Data on fishing and stock dynamics in the Shark Bay crab fishery have been obtained from a number of ongoing programs run by the Western Australian Department of Fisheries (DoF). There is conflicting evidence in the data collected from the crab trap fishery in Shark Bay about the sustainability of current overall catch levels.
Introduction
Management history of trawling for blue swimmer crabs in Shark Bay
Map showing the location of Shark Bay in Western Australia, and the boundaries and exclusion zones of the Shark Bay Prawn Managed Fishery.
Management history of trap fishing for blue swimmer crabs in Shark Bay
The Carnarvon Experimental Crab Trap Fishery (CECTF) was established in 1998 to determine the potential for a sustainable blue swimmer crab fishery in the waters of Shark Bay north of Cape Inscription. There are currently a total of 1,500 traps in the Shark Bay crab fishery, with a maximum of 400 of the 1,500 traps that can be used in East and West Bay south of Cape Peron.
Recreational fishing for blue swimmer crabs in Shark Bay
Efforts have also been made in the marketing of Shark Bay blue swimmer crabs as a premium seafood product, with fishermen using the World Heritage List of the Shark Bay region to emphasize the pristine nature of the habitat and the sustainability of the managed crab fishery .
Physical description of Shark Bay
The limited exchange of oceanic water, minimal freshwater input and high evaporation rates have resulted in Shark Bay containing three distinct water body types: oceanic (salinity of 35 – 40‰) in the northern waters and upper gulf regions, metahaline in the middle gulf regions and hypersaline in the lower canyons. These distinct salinity regimes affect habitat and species distribution, resulting in three distinct biotic zones in Shark Bay (Francesconi and Clayton, 1996).
Biology of blue swimmer crabs
Report objectives
Kangas (2000) summarized existing studies that had provided information on the biology, population characteristics and exploitation of the blue swimmer crab, Portunus armatus, in Western Australia, and identified gaps in existing knowledge. Data from commercial fishermen's statutory returns and catch monitoring surveys were analyzed to describe commercial trap catch and effort and to assess the sustainability of current commercial harvest levels, while research log and tagging survey data were used to provide an overview of the spatial distribution of crab stocks. in Shark Bay.
Methods
Study Location
Commercial catch and effort
Trap
Trawl
Commercial monitoring
Fishery-independent trawl surveys
November trawl survey
Map showing the location of fishing grounds sampled each year during the annual November Independent Trawl Survey in Shark Bay between 2002 and 2010 and the corresponding fishing grounds referenced in the data analysis.
Pilot sampling in eastern Shark Bay (ENA)
Commercial trap factory unloads
Daily research logbooks
Trap
Trawl
Recreational Fishing
Statistical Analysis
Commercial trap catch and effort (CAES)
CPU (kg / step lift) was calculated as landed weight / average number of pots serviced each day in a month / number of fishing days in fishing block in that month. Analysis of interannual trends in effort versus catch is based on the technique of Hall and Penn (1979), taking into account the spatial and temporal variations in fishing effort and efficiency between fishermen by standardizing CPU with the variables: financial year month, fisherman (3 fishermen working in the northern fishing grounds) and block (Western Gulf, Eastern Gulf, Carnarvon, N Carnarvon).
Commercial trap catch monitoring
Nominal catch was then divided by the standardized CPUE to give the standardized effort, which was plotted against nominal catch. Subsequent analysis has revised the male size at maturity in Shark Bay to 97.0 mm CW (de Lestang et al., 2003a), and this value will be used in all future statistical analyzes of the Shark Bay crab population.
Fishery-independent November trawl survey
Crab carapace widths were also analyzed, except that no transformation was applied to the data and no weighting was applied.
Results
Trap Sector
Fishers’ Catch and Effort Returns (CAES)
Annual catch ( ), effort ( ), and catch per unit effort (−−) for commercial blue swimmer crab fishermen in Shark Bay by calendar year from 1990-2011. Plot of nominal effort (traplifts x 1000) against total annual catch (t) by financial year 1990/91 to 2010/11* for all commercial trap fishermen operating in Shark Bay, derived from monthly returns of fishermen. Plot of standardized effort (traplifts x 1000) against total annual catch (t) by financial year from 1990/91 to 2010/11* for commercial trap fishermen operating in the Shark Bay fishing grounds north of Cape Inscription derived from fishermen's monthly legal catch and effort returns.
Factory unloads
Proportions of extra-large males were also highest during the summer months from October to April, accounting for between 37% and 51% of the catch. This period coincides with the closure of the trawl fishery, which operates from April to September. However, there was a similar intra-annual monthly pattern of depletion in the proportion of extra large crabs, peaking at 65% of the retained catch in February before steadily declining with fishing pressure to 34% in August (Fig. 20).
Catch monitoring
Standardized catch rates of sexually mature female sublegal crabs for July/August/September (-), October/November/December (---) and March/April/May (····) by depth derived from monitoring commercial trap surveys in Shark Bay between 2000 and 2010. Standardized catch rates of commercial males and sexually mature male juvenile crabs for July/August/September (—), October/November/December (---) and March /April/May ( ····) by latitude derived from commercial trap monitoring surveys in Shark Bay between 2000 and 2010. Standardized catch rates of live female crabs for July/August/September (—) , October/November/December (---) and March /April/May (····) by depth derived from commercial trap monitoring surveys in Shark Bay between 2000 and 2010.
Fishery independent trawl surveys
- November trawl
- Pilot sampling in eastern Shark Bay (ENA)
Mean annual standardized catch rates of wild blue swimmer crabs derived from annual November angler surveys in Shark Bay between 2002 and 2010. Standardized catch rates of: (A) commercial male, (B) male sexually mature juvenile, (C ) sexually immature male, (D) commercial female, (E) sexually mature juvenile female, and (F) sexually immature female blue swimmer crab from the fishery, derived from annual fishery surveys of fishermen in Shark Bay between 2002 and 2010. Standardized catch rates of live female blue swimmer crabs by fishing ground derived from annual November catch surveys of anglers in Shark Bay between 2002 and 2010.
Daily research logbook data
- Trap
- Trawl
Scallop vessels carry a very small amount of crab and total annual landings are in the order of 10 t. In the years 1998-2005 inclusive most crabs were kept early in the season, while from 2006 most crab numbers were kept in the middle of the season May-August (Fig. 10). This tends to accommodate the late shrimp strategy of opening the eastern portion of the fishery later and rotating the opening of areas where crabs are mostly abundant early in the season.
Recreational Sector
The recreational component of the total catch for lower Shark Bay was 1% to 4% over four years (Table 7). Subsamples of blue swimmer crabs retained by recreational fishers in the eastern gulf were measured (carapace width) during each pink snapper survey. Length frequency distribution of blue swimmer crabs caught by recreational anglers surveyed in eastern Shark Bay during the 2005-2007 pink snapper surveys.
Discussion
Commercial fishing history
Trawl
Trap
Standardized catch rates in the Eastern Gulf initially showed a similar trend to that experienced in the northern fishing grounds. Monthly standardized catch rates in the northern fishing grounds were highest over the summer and autumn months, peaking in March, April and May (Fig. 10). However, catch rates in the Eastern Gulf were highest in late winter/spring, peaking in July, August and September.
Commercial catch composition
Trawl
Standardized trap catch rates increased by approximately 50% in the first three years of the CECTF, representing an increase in fishing efficiency as trappers identified productive areas for crabbing in the waters north of Cape Inscription. Trap fishermen have identified prime fishing grounds in the northern waters of Shark Bay and concentrated their fishing activities in the more productive months of November to June (Figure 10). It is not clear whether this decline represents a depletion of crab stocks in the eastern gulf or is more reflective of a decline in fishing efficiency.
Trap
A decline in the proportion of extra large male crabs throughout the fishing season was also observed in the retained commercial trap catch of the Eastern Gulf. These changes in size were also reflected in the proportion of extra large (150+ mm CW) male crabs in the commercial trapping from the main crab grounds north of Cape Inscription. Similar trends were identified in the size of commercial female and berry female crabs caught during commercial trap monitoring in Shark Bay.
Spatial/depth trends
Sub-legal male crabs accounted for a quarter of the commercial trap monitoring catches in the fisheries north of Cape Inscription in 1999/00. In contrast, almost half (mean: 46±2.7%) of the crabs caught during fishery-independent trawl surveys in November were sub-legal. 2003) reported similar findings in the Moreton Bay crab fishery in Queensland. Statistical analysis of commercial trap monitoring data suggests that commercial female crabs are most abundant in the deeper waters (>17m) of Shark Bay throughout the year (Fig. 29).
Reproductive biology/breeding stock
Despite the high proportion of commercial crab catches coming from the northern fishing grounds, and the significant level of exploration by trappers in Shark Bay over the past 10 years, little effort has been made to commercialize crab stocks in the northern waters of Shark Bay. between Cooks Island and Point Quobba. While 8% of female crabs trapped in November surveys were berries, 39% of female crabs caught with trawls were berries. Although the use of commercial traps to determine spawning levels should be treated with caution, assuming that the behavior of crab trappers to avoid areas with a high abundance of berry females is consistent throughout the year, the assumption that the peak in number of berry females in commercial trap monitoring catches in the Jul/Aug/Sep period coincides with a peak spawning period in Shark Bay remains valid.
Life cycle of the blue swimmer crab in Shark Bay
Conclusions
Stock status/sustainability risk
Plots of annual standardized effort versus catch for the fyke fleet based on fishing charters. Average annual standardized catch percentage for the tonna fleet based on the fisherman's Statutory Monthly (CAES) declarations;. Annual standardized catch rate for male and female crabs for the trap fleet based on commercial monitoring data;
Future Research
Annual standardized catch rate for commercial (mm CW) and large (150+ mm CW) male crabs and female crabs for the trap fleet based on factory releases;. Trends in mean annual standardized shell width for male and female crabs without berries and female crabs for the trap fleet based on commercial monitoring data; Annual standardized catch rates for male, female crabs without berries and female crabs with berries based on fishery independent trawl surveys;
Management issues for future consideration
Monitoring of the commercial crab fishery in Shark Bay should continue to determine whether catch levels from the combined trap and trawl sectors remain at or below current levels, and that catch composition and crab size remain relatively constant. Additional resources will also be required to complete the comprehensive stock assessment required to adequately determine a sustainable harvest level for the Shark Bay crab stock. Determine the socio-economic significance of the blue swimmer crab resource in the commercial trap and trawl sectors in Shark Bay.
Acknowledgments
Synopsis of the biology and exploitation of the blue swimming crab, Portunus pelagicus Linnaeus, in Western Australia. Bureau of Resource Sciences and Fisheries Research and Development Corporation, Canberra, A.C.T. Proceedings of the First National Workshop on Blue Swimmer Crab Portunus pelagicus. The biology of the blue swimming crab Portunus pelagicus in the Leschenault Estuary and Koombana Bay in south-western Australia.
