POPULATION-DYNAMICS OF JUVENILE TIGER PRAWNS (PENAEUS-ESCULENTUS AND P-SEMISULCATUS) IN SEAGRASS HABITATS OF THE WESTERN GULF OF CARPENTARIA, AUSTRALIA

The population dynamics of small tiger prawns (Penaeus esculentus and P. semisulcatus) were studied at three sites around north-western Groo te Eylandt, Gulf of Carpentaria, Australia, between August 1983 and Au gust 1984. Seagrasses typical of open-coastline, reef-flat and river-m outh communities were found in the shallow depths (less than or equal to 2.5 m) at these sites. The temperature and salinity of the bottom w aters did not differ among the shallowest depths of the three sites an d mean values at night ranged from 21.9 to 32.0 degrees C, and from 30 .1 to 37.5 parts per thousand S. Data from fortnightly sampling with b eam trawls showed that virtually all postlarvae (similar to 90%) were caught in the intertidal and shallow subtidal waters (greater than or equal to 2.0 m deep). At one site, where the relationship between seag rass biomass, catches and depth could be studied in detail, high catch es were confined to seagrass in shallow water, within 200 m of the hig h-water mark. This was despite the fact that seagrass beds of high bio mass (>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 waters 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. semisulca tus) and juvenile P. esculentus on the seagrass in the shallowest wate rs at each site were higher in the tropical pre wet (October-December) and wet (January-March) seasons than at other times of the year. Juve nile P. semisulcatus catches were highest in the pre-wet season. While seasonal differences accounted for the highest proportion of variatio n in catches of tiger prawn postlarvae and juvenile P. semisulcatus, s ite was the most important factor for juvenile P. esculentus. In each season, catches of juvenile P. esculentus were highest in the shallow, open-coastline seagrass, where the biomass of seagrass was highest. T he fact that the type of seagrass community appears to be more importa nt to juvenile P. esculentus than to postlarvae, suggests that charact eristics of the seagrass community may affect the survival or emigrati on of postlarval tiger prawns. Few prawns (<10%) from the seagrass com munities in shallow waters exceeded 10.5 mm in carapace length. Despit e the intensive sampling, growth was difficult to estimate because pos tlarvae recruited to the seagrass beds over a long period, and the res idence times of juveniles in the sampling area were relatively short ( similar to 8 wk).