The relationship between sediment type and the distribution of the commercial prawn species of the western Gulf of Carpentaria was examined. The distribution of sediments was described on the basis of the mud content while the spatial distributions of the adult populations of all the commercial species were described from the results of trawl surveys. The main species caught were the tiger prawns Penaeus esculentus and P. semisulcatus, the endeavour prawns Metapenaeus endeavouri and M ensis, and king prawns P. latisulcatus and P. longistylus, the banana prawn P. merguiensis, and the coral prawn Solenocera australiana.The individual species were caught in varying depth ranges and, in order to assess the influence of sediment type on the spatial cii~iiibiiiioii~, a stepwise m.;!tip!::was carried out in which variation due to depth was considered before that due to percentage mud. Although depth generally accounted for most of the variation in catch-per-unit-effort (CPUE), percentage mud was also found to be a significant factor for all species except P. merguiensis. Three species, P. semisulcatus, M. ensis and S. australiana, showed a preference for sediments with a high mud content while the abundances of P. esculentus, endeavouri and P. latisulcatus were each negatively correlated with percentage mud. P. longistylus showed a strong depth-mud interaction, being largely found on sediments of 40-60% mud in depths of 40-50 m. P. merguiensis was found in depths <20 m but, because the trawl stations in this depth range were all high in mud content, there was no significant correlation with sediment type. The distribution of the CPUE of all the commercial species combined was relatively even and showed no correlation with sediment type (either percentage mud or percentage organic carbon) and only 13% of the variation could be explained by a preferred depth range. Unlike the adults, juveniles were largely confined to shallow inshore waters (<20 m). Tagging experiments carried out on the major commercial species, P. esculentus and P. semisulcatus, in common inshore nursery grounds demonstrated the preferences for different sediment types; P. semisulcatus recaptures were mainly in areas with the finest sediments (>75% mud) whereas those of P, esculentus were associated with coarser sediments (50-75% mud).

Concurrent trawl surveys and tag-recapture studies carried out in the north-western Gulf of Carpentaria between August 1983 and March 1985 provided a detailed description of the growth, movement and age structure of the population of grooved tiger prawns, Penaeus semisulcatus. Growth curves based on the tag-recapture data were used to interpret the length-frequency data collected from the trawl surveys and to determine the number of year classes present. The length-frequency data pertaining to the 1984 year class provided the basis for an estimate of the longevity of the species and a description of the offshore movement patterns. The effect of infestation by the bopyrid parasite Epipenaeon ingens on growth and movement was also examined. Individuals of P. semisulcatus may live for about 2 years, but in the north-western Gulf of Carpentaria very few survive beyond 18 months. Subadults recruit to the offshore fishing grounds during summer and autumn (November-March) at sizes between 20 and 25 mm carapace length (CL) (between 4 and 6 months). By 18 months of age, males reach a size of about 39 mm CL and females about 50 mm CL. The growth rates of both sexes are affected by the presence of E. ingens: males grow faster and larger, the females slower and smaller, with both sexes attaining a size of around 43 mm CL by 18 months of age. The recruiting year class continues to disperse offshore during autumn (from March to May) in such a way that by winter (June and July), although highest abundance is in depths of 35-40 m, the population extends well beyond the commercial fishery into depths greater than 50 m. Although prawns infested with E. ingens show a similar offshore movement, they do not venture beyond depths greater than about 30 m. The sex ratio within the year class remains at around 1 : 1 from the time of recruitment to about 1 year of age. Thereafter, the percentage of females declines steadily; by about 18 months of age very few prawns remain in the population, and of these only about 25% are female. In contrast, the sex ratio for prawns infested with E. ingens remains at about 1 : 1 throughout.

Trawl surveys in the north-western Gulf of Carpentaria were carried out each lunar month from August 1983 to March 1985 to assess the temporal and spatial distribution and abundance of P. esculentus and P. semisulcatus. The information obtained was then compared with that from fishermen's logbooks. Water temperature and salinity were monitored during the study and their possible influence on the distributions has been inferred. The distributions of juveniles of less than 20 mm carapace length indicated that, for both tiger prawn species, the main nursery areas in the region were in Blue Mud Bay and in the bays along the northern coast of Groote Eylandt. Although the two species shared the same nursery areas, the juveniles were concentrated in different parts of Blue Mud Bay and were most abundant at different times. Catches of juvenile P. esculentus increased substantially in October and peaked in November, whereas catches of juvenile P. semisulcatus increased in November and peaked in January. Catches of both species showed a secondary peak in March 1984, coincident with the heaviest monthly rainfall of the summer monsoon season. The distribution of larger prawns showed spatial separation of the two species in the offshore fishery. The monthly pattern of the catch-per-unit-effort (CPUE) of the two tiger prawn species combined was similar to that obtained from fishermen's logbooks; the logbook data are therefore generally reliable. Monthly CPUE from the trawl surveys for the individual species showed distinct peaks in late summer, which were apparently related to recruitment of small prawns into the fishery. CPUE for P. semisulcatus peaked in February and April (juvenile abundance had peaked in January and March); CPUE for P. esculentus peaked in January and May (juvenile abundance had peaked in November and March). However, a distinct CPUE peak in spring (August/September) for P. semisulcatus could not be related to a previous peak in juvenile abundance; this was presumably a result of an increase in catchability. Although there was evidence linking changes in the catchability of P. semisulcatus to changes in water temperature, a similar link was not as evident for P. esculentus.

Growth data were obtained for the two tiger prawn species P. esculentus and P. semisulcatus from a tagging experiment carried out in February 1981 in waters adjacent to Groote Eylandt in the western Gulf of Carpentaria. A von Bertalanffy growth curve was fitted to these data and least squares estimates of the parameters L∞ and K and joint 95% confidence regions were calculated for males and females of both species. Tests on the residuals from the fitted curve were carried out to check the adequacy of the fit of the von Bertalanffy model, and differences in the parameters L∞ and K between sexes and species were examined. An alternative von Bertalanffy-type model was used to test the consistency between the estimated L∞ values and the carapace leilgths of the largest individuals observed in samples from the populations. For males of each species, the fit of the von Bertalanffy model was satisfactory, and the estimates of L∞ were consistent with the largest observed carapace lengths in catch samples. For females, however, the fit of the von Bertalanffy model was not entirely satisfactory, especially for P. semisulcatus, for which additionally the estimated L∞ was much higher than the largest carapace length in catch samples. Possible reasons for this lack of fit are discussed. Estimates of L∞ all differed significantly, with the exception of P. esculentus males and P. semisulcatus males. No significant difference was found in estimates of K for P. esculentus males and females, and only a marginally significant difference in K between P. esculentus males and females and P. semisulcatus males. Presence of a bopyrid parasite in P. semisulcatus did not affect growth.

The potentially detrimental side-effects of prawn trawling are coming under increasing scrutiny in Australian waters, particularly in such ecologically sensitive areas as Queensland's Great Barrier Reef, and various restrictive measures are being suggested. Before changes are imposed on the prawning industry, the effects of trawling on the target prawn species and the long-term management of these effects need to be fully understood. Using a simulation model of a simplified prawn fishery, this paper describes the basis for the current regulatory mechanisms for Australian's prawn fisheries, in particular the manipulation of both the level and pattern of fishing effort. It is shown that even in moderately fished stocks, the fishery manager has several options, such as seasonal and nursery area closures, that are consistent with the goal of minimizing the impact of prawn trawling, while in no way penalizing the industry economically. With these in mind, possible ways of resolving or reducing the conflict with groups outside the prawning industry are discussed.