The following report is a description of the two FIRTA projects (85/85 & 89/13) which examined recruitment processes in penaeids at Albatross Bay in the northeastern Gulf of Carpentaria over the years 1985 to 1992. The aim of the first of the two projects was to identify the main factors which control the recruitment of prawns, while the aim of the second project was to build on the results on the first to provide managers with explanations for declines in commercial catches that had been experienced in the fishery.

The work was focused in three main areas: measuring year-to-year variation in numbers of the main life history stages (eggs, larvae, juveniles and adults in the Albatross Bay region) and correlating the abundances with changes in the environment; measuring the year-to-year variation in the extent of predation by fish on juvenile and adult prawns; and examining relationships between commercial fishery catches throughout the Northern Prawn Fishery and meteorological data.

The results of the projects have enabled us to better define the life history dynamics of the banana prawn Penaeus merguiensis and, to a lesser extent, the grooved tiger prawn Penaeus semisulcatus. Life cycles of both species were found to be based on two cohorts per year. Comparison with P. merguiensis life cycles throughout the Indo-West Pacific region has shown that two cohorts per year was the common pattern, and that the relative contribution to offshore commercial fisheries of each cohort in the various regions was governed to a large extent by the local pattern of rainfall. In the case of P. merguiensis in Albatross Bay, differential mortality results in only one of these cohorts contributing significantly to the commercial fishery.

Like P. merguiensis, two cohorts per year was also found to be the more typical recruitment pattern for P. semisulcatus in Albatross Bay. However, unlike P. merguiensis, both cohorts may contribute to the annual commercial fishery, though the respective contributions may fluctuate from year to year.

For banana prawns, the population models developed in the south-eastern Gulf have now been . fine-tuned for the north-eastern Gulf environment and we have been able to explain, and even predict, the interannual variation in banana prawn commercial catches to a reasonable degree. However, the work on P. semisulcatus has not yet progressed to the stage where we can unequivocally separate the environmental and fishery-induced effects on recruitment Nevertheless, the advances made during these two projects have enabled us to contribute significantly to management of the fishery at a time of major structural change within the fishing industry. In particular, our estimates of average long-term sustainable yields have been very important in the fleet-rationalisation process, while our understanding of life-cycle timing and migration patterns was critical in setting seasonal closures to optimise size composition of the catch.