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In recent decades the eastern stock of Atlantic bluefin tuna Thunnus thynnus thynnus has declined, driven by excessive catches partially mediated by the growth of the capture-based aquaculture of bluefin in the Mediterranean. This study addresses the potential direct and indirect food-web effects on trophic linkages in the ecosystem through the removal of both small pelagic fish species and wild bluefin tuna for aquaculture operations in the Mediterranean. A mass-balanced model representing the southern Catalan Sea during the 1990s was modified to include a bluefin tuna farm partially supplied from fish captured in the area modeled. Six scenarios were developed to simulate possible changes to the capture-based aquaculture operations and possible impacts to the ecosystem. The addition of one bluefin tuna farm in the Catalan Sea did not produce substantial effects in the ecosystem. Simulation scenarios that include the level of production already present in a similar ecosystem in the Murcia region resulted in large fluctuations of both biomass and yield for bluefin tuna, as well as for many species in the modeled ecosystem. Increases in biomass of lower trophic level functional groups were observed with reductions in biomass from higher trophic level predators. These outcomes demonstrate that removal of biomass at top and intermediate trophic levels can have direct and indirect outcomes on the structure of the ecosystem due to the complexity of the food web. Our results suggest, in the case of the Western Mediterranean Sea, increasing bluefin tuna farming activities will likely contribute towards further degradation of an already highly exploited ecosystem

Linear mixed models were used to test the null hypothesis that there were no differences between seasons and locations in the reproductive potential of female eastern king prawns, Melicertus plebejus along the east coast of Australia. Three samples were collected in each season between autumn 1991 and winter 1992 (inclusive). Females capable of spawning were found at all locations but proportions were greater in lower than higher latitudes. Females capable of spawning were not found at the southern (highest latitude) most location in all seasons. There was a significant interaction in reproductive potential between seasons and locations suggesting that patterns among seasons differed between locations and vice versa. Reproductive potential was greatest amongst the northern (lower latitudes) most locations and was greatest in autumn at these locations. Seasonal patterns were less pronounced further south (higher latitudes). The length composition of females in catches differed between locations with more larger prawns found in samples from northern locations. The challenge that remains is to quantify the oceanic sources of larvae that contribute to recruitment in each nursery area and the estuarine sources of juveniles that contribute adults back to the effective spawning stock. Maintaining the effective spawning stock and important nursery areas are crucial to the sustainability of this resource.

The effect of different combinations of tagging and release methods was examined on the survival, moulting, growth increment and recapture rates of slipper lobsters Thenus spp. on the Queensland (Australia) east coast using generalised linear modelling methods. Laboratory experiments indicated that while tagging is unlikely to significantly affect the survival rate or growth increments of lobsters, it is likely to lower the incidence of moulting. In the field, recapture rates of tagged lobsters increased markedly by applying an antibiotic/antifungal ointment to the tag wound and by minimising the length of time lobsters were held on board prior to release. Recapture rates for Thenus orientalis declined by about 0.5% for every hour lobsters were held on board prior to release, even though they were kept in aerated seawater. About twice as many male T. orientalis were recaptured compared to females. Reasons for this may be related to size differences between the sexes and how the fishers valued the rewards for different size classes. The size of the T-bar anchor tag affected recapture rates of Thenus indicus; recapture rates of lobsters tagged with small tags were about 33% higher than those with large tags. Two different methods of release were also compared; surface release and bottom cage release. Recapture rates of large lobsters and those that were held on board for several hours prior to release were improved by using the bottom cage method. However, under some conditions, the cage may lower recapture rates relative to the surface release method, possibly because of increased stress and trauma from crowding in the cage. For both species, growth increments of recaptured lobsters increased with the period at liberty and declined with increasing lobster size. Females had significantly larger growth increments than males.

A prawn larval behavioural model was coupled to a hydrodynamic model of the Gulf of Carpentaria in northern Australia to provide estimates of the size of the spawning area from which nursery ground populations are drawn, referred to here as the advection envelope. We have assumed that, during the first 8 d after the nauplii hatch, the larvae undergo a diel vertical migration in the water column, without spending any time on the bottom. After 8 d, larvae in waters shallower than a preset transition depth were assumed to switch to vertical migration cued by the tidal cycles-remaining on the bottom during outgoing tides, and swimming into the water column during flood tides. This tidal behaviour generated a net advection of postlarvae into the coastal zone and local estuaries. The model demonstrated that this mechanism is very efficient at accumulating larvae along the coastal zone. Changes in the timing and magnitude of the tides through the year generated a strong seasonal signal in the size and shape of the advection envelope, with typically a 2-fold difference in the size of the envelope between October and March. However, winds had little effect on the size of the advection envelopes, and interannual variation in the size and shape of the advection envelopes was small (<10%). The model also demonstrated that advection envelopes are very sensitive to the postlarval transition depth, which has not yet been adequately constrained by either field or laboratory studies. For example, changing the transition depth from 7 to 30 m typically resulted in a 2-fold increase in the size of the advection envelope. The results of the model may also have significant implications for the management of the prawn fishery. Comparisons of the advection envelopes with the distribution of tiger prawn catches indicate regions where fishing is most likely to have an impact on the spawning stock and subsequent recruitment to the fishery. The results also suggest that there are 3 discrete su

The size-frequency distribution of the commercial catch is often used as the basis of fisheries stock assessments (Paul and Morgan, 1987; Gulland and Rosenberg, 1992) because most dynamic processes of populations (growth, survival, recruitment) are reflected in changes in this distribution. The data are generally collected, often at great expense, by sampling the catch at landing sites and markets, or onboard fishing vessels. Size-frequency distributions of prawns (Penaeus esculentus and P. semisulcatus) can also be obtained from fish processors, who grade landings by size. These data are easier and cheaper to obtain than research samples, but unfortunately they are also considered less accurate and lack spatial information. However, they have been used in stock assessment of prawns in Kuwait (Jones and van Zalinge, 1981) and Malaysia (Simpson and Kong, 1978). It is often difficult to relate size data obtained from a processor to time and place of capture of the prawns, but this is not the case when the product is packed onboard, as in Australia's northern prawn fishery (NPF). Trawler operators in the NPF have voluntarily recorded size composition since 1985, when provision for this was made in operators' daily logbooks (between 30% and 45% of the tiger prawn catch reported in the logbooks contain size information). These books are therefore the most comprehensive source of information on the spatial and temporal size distribution of the commercial catch of the NPF. Present assessments of the fishery are based on deterministic growth and deterministic seasonal recruitment patterns (Wang and Die, 1996) and do not use size-structured data. If available, these data would help relax the recruitment and improve current stock assessments of the NPF. Before the size data recorded in the logbooks can be used, however, the accuracy of size grading at sea needs to the assessed. This paper examines the accuracy of grading tiger prawns, by using data collected from a private fi

The prawn trawl fishery in Exmouth Gulf, Western Australia, is well managed and harvests a mixture of penaeid prawns. Catches of the high value, brown tiger prawn Penaeus esculentus have comprised about 36% of the annual catch in the 1990s. However, annual catches of tiger prawns are now about half the level they were in the 1970s and fluctuate markedly, from about 200 to 680 t. These changes in catch create uncertainty in the supply of prawns for export markets and force fishing and processing operators to have excess capacity to deal with good years. Managers, fishing industry and researchers are considering the option of enhancing the natural recruitment of brown tiger prawns by releasing juveniles in wild nursery areas to reduce natural fluctuations and increase the average annual catch. This collaborative project (CSIRO, Fisheries WA, MG Kailis Group of Companies) assessed the feasibility of stock enhancement of tiger prawns in Exmouth Gulf by: • developing a bioeconomic model • examining the risks of changes in the genetic composition and introducing disease to the wild population of tiger prawns, and • identifying further work that would be needed before stock enhancement should proceed. This is the first stage in several stages that would lead to stock enhancement of tiger prawns in Exmouth Gulf. The project was initiated through a workshop of all project participants in Perth in July 1998.

This study examined the lunar and diel variation in catch rates and reproductive condition of adult eastern king prawns, Penaeus plebejus, in relatively deep (160 m) coastal waters off southeastern Queensland. Females numerically dominated catches over most of the lunar cycle and constituted 76% of the weight of the catch. Analysis of variance (ANOVA) revealed an interaction between lunar phase and sex; catches peaked during Lunar Phase 3 (full moon plus or minus 3 days) and were particularly marked for males. This was the only period during the lunar cycle when the sex ratio approached 1:1. There was also an interaction between trawl-time and sex; male catch rates were at a minimum early in the evening, whereas female catch rates were at a maximum then and declined throughout the night. Trawler logbook catch rate data from the same area over a similar period indicated an interaction between lunar cycle and lunar phase. ANOVA revealed an effect of the interaction between phase and sex on the incidence of soft prawns; the incidence of soft males increased during Phase 4 (half moon waning to new moon plus or minus 3 days). Ovary weight also varied between phases and was higher during Phases 2 (half moon waxing to full moon plus or minus 3 days) and 4 (half moon waning to new moon plus or minus 3 days). Trends in the ovary weight and the incidence of histologically mature and ripe females suggested there are two periods of increased spawning activity during each lunar cycle. A cyclic regression fitted to the data explained 93% of the variation in the incidence of ripe females between samples. The influence of these cyclic trends in catch rate and reproductive condition should be considered when monitoring the spawning stock in the fishery and when planning sampling strategies in any future reproductive studies.

A quantitative method of staging reproductive development based on the use of cell measurements and computerized image analysis of histological sections is described. The method has been applied to samples of eastern king prawn, Penaeus plebejus, collected in Queensland, Australia. Luminance measurements of selected tissue structures within the ovary were used to determine three luminance ranges which characterized ovarian development in histological sections. Cell diameter, average luminance and percent area of an ovary section within a luminance range of 200–245 were the most useful variables to discriminate between the different ovarian developmental stages. Discriminant analysis successfully separated maturing, mature and resorbing groups in 94% of the samples, but discrimination of stages within the above groups was as low as 30%. The estimated discriminant functions suggest there is a continuum of development which can be characterized by a combination of measurements obtained from histological sections.

The size at recruitment, temporal and spatial distribution, and abiotic factors influencing abundance of three commercially important species of penaeid prawns in the sublittoral trawl grounds of Moreton Bay (Queensland, Australia) were compared. Metapenaeus bennettae and Penaeus plebejus recruit to the trawl grounds at sizes which are relatively small (14-15 mm carapace length, CL) and below that at which prawns are selected for, and retained, in the fleet's cod-ends. In contrast, Penaeus esculenlus recruit at the relatively large size of 27 mm CL from February to May, well above the size ranges selected for. Recruitment of M. bennettae extends over several months, September-October and February March, and was thus likely to be bi-annual, while the recruitment period of P. plebejus was distinct, peaking in October-November each year. Size classes of M . bennettae were the most spatially stratified of the three species. Catch rates of recruits were negatively correlated with depth for all three species, and were also negatively correlated with salinity for M. bennettae.

ICP-MS is being used to determine if prawns, like some species of fish, possess an "environmental imprint" attained in their juvenile nursery habitats. Such an imprint could act as a naturally occurring tag and provide useful information on nursery grounds, migration patterns, and stock structure. Samples of juvenile eastern king prawns, Penaeus plebejus, from four different estuarine nursery areas several hundreds of kilometers apart were distinguished from one another using combinations of the concentration of elements in their body tissues. Four different body tissues were used: eyes, hepatopancreas, abdominal muscle, and exoskeleton. Canonical-variate (discriminant) analyses showed that each of the four body tissues could be used to correctly classify the samples with high (100%) predictability. The results although helpful, should be treated with caution as they only provide a static and narrow representation of the prawns' elemental profiles in space and time. Further experiments are in progress to determine if the prawns retain these differences through time in the wild and in the laboratory.