Modelling the recruitment of tiger prawns Penaeus esculentus and P. semisulcatus to nursery grounds in the Gulf of Carpentaria, northern Australia: Implications for assessing stock-recruitment relationships

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 sub-stocks of Penaeus esculentus and P. semisulcatus in the Gulf of Carpentaria and, therefore, challenge the assumption that there is a single tiger prawn stock covering the entire region

Developing and assessing techniques for enhancing tropical Australian prawn fisheries and the feasibility of enhancing the brown tiger prawn (Penaeus esculentus) fishery in Exmouth Gulf

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

Lunar periodicity in catch rate and reproductive condition of adult eastern king prawns, Penaeus plebejus, in coastal waters of south-eastern Queensland, Australia

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

Lunar periodicity in catch rate and reproductive condition of adult eastern king prawns, Penaeus plebejus, in coastal waters of south-eastern Queensland, Australia

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

Discriminating Populations of the Eastern King Prawn, Penaeus plebejus, From Different Estuaries Using ICP-MS Trace Element Analysis

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

Discriminating Populations of the Eastern King Prawn, Penaeus plebejus, From Different Estuaries Using ICP-MS Trace Element Analysis

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

Temporal and Spatial Patterns in Recruitment of Three Penaeid Prawns in Moreton Bay, Queensland, Australia

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