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.

The proposed work is fundamental for assessment and sustainable, optimal harvest of Australia’s Spanish mackerel resources. This goal requires accurate information on which management decisions can be based. This project therefore seeks to describe the stock structure of a national shared resource, with a view to the development of complementary management approaches.

The NT, WA, Qld, Torres Strait and NSW have separate management regimes for the mackerel fisheries in their waters. However, our lack of information on stock structure means that the appropriate scale of management units is just not known. It is unlikely that it will coincide with current administrative boundaries. Basic questions such as whether management actions in one state will impinge on the fisheries of others cannot yet be answered. With such uncertainty, for example, would declines in one area reflect interception during migration, or over-fishing of spawners in another area? Different responses to such questions may require fundamentally different management approaches. Hence the Northern Australia Fisheries Management meeting of May 1997 recognized that stock definition was required for effective assessment and management of this species.

Most fishery assessments assume a randomly mixed unit stock; an alternative is to explicitly include spatial dynamics. Possibly with the exception of the east coast, there is no real basis for defining Spanish mackerel unit stocks. In none of the Australian fisheries are spatial relationships sufficiently understood to be addressed in assessments. The proposed research is requisite for basic stock assessment, and the first step in developing spatially structured models and management.

Spanish mackerel are also taken across our northern boundaries, in Indonesia, Papua New Guinea, and Pacific Island states. The proposed research is the basic work necessary to develop the methodology and information base for future research into these shared stocks, and for future studies into fine-scale spatial dynamics.

Recently available information suggests the Spanish mackerel fishery is growing rapidly in both commercial and recreational sectors. This underlines the need for this work as a basis for rational management. Commercial catches in WA and NT have increased substantially in recent years, and prices continue to rise. A recent recreational survey in NT revealed that recreational Spanish mackerel catches are of similar order to commercial catches. The species is a favoured target in the rapidly-growing and lucrative fishing tour sector.

The need for good for stock assessment is thus growing. Each state has responded with FRDC- or internally-funded programs. The results of this project could substantially change the directions of these projects, by establishing whether the assessment and management should be on a joint basis across states, or whether they should be on a more regional basis.