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.