Commercial catches of blue swimmer crabs have declined significantly since 2000 due to low stocks, resulting in the closure of the fishery in December 2006. The 2009/10 Egg Production Index suggests for the first time since the fishery closed that the breeding stocks may be sufficient.
Introduction
Background
Blue Swimmer Crab Biology in Cockburn Sound
Map of Cockburn Sound showing the extent of the commercial blue swimmer crab fishery of Cockburn Sound (■), Warnbro Sound (■) and (Area I of the West Coast Estuarine Managed Fishery (Swan-Canning Estuary)). Genetic studies have shown that the blue swimmer crab population in Cockburn Sound is generally independent of other stocks in the state, for example, the Peel-Harvey Estuary, Geographe Bay and Dongara (Chaplin et al., 2001).
Physical Description of Cockburn Sound
In Cockburn Sound, mating occurs in the late austral summer–autumn (January to April) when females have finished spawning and recently matured young have soft armor (Kangas, 2000). Therefore, adverse changes in environmental conditions or high levels of estuary fishing pressure could have very detrimental and long-term effects on crayfish stocks in Cockburn Sound (Chaplin et al., 2001).
History of the Fishery - Management
Commercial Fishery
The crab trap when set has a height between the rings of not more than 450 mm;. The crab trap has a tag not more than 15 cm long and not more than 6 cm high firmly attached to it which clearly displays (in black letters not less than 6 cm high) the licensed fishing boat number of the authorized boat from which the crab trap used;
Spatial Distribution of Commercial Crab Fishing
Before the closure of the fishery in December 2006, there were 12 permit holders sharing a total allocation of 800 crab traps. The crab trap or another crab trap attached to the first crab trap by means of at least 20 m of negative buoyancy rope has attached a negative buoyancy rope to which is attached a float of at least 190 mm diameter and which floats clearly in letters of at least 80 mm in height and 15 mm in width, the number of the approved boat from which the crab trap is used, and 5.
Recreational Fishery
While commercial catches can be measured from monthly fishermen's returns, the data available for recreational fishermen in Cockburn Sound is limited to a limited number of surveys. A summary of recreational survey data for the Cockburn Sound crab fishery is presented in Appendix 2 of this report.
Resource Sharing Agreements
Because of its proximity to Perth, Cockburn Sound is popular with recreational crab fishermen. A boat-based flush survey conducted over the peak recreational period (Jan–Mar) in 2006 yielded a recreational catch estimate of 3 t (Sumner, unpubl. data).
Swan River and Warnbro Sound
The spawning closure covers the months of October and November in Warnbro Sound, while fishing is permitted all year round in the Swan-Canning Estuary. While commercial effort levels have fallen in the Swan River and remained flat in Warnbro Sound, the number of recreational angling participants continues to rise.
Objectives of DBIF Project
Compared to Cockburn Sound, the commercial fishery for blue swimmer crab in the Swan River and Warnbro Sound is small. A genetic assessment of the relationships between communities of the blue swimming crab, Portunus pelagicus, in Cockburn Sound, the Swan River Estuary and Warnbro Sound.
Decline in the blue swimmer crab stocks of Cockburn Sound – history,
Abstract
Introduction
Methods
- Study site
- Commercial catch and effort
- Commercial monitoring
- Fishery-independent sampling – breeding stock
- Fishery-independent sampling – juveniles
- Catch prediction model
- Decision-Rule Framework
LSMs were used due to the unbalanced nature of the data (unequal number of trawls for each area, month and year). The juvenile index (0+) and residual (1+) generated for the catch prediction model form the x-axis of the decision rule framework.
Results
- Commercial catch history
- Trawl surveys
- Catch monitoring
- Catch prediction model
- Spawning Stock
- Management - Decision Rule Framework
Most of the annual commercial catch in the Cockburn Sound fishery is taken during the summer months (Fig. 2.3). A draft decision framework for the managed Cockburn Sound Crab fishery which will assist in the management of the annual commercial catch.
Discussion
- Factors Contributing to Collapse
- Fishery near edge of species distribution – vulnerable to
- Self-recruiting population
- Change in fishing method – increased fishing pressure on
- Juvenile Index
- Recovery
- Management
This extended pre-spawn cool period was an unusual event and a major contributing factor to the decline in the Cockburn Sound crab fishery. The reduction in male size (Abbe, 2002) and female size and mean size at maturity that occurred during the Chesapeake Bay decline (Lipcius and Stockhausen, 2002) needs to be verified for the Cockburn Sound crab fishery.
Stock-Recruitment-Environment Relationship
Abstract
Introduction
They suggested that water temperatures have a significant impact on egg production in blue swimmer crabs, and therefore on the timing and longevity of the spawning period. Therefore, water temperatures during any part of the life cycle can significantly alter population size and structure over a much larger time frame.
Methods
Standardized egg production index
The impact such a management response can have on fishermen's livelihoods and recreational interests highlights the urgent need to understand the cause of the decline in recruitment and better management of the fishery. The aim of the current study was to determine the combination of spawning stock and environmental impacts that contributed to the collapse of this apparently robust stock, in order to assess whether other similar fisheries may also be vulnerable.
Water temperature
Stock-recruitment relationship
Results
The best fit explained approximately 76% of the catch variation and was generated using August water temperatures in the model (Figure 3.2), although good fits were generally obtained for the months August to December. The best fit explained approximately 76% of the catch variation and was generated using August water.
Discussion
This model, which quantifies the influence of water temperature on crab recruitment, will aid in fishery management. The tiger shrimp stocks had two additional factors contributing to their overfishing that are not found in the crab fishery.
Warnbro Sound and Swan River Crab Fisheries
Genetic study
Management Implications
However, the amount of gene exchange between the Cockburn Sound assemblage of this species and those in the Swan River Estuary and Warnbro Sound is likely to be temporally variable and generally insufficient to have a major impact on the abundance of this species in either of these water bodies. In fact, based on (simplified) inferences about the distribution of likely barriers to the spread of this species in these waters, we predict that the group of P.
Commercial monitoring
Methods
Annual commercial catch and efforts for the Swan conservation estuary and Warnbrosund are presented per calendar year. Monitoring of commercial catches and efforts of blue swimmer crab began in the Swan-Canning Estuary in November 2007 and in Warnbro Sound in March 2007.
Results
Annual commercial catch of blue swimmer crab (t) and effort (number of anglers and total fishing days) from West Coast Estuary Managed Fisheries Area I (Swan-Canning Estuary) from 1990 to 2009. Annual commercial catch of blue swimmer crab (t) from Warnbro Sound by fishing method and total effort (fishing days) regardless of method.
Discussion
The reasons for the large size of Swan River crabs may be that Cockburn Sound and the Swan River are a single population, with the larger crabs in the Swan River comprising the 2+ age group. As with Swan River, commercial catch monitoring data from Warnbro Sund is not necessarily a reliable indicator of stock parameters in itself.
Recreational crabbing
- Recreational fishing survey in Swan-Canning Estuary Basin 1998-99
- Recreational boat-based fishing survey Augusta and
- Angler’s Daily Log Book and Fishing Tournament
- Volunteer Fisheries Liaison Officer program: recreational fishing
Analysis of these data sets can provide recreational catch rates of blue swimmer crabs for the Swan River and Warnbro Sound. The logbook program is ongoing, so analysis of logbook data can provide recreational catch rates of blue swimmer crabs for the Swan River and Warnbro Sound from 2004 onwards.
Stock Status of the Cockburn Sound Crab Fishery
Introduction
Fishery Overview
Methods
- Commercial monitoring
- Fishery-independent sampling – breeding stock
- Fishery-independent sampling – juveniles
- Catch prediction model
Following the closure of the fishery in December 2006, the RV Naturaliste trawl surveys in Cockburn Sound expanded their spatial coverage. Department of Fisheries research sampling and Murdoch University juvenile trawling intensity in Cockburn Sound since 1998.
Results
- Catch and Effort
- Commercial monitoring
- Research Trawl Surveys
- Juvenile recruitment - Catch prediction model
- Egg Production
- Stock-recruitment-environment relationship
Commercial blue swimmer crab catch (t), effort (trap lift x 1,000) and catch per unit effort (kg/trap lift) per month for the 2010 season of the Cockburn Sound Crab Fishery. The correlation is based on the occurrence of the breeding stock (Egg Production Index) over the summer (year t/t+1), the following season's commercial catch (year t+1/t+2) and the average water temperature (from ) before the start of spawning (August-September year t).
Decision Rule Framework - Management
Draft decision rule framework for the Cockburn Sound managed crab fishery, which includes both the recruitment strength (young 0+) from the previous season's spawn (September to January) and the residual stock (1+) remaining in the water until the end of the current fishing season, to estimate the percentage of catch per month in the season. Draft decision rule framework for the managed Cockburn Sound Crab fishery incorporating both recruitment strength (juvenile 0+) from the previous season's spawning. from September to January) and the residual stock (1+) remaining in the water until the end of the current fishing season to estimate the catch percentage per month of the season.
Management advice
Following consultation with RecFishWest and WAFIC and advice from the Department, the Minister decided to reduce the commercial size limit from 140mm CW to 135mm CW and maintain the same season length between 15 December 2010 and 30 March 2011 (option 1). The recreational fishing season was extended by 1 month to the end of April to compensate for the anticipated increase in the share of the catch by commercial fishermen in 135 mm CW.
Future Research
Acknowledgements
Reproductive biology of the blue swimmer, Portunus pelagicus (Decapoda: Portunidae) in five bodies of water on the west coast of Australia. A summary of the biology and exploitation of the blue swimming crab, Portunus pelagicus Linnaeus, in Western Australia.
Appendix 1
Recreational Catch
Recreational Effort
Some 19,725 hours were spread over just 11,876 days, however, increasing the average crabbing trip to 3.84 fishing hours. Extrapolating these estimates predicts a recreational crabbing effort of 16,176 fishing days for calendar year 2004 (Table 9.2).
Sex ratio
Effort remained fairly constant in 2004, with recreational crabbers fishing an estimated 18,466 hours during 12,058 fishing days in the January to March period. Numbers of blue swimmer crabs recorded from surveys of recreational boaters on five boat ramps in Cockburn Sound between January and March 2002-2004.
Size distribution
Average length of blue swimmer crabs caught by recreational anglers interviewed at boat ramps in Cockburn Sound between January and March.
Cockburn Sound recreational blue swimmer crab survey - 2009/10 season
Allocation of time to survey ramps in Cockburn Sound during the West Coast Creel survey. Allocation of time to ramps in Cockburn Sound during the Cockburn Sound (Early Morning) Creel Survey.
