x106 eggs/stairlift), except in 2017 when egg production fell to 9.3x106 eggs/stairlift (Figure 9.31). Annual catch rates of sexually mature females in Britain fell from 13 to 6.6 crabs/trap catch, before returning steadily to 13 crabs/trap catch in 2017 (Figure 9.32). Egg production in LE was relatively consistent between years (14-22x106 eggs/trap lift), except in 2017 when egg production fell to 9.3x106 eggs/trap lift.

Annual catch rates of mature females in GB dropped from 13 to 6.6 crabs/trap before steadily returning to 13 crabs/trap in 2017.

Figure 9.31 Mean annual catch rates (numbers of crabs / traplift; ± 95% confidence intervals) of  sexually  mature  female  blue  swimmer  crabs  and  associated  egg  production  (numbers of  eggs / traplift; ± 95% confidence  intervals) recorded during b
Figure 9.31 Mean annual catch rates (numbers of crabs / traplift; ± 95% confidence intervals) of sexually mature female blue swimmer crabs and associated egg production (numbers of eggs / traplift; ± 95% confidence intervals) recorded during b

Productivity Susceptibility Analysis

9.3.6.2.1 Warnbro Sound and the Mandurah to Bunbury Developing Crab Fishery Given the sensitivity results for crab stocks in WS and MBDCF, WSCMF scored a total sensitivity of 1.58 compared to 1.88 for MBDCF. HI for the purpose of PSA analysis is considered the same as in WSCMF and MBDCF and is estimated at 1.88 for sensitivity for both commercial and recreational sectors (Table 9.4). Sensitivity scores of 1.58 and 1.88 for WSCMF and MBDCF's commercial fisheries, respectively, and 1.88 for their recreational fisheries were based on moderate to high availability, affordability, selectivity, and post-release mortality.

A susceptibility score of 1.88 for both the commercial and recreational sectors in the HI was based on moderate to high availability, encounterability, selectivity and post-release mortality.

Table  9.3  PSA  susceptibility  scores  for  each  sector  that  impacts  on Portunus  armatus in  Warnbro Sound (WS) and the Mandurah to Bunbury Developing Crab Fishery (Area  1 - Comet Bay and Area 2 - Mandurah to Bunbury; MBDCF) which for the purpose o
Table 9.3 PSA susceptibility scores for each sector that impacts on Portunus armatus in Warnbro Sound (WS) and the Mandurah to Bunbury Developing Crab Fishery (Area 1 - Comet Bay and Area 2 - Mandurah to Bunbury; MBDCF) which for the purpose o

Stock Status Summary

Weight of Evidence Risk Assessment

Furthermore, the recreational fishery for blue swimmer crabs in the WS is currently considered to be minimal. Furthermore, the recreational fishery for blue swimmer crabs in WS is currently considered to be negligible. Issues affecting the inshore crab fishery of Western Australia: Blue swimming crab (Portunus Pelagicus), Sand crab (Ovalipes Australiensis).

Environmental determinants of commercial blue swimmer crab (Portunus armatus) catch rates in Western Australian fisheries.

Table A1.1. Performance indicators and associated reference levels used to evaluate the status  of indicator species and secondary indicator species in the Pilbara and Kimberley
Table A1.1. Performance indicators and associated reference levels used to evaluate the status of indicator species and secondary indicator species in the Pilbara and Kimberley

Figure

Figure  2.1.  An  outline  of  the  risk-based  planning  cycle  used  for  determining  Departmental  priorities and activities
Figure  3.1.  Bioregions  of  Western  Australia  and  the  locality  of  blue  swimmer  crab  stocks  in  Warnbro  Sound,  Comet  Bay,  Mandurah  to  Bunbury,  the  Leschenault  Estuary,  Geographe  Bay  and  the  Hardy  Inlet,  within  the  West  Coast
Figure  5.1  The  blue  swimmer  crab,  Portunus  armatus.  Illustration  ©  R.  Swainston  (www.anima.net.au)
Table 6.1 Summary of key attributes of the commercial Warnbro Sound Crab Managed Fishery
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References

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