Fisheries long term monitoring program : Mud Crab (Scylla serrata) Report: 2000-–2002

Abstract

The Queensland Department of Primary Industries and Fisheries (DPI&F) manages the harvest of Queensland’s fish, mollusc and crustacean species and the habitats they live in and is committed to monitor the condition and trends in fish populations and their habitats. This information is used to assess the effectiveness of fisheries management strategies ensuring that the fisheries continue to be ecologically sustainable.
Two species of mud crab, Scylla serrata and S. olivacea, are found in Queensland waters (Keenan et al. 1998). The most abundant of these species in Queensland is S. serrata. The annual Queensland harvest of mud crabs, primarily S. serrata, has been steadily increasing from approximately 200 t in 1988 to around 1000 t in 2000 - 2001. Commercial catch rates remained relatively constant at about 20 kg/day until 1997, increasing to 26 kg/day in 2000 - 2001. The high value of the fishery ($10.4 M) (Williams 2002), increasing catch rates and increasing total effort combined with general public pressure to review the suitability of current management arrangements, resulted in mud crabs being included in the DPI&F, Long Term Monitoring Program (LTMP).
The objectives of the mud crab monitoring was to obtain fishery-independent catch per unit effort data to estimate annual changes in relative abundance, record size frequency and sex ratios for long-term comparison of population structure and population sustainability indicators, and to record changes in habitat, water quality, effects of fishing pressure and several additional abiotic variables.
The sampling design included 17 regions statewide, from the Gulf of Carpentaria in north-western Queensland to Moreton Bay in south-eastern Queensland. Each region was stratified into four locations - foreshore, mouth, mid- and upper-estuarine. Standard commercially available Munyana© brand crab pots are used to conduct 20 pot sets annually at each location.
Bycatch (species not targeted and not kept) in the mud crab fishery, is relatively low in both amount and diversity when compared to other fisheries (Barker et al. 2004). Bycatch retained in pots is predominantly alive and can be released in good condition at the point of capture. The setting of pots to reduce exposure during periods of low tide can reduce the mortality of bycatch species.
The statewide summary of water quality data yields few correlations between the various water quality parameters and mud crab catches. However, there is anecdotal evidence from LTMP staff, researchers and recreational and commercial fishers that water quality does effect the location, sex and number of crabs that can be caught.
Significant differences in mean carapace width were observed between male and female crabs, between regions and between locations. However, the magnitude of these differences was often small. The size frequency of male crabs declines sharply above the 150 mm minimum carapace width where as female crabs show a bimodal size frequency distribution. This correlates with the minimum legal size, and it is likely the size frequency of male crab is influenced by fishing mortality. The difference in mean carapace width between locations showed a trend towards smaller crabs the further upstream the sampling occurred.
Investigation of mean standardised catch rates shows lower catch rates and lower proportions of female crabs in the Gulf of Carpentaria than on the east coast. There were higher proportions of sub-legal male crabs than legal male crabs in close proximity to major urban areas a result that suggests fishing pressure (commercial and recreational) is higher in these areas.