Definition of effective spawning stocks of commercial tiger prawns in the NPF and king prawns in the eastern king prawn fishery: behaviour of post-larval prawns
CSIRO Oceans and Atmosphere Hobart
In the Northern Prawn Fishery and the East Coast King Prawn Fishery managers have concerns about declining levels of recruitment to the fishery, and the lack of knowledge of the relationship between spawning stock and recruitment to the fishery. In both fisheries, the increase in effective fishing effort due to the use of GPS plotters and other modern technologies means that measures will inevitably have to be taken to reduce effort in the future. A more accurate definition of the real spawning stock would allow management to more effectively protect spawning stock and maximize catches by ensuring that critical areas were not overfished while allowing fishing in non-critical areas. Clear identification of the critical spawning areas would also allow managers to determine if changes in annual recruitment were due to changes in levels of spawning stocks. The CSIRO hydrodynamic model of Albatross Bay (and models being developed for the entire Gulf of Carpentaria) have the potential to allow managers to more accurately define the effective spawning stocks for tiger prawns, but are limited by our lack of knowledge of a critical piece of postlarval behaviour: the timing of the change in vertical migration behaviour from being day-night cued to tidally cued. The research proposed for eastern king prawns is particularly important, not only because it provides information relevant to improving the management of the East Coast King Prawn Fishery, but also because it will allow us to validate the techniques used in obtaining the behavioural data on tiger prawns for the Northern Prawn Fishery (see Methods).
1. Measure the critical vertical migration behaviour of postlarval tiger and king prawns that determines their inshore advection patterns.
2. Incorporate this behaviour into hydrodynamic models to accurately estimate the effective spawning stocks of tiger and king prawns.
To effectively manage most fisheries, including penaeid prawn fisheries in northern and eastern Australia, it is important to know the relationship between the size of the spawning population and the number of young adults that recruit to a fishery in the next generation. In the tiger prawn fishery in the Gulf of Carpentaria, it has been assumed for management purposes that the total adult population at a particular time is the effective spawning stock, i.e. all spawners contribute to the next generation's stock. This is not necessarily correct and is particularly unlikely for many species of penaeid prawns, whose larvae and postlarvae have to migrate from offshore spawning areas to coastal and estuarine nursery areas. The area that contains spawners that actually contribute offspring to subsequent adult populations has been termed the effective spawning area.
The main aim of our project was to investigate the vertical migration behaviour of postlarval penaeid prawns that enables them to recruit from offshore spawning areas to the coastal nursery areas. We used two strategies to achieve this aim: field sampling in estuaries in southern and northern Queensland and laboratory experiments.
Keywords: Effective spawning, prawns, postlarvae, behavior, vertical migration.