An estimate of stock size is a fundamental requirement in predicting a fishery's production potential and subsequently in developing ecologically sustainable management practices. As yet no stock estimates are available for Australian mud crab fisheries.
A number of factors make traditional stock assessment methods inappropriate for this fishery. Dr Carl Walters, in a 1996 review of the NT mud crab fishery, found that catch and effort models and assessment methods based on catch per unit effort data (CPUE) were inappropriate for assessment of this fishery due to non-randomness (hyperstability) in the spatial pattern of fishing effort. Mud crab fishers consistently fish down areas before moving to a new unfished or previously spelled area. Catch rates from this systematic local depletion process remain high, providing no evidence of stock decline. Likewise, use of traditional length-based models for estimating mortality and growth rates are problematic due to the non-continuous pattern (moult process) of crustacean growth. ( Walters 1996 FRDC Project No 96/158). Visual assessment techniques are also inappropriate due to the high turbidity of northern Australian tropical estuarine waters and fishing gear biases combined with species specific behavioral characteristics (i.e. burrowing) permits only the use of baited pots as a sampling tool. (pers. Comm. B. Hill 1999).
Recent upward trends in mud crab catch rates suggest that a degree of urgency in gaining estimates of mud crab stock size is warranted. Dr Walter's 1996 assessment results, based on the limited available data, suggested that the NT fishery was fully exploited (70-90% of available stock) and that there was little room for further development. However, in the following year the total NT mud crab catch doubled reaching 595 tonnes, with only a small increase in reported effort ( high levels of non-compliance were also reported). Qld total commercial catch has also substantially increased from approximately 400t in 1995 to 660t in 1998. Recreational and Indigenous activity trends also suggest increased activity.
Also of importance is recent work by Knuckey (1999) confirming Dr Walters conclusions that the NT Fishery (and probably Qld) heavily exploits the year one recruits. Considering the short lifespan (4 years) of this portunid crab, gaining an annual estimate of stock size is a high priority.
Anecdotal evidence from commercial fishers suggests that crab abundance follows a lunar cycle. This fine scale detail in catch variability is not evident from current logbook data and this may be a crucial factor in determining the optimal sampling period.
Given the accelerating pace of coastal development in northern Australia, identification and quantification of critical mud crab habitat is a priority for future protection of the ecosystem on which mud crab and various other stocks depend. Our ability to provide achievable biological reference points for the sustainable management of the mud crab fishery is limited by the nature of the fishing operation and the seasonal variability of catch rates.
A fishery-independent measure of stock abundance is needed to determine the size of the mud crab resource and therefore its sustainable harvest, including the potential for future development in areas such as Western Australia.
A significant achievement of this project has been the completion of mapping of coastal wetland habitats using remote sensing techniques, which provided a complete broad-scale coverage of mud crab habitats in the NT and Qld. A major outcome/output of this work has been the incorporation of the mapping into a geographical information system (GIS) permitting a much wider application across a variety of natural resource management agencies and issues. The updated Qld maps are now available electronically to the public via the QDPI&F website CHRIS. The identification and quantification of northern Australian coastal wetland habitats will benefit a broad range of northern Australian inshore fisheries.
Survey and analysis methodologies, based on mark-recapture techniques, have been developed to estimate mud crab density for two key habitat types in northern Australia. Density estimates for each habitat type were extrapolated up across adjacent regions in each state providing the first broad scale estimates of mud crab stock size. A direct and recent output from this work has been the use of preliminary biological and fishery data, to compare trends between years for Qld and NT mud crab fisheries, during a recent fishery assessment. This fishery assessment was convened in July 2004, to investigate the reduction in commercial mud crab catch in the NT. Negotiations on adjustment to the NT management arrangements are currently in progress.
Declines in catch and catch rate were observed in both the NT and Qld Gulf of Carpentaria (GOC) surveys over the two years of this study. This suggests large-scale environmental drivers influence mud crab recruitment success, at least for Gulf region. Estimated abundance for this region in the NT indicates a very high proportion of the legal sized mud crab stock was removed in 2003. Provision of information such as this may be far more useful for management purposes than logbook catch per unit effort (CPUE) data alone. The assessment techniques developed during this project provide a means to increase the value of CPUE data, setting up a benchmarking process that will ultimately assist in making well informed and timely management decisions.
Keywords: mud crab, abundance, habitat mapping, depletion, mark recapture, removal, trapping web