Apart from the work by Bryars & Adams (1997) on three assemblages of Potunus pelagicus in South Australia, there have been no studies aimed at determining the extent to which blue swimmer crabs are represented by different stocks in spatially-isolated habitats within a given geographical region in Australia. Furthermore, the studies in South Australia were carried out using allozymic analyses, which typically do not have the same ability to discriminate precisely between the boundaries of stocks, as can be achieved with the microsatellite techniques we intend to use in our proposed study (see Wright & Bentzen, 1995; OConnell & Wright, 1997).
As with all fisheries, a basic prerequisite for managing the fishery for blue swimmer crab is the identification of production units or stocks of a species since inadequate knowledge of stock structure may lead to over- or under-exploitation (Smith et al., 1997). For the following specific reasons, information as to whether there are one or more stocks of swimmer crabs, ie. genetically discrete assemblages, in a given region is crucial for managing the fishery for this portunid.
1. The ability of managers to respond appropriately to any marked decline in the abundance of blue swimmer crabs within part of the range of this species will depend on knowledge of whether or not the crabs in that region constitute a single stock or are part of a larger and more widely-distributed stock. In this context, it is highly relevant that the work of Bryars & Adams (1997) showed that, in spite of the high vagility of blue swimmer crabs, the assemblages of this species in relatively nearby localities can be genetically different and thus constitute different stocks. It should also be recognised that, as the fishery for blue swimmer crabs increases, the mortality produced by repeated captures and releases of undersized crabs could place pressure on those stocks that are being heavily fished.
2. There is a need to identify the different stocks of blue swimmer crabs that exist within a region in order to facilitate adaptive management (and opportunities for research) of separate populations that are exposed to (or experience) different levels of fishing mortality.
3. Any modelling of the blue swimmer crab fishery in a region is dependent on knowing whether the assemblages in that region constitute one or more stocks.
4. Fisheries managers need to know the extent to which the biological characteristics such as growth rates and fecundity, vary amongst stocks. This can be explored in Western Australia because biological data are continuing to be accumulated for different assemblages of blue swimmer crabs in this state under the auspices of past and current FRDC grants (FRDC Project Nos 95/042 and 97/137).
As well as responding to the immediate need for stock identification of blue swimmer crabs, the proposed project will generate detailed information on the population genetic structure of Portunus pelagicus over virtually its entire range in Australia. The resultant information will help to identify the importance of various factors, such as climate, current flow and physical isolation, in determining the stock structure of this species. In turn, this information will be useful should genetic management of this species become a priority, e.g. for conserving genetic diversity within stocks and for identifying stocks possessing desired performance traits.

Literature Cited

Bryars, S. & Adams, M. (1997) Population structure of the blue swimmer crab, Portunus pelagicus, in South Australia. In: Kumar, M. S. (ed.) Proceedings of the First National Workshop on Blue Swimmer Crab Portunus pelagicus. SARDI Reseach Report, Series Number 16, SA.
OConnell, M. & Wright, J. M. (1997) Microsatellite DNA in fishes. Reviews in Fish Biology and Fisheries, 7, 331 - 363.
Smith, P. J., Benson, P. G. & McVeagh, S. M. (1997) A comparison of three genetic methods used for stock discrimination of orange roughy, Hoplostethus atlanticus - allozymes, mitocondrial DNA, and random amplified polymorphic DNA. Fishery Bulletin, 95, 800-811.
Wright, J. M. & Bentzen, P. (1995) Microsatellites: genetic markers for the future. In: Carvhalo, G. C. & Pitcher, T. J. (eds.) Molecular Genetics in Fisheries. Chapman & Hall, London.