Marine Biology (1994) 119:133-143 9 Springer-Verlag 1994 N.R. Loneragan - R.A. Kenyon 9 M.D.E. Haywood D.J. Staples Population dynamics of juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) in seagrass habitats of the western Gulf of Carpentaria, Australia Received: 15 November 1993 / Accepted 22 November 1993 Abstract The population dynamics of small tiger prawns (Penaeus esculentus and P. semisulcatus) were studied at three sites around north-western Groote Eylandt, Gulf of Carpentaria, Australia, between August 1983 and August 1984. Seagrasses typical of open-coastline, reef-flat and river-mouth communities were found in the shallow depths (_<2.5 m) at these sites. The temperature and salinity of the bottom waters did not differ among the shallowest depths of the three sites and mean values at night ranged from 21.9 to 32.0 ~ and from 30.1 to 37.5%0 S. Data from fortnight- ly sampling with beam trawls showed that virtually all post- larvae (-90%) were caught in the intertidal and shallow subtidal waters (_>2.0 m deep). At one site, where the rela- tionship between seagrass biomass, catches and depth could be studied in detail, high catches were confined to seagrass in shallow water, within 200 m of the high-water mark. This was despite the fact that seagrass beds of high biomass (>100 g m -2 between August and February) were found nearby, in only slightly deeper water (2.5 m). It is likely, therefore, that only the seagrass beds in shallow wa- ters of the Gulf of Carpentaria act as important settlement and nursery areas for tiger prawns. In general, catches of tiger prawn postlarvae (both P. esculentus and P. semisul- catus) and juvenile P. esculentus on the seagrass in the shal- lowest waters at each site were higher in the tropical pre- wet (October-December) and wet (January-March) sea- sons than at other times of the year. Juvenile P. semisulca- tus catches were highest in the pre-wet season. While sea- sonal differences accounted for the highest proportion of variation in catches of tiger prawn postlarvae and juvenile P. semisulcatus, site was the most important factor for juve- nile P. esculentus. In each season, catches of juvenile P. es- Communicated by G. E Humphrey, Sydney N. R. Loneragan ([]). R. A. Kenyon. M. D. E. Haywood 9 D. J. Staples CSIRO Division of Fisheries, Marine Laboratories, EO. Box 120, Cleveland, Queensland 4163, Australia 1 Present address: Bureau of Resource Sciences, Department of Pri- mary Industry and Energy, EO. Box E111, Queen Victoria Terrace, Canberra, ACT 2600, Australia culentus were highest in the shallow, open-coastline sea- grass, where the biomass of seagrass was highest. The fact that the type of seagrass community appears to be more important to juvenile P. esculentus than to postlarvae, sug- gests that characteristics of the seagrass community may affect the survival or emigration of postlarval tiger prawns. Few prawns (<10%) from the seagrass communities in shallow waters exceeded 10.5 mm in carapace length. De- spite the intensive sampling, growth was difficult to esti- mate because postlarvae recruited to the seagrass beds over a long period, and the residence times of juveniles in the sampling area were relatively short (-8 wk). Introduction Penaeid prawns (shrimp) conU'ibute to important commer- ciN fisheries in many parts of the world, particularly in tropical and sub-tropical regions (Garcia and LeReste 1981, Dall et al. 1990). In general, prawns in the genus Pe- naeus breed in offshore waters. The planktonic larvae are transported (or advected) inshore within 2 to 3 wk and set- tle in coastal and estuarine nursery grounds as postlarvae, the smallest juvenile stage. The juvenile nursery habitat can vary according to species: for example, juvenile tiger prawns (P. esculentus and P. semisulcatus) are found in beds of seagrass and algae (Staples et al. 1985, Coles and Lee Long 1985), brown shrimp (P. aztecus) in saltmarsh- es (Zimmerman et al. 1984), and banana prawns (P. mer- guiensis) in mangrove-lined creeks and rivers (Staples et al. 1985, Vance et al. 1990). In tropical Australia, two species of tiger prawn, one endemic to Australia (Penaeus esculentus) and the other found throughout the Indo-west Pacific region (P. semisul- catus) (Grey et al. 1983), form a major component of the prawn fishery in the Gulf of Carpentaria (the Northern Prawn Fishery, Somers 1994). These species are fished commercially in waters offshore from those where seagrass beds are found (Coles and Lee Long 1985; Staples et al. 1985; Poiner et al. 1987; Somers 1987).