DIETARY STUDIES ON THE PREDATORY FISHES OF THE NORMAN RIVER ESTUARY, WITH PARTICULAR REFERENCE TO PENAEID PRAWNS

The predatory fish community and their prey in the Norman Rivere estua ry, Gulf of Carpentaria Australia, are compared with the communities o f other tropical inshore areas, to investigate patterns of predation i n tropical inshore areas particularly in relation to penaeid prawns. A biotic factors (turbidity, freshwater input) and diversity of habitat types affect the composition of both prey and predator communities, re sulting in large differences in the communities of tropical inshore wa ters. The stomach contents of 2059 predatory fish from the Norman Rive r estuary were analysed over four sampling trips in the wet and dry se asons. The stomachs of 61% (1255 fish) of 54 species contained a total of 676.2 g (dry weight) of food, while 39% (804 fish) were empty. Tel eosts were the main component of the diet (by dry weight) of 13 of the 22 species analysed, followed by annelids for two species. Five speci es had only teleosts in their stomachs. Most predator species ate bent hic or bentho-pelagic prey, while three species-Rhizoprionodon taylori , Scomberoides commersomianus and Leptobrama mulleri-ate mainly pelagi c prey. Although 19 species ate some penaeids, only Polydactylus sheri dani ate little else. This species, Lates calcarifer and Eleutheronema tetradactylum ate 94.5% of all the penaeid prey and 97.9% of all the commercially important penaeid prey recorded in the study. Penaeid pre dation indices (calculated from gillnet catch rates, proportion of pen aeids in the diet and a consumption rate of 3% body weight per day) we re 0.23 g of penaeid per net-metre per day for P. sheridani, 0.15 for L. calcarifer and 0.03 for E. tetradactylum. Commercially important pe naeid predation indices were 0.11, 0.13 and 0.01, respectively. These values are intermediate between those previously recorded for the main penaeid predators in other inshore areas of the Gulf of Carpentaria ( Embley River estuary and Groote Eylandt). (C) 1998 Academic Press Limi ted.