Until recently management of prawns in the Joseph Bonaparte Gulf (JBG) has been a “byproduct” of measures designed for other NPF stocks. In recent years, the JBG has been fished with the two seasonal openings (April to June, August to November), however in 2007 this arrangement changed, and fishing has only been allowed August to November. The rationale of this change was to see if average size of prawns, and total value of the catch could be increased. However, no clear objectives, performance measures or monitoring system were established to evaluate whether this was achieved.

Little targeted research has been carried out on red-legged banana prawns (Penaeus indicus), the main JBG species. In recent years only rudimentary catch and effort, and commercial grading data have been collected. Even this has not been subject to systematic analysis. In these circumstances anecdotal accounts of recent trends can be argued to support five plausible explanations with differing management implications:
1. The late season has optimized value of production.
2. Value is being forfeited because the season is too late and peak annual biomass is being missed.
3. Value is being forfeited because poor weather late in the year and other fishing commitments prevent limit boats days in the JBG.
4. The late season intensifies fishing on the aggregated spawning biomass risking recruitment over-fishing.
5. Natural variability has produced lower catches than normal.

The NPF Industry Pty Ltd needs its discussion of management in the JBG, and other localized stocks, to be based on sound scientific data. The immediate need is to provide a scientific basis for discussions about the 2010 season, which will occur in late 2009. The medium term need is for NPF Industry Pty Ltd develop its capacity to conduct cost-effective data collection programs.

Through the research being undertaken in FRDC Project 2000/311, “Development of research methodology and quantitative skills for integrated fisheries management in WA”, it has been recognised that the dietary data available for Western Australian fish are limited and may result in an imprecise specification of the food web. An urgent need to collate the existing dietary data in order to assess their adequacy in developing information on the extent to which fish species predate upon fish and invertebrates, or ingest plant material, has been identified. Ecosystem modelling and monitoring of the “health” of the non-landed prey species will require the development and maintenance of a database containing consistent and comprehensive details of the relative proportions of the prey species in the diets of their predators. Gaps within the resulting database need to be determined and the research methods, that are required to fill these gaps, need to be identified. Without such data on dietary compositions, it will be impossible to ascertain accurately the relationships between species, thereby hampering the development of accurate ecosystem models. The utility of the existing data sets will be considerably enhanced by their collation and synthesis, and the resulting data are considered to be essential for the development of ecosystem models capable of providing the information required to manage fisheries in accordance with the principles of ESD.

There is a critical need to develop a management policy for the blue swimmer crab fishery in Western Australia, that is based on sound biological data collected for this species in this state. Since the environments in which blue swimmer crabs are caught in Western Australia are unique, it is essential that detailed biological data of this species are collected specifically for these waters. The type of biological data that are required include information on the habitat requirements, size and age composition, growth and reproductive biology of this crustacean species. Reliable data on some of these aspects (size and age composition and reproductive biology) are essentially confined to those obtained for the Peel Harvey Estuary and to those derived from catches obtained in Cockburn Sound, using a trawl net with a large mesh (Potter et al., 1983; Penn, 1977; FRDC project 95/042). From a comparison of data in these latter studies, it would appear that crabs grow to a larger size or live longer in marine waters than in estuaries. There is thus an urgent need to obtain more precise information on the biology of blue swimmer crabs in the different habitats in the marine habitats in south-western Australia.

There are no biological data for blue swimmer crabs in Shark Bay, where the fishery is developing and has great potential. Shark Bay is located at a far more northern latitude than the Peel-Harvey Estuary and Cockburn Sound, and is characterised by far warmer water temperatures. Furthermore, there are essentially no estuaries near Shark Bay, which means that this type of area which is often used as a nursery ground is not available to this portunid in the region of Shark Bay. The presence of far warmer temperatures in Shark Bay, which will presumably influence the activity, feeding, growth and reproductive biology of this species (see e.g. Hill, 1980; Sukumaran & Neelakantan, 1996), together with the type of environment found in that region, mean that it would be very dangerous to extrapolate from the results of studies further south in temperate Western Australia, or indeed from elsewhere in Australia.

It should be noted that the biological data produced by this proposal will complement a separate study of the dynamics of the fishery (both commercial and recreational) and exploitation of the series of stocks which contribute to the catch of blue swimmer crabs in Western Australia. This project proposal by the Fisheries Department Research Division, will be submitted to the FRDC in December 1997 for the 1998/99 round of grant proposals.

References

Hill, B.J., 1980. Effects of temperature on feeding and activity in the crab Scylla serrata. Marine Biology 59: 189-192.

Penn, J.W., 1977. Trawl caught fish and crustaceans from Cockburn Sound. Rep. Dep. Fish. Wildl. West. Aust. 20: 1 24

Potter, I.C., P.J. Chrystal & N.R. Loneragan, 1983. The biology of the blue manna crab
Portunus pelagicus in an Australian estuary. Mar. Biol. 78: 75 85.

Sukumaran, K. K. & B. Neelakantan, 1996. Relative growth and sexual maturity in the marine crabs, Portunus (Portunus) sanguinolentus (Herbst) and Portunus (Portunus) pelagicus (Linnaeus) along the southwest coast of India. Indian J. Fish. 43: 215-223.

Sound estimates of natural and fishing mortality form the basis of fishery stock assessment and modelling. Without these data, the extent to which the spawning biomass has been reduced or the effectiveness of management measures in sustaining wild fish stocks cannot be determined. These estimates of mortality are crucial if the commonly-used reference points for fishing mortality and biomass are to be used in managing fisheries. Considerable advances have been made in obtaining estimates of mortality when data on the age composition of the annual catch are available. However, when there are only data on the length composition of the catches, analyses become more complex and the algorithms are not as well defined. Although information is usually available within the biological and fishery data that allows estimation of total mortality, the information on natural mortality must usually be obtained from empirical models that relate natural mortality to characteristics of the life history for other fish species or by estimating the relationship between total mortality and fishing effort, thereby obtaining an estimate of natural mortality by subtraction. Furthermore, there is a need to understand how natural mortality varies with size. Although the study of mortality deserves to be a primary focus of fishery research, it is difficult and thus often set aside. For many of Australia’s finfish stocks, there is a need to develop length-based methods to estimate mortality, which can be used to monitor the stock status of the recreational fisheries, and to obtain improved estimates of natural mortality that can be used in assessing sustainability. The project falls within FRDC’s Natural Resources Sustainability Program and is intended to increase and apply knowledge of stock assessment methods by developing methods of estimating mortality using length composition data which, compared with age composition data, are relatively inexpensive to collect.