Ecology of juvenile and adolescent banana prawns, Penaeus merguiensis, in a mangrove estuary and adjacent off-shore area of the Gulf of Carpentaria. I. Immigration and settlement of postlarvae

The hydrology of the Norman River and adjacent off-shore region in the Gulf of Carpentaria is dominated by the alternation of a summer north-west monsoon and a winter south-east trade wind. The seasonality in both air temperature and rainfall is reflected in marked annual cycles of water temperature, salinity, and dissolved oxygen and nutrient (nitrate and silicate) levels. Near the mouth of the river; water temperature ranged from 15.0�C in June to 32.7�C in February, while salinity ranged from c. 0‰, in December and February to 38.2‰, in September. Postlarval banana prawns, P. merguiensis, were found in the river from November to May. but immigration was essentially restricted to the period immediately preceding the wet season (mid November). Maximum numbers ofjuvenile prawns occurred approximately 2 weeks later. A marked tidal periodicity in the movement of postlarvae was observed; postlarvae were present in the water column at the mouth of the river only during the flood tide with maximum numbers moving into the river at the time of maximum flood tide velocity. Incoming postlarvae settled out in the shallow water along the banks of the river (within 10 m of the shoreline), inland as far as 85 km. A lager catch of smaller prawns 48-72 km up-stream (salinity 10-20‰,) suggested that this area was the main settlement area for postlarvae. The population, however, contracted towards the river mouth during the wet season. concentrating the prawns in an area extending only a few kilometres up-river. Most prawns were caught on the muddy substrate adjacent to the mangrove (Avicennia sp.) fringe which lines the numerous bends in the river. Mechanisms for transporting larvae into the estuary and the relationship between the seasonality of physical and chemical parameters and immigration of postlarvae are discussed.