Visual surveys reveal high densities of large piscivores in shallow estuarine nurseries

Shallow estuarine nurseries are widely believed to provide juvenile fishes with refuge from predation due to the low numbers of piscivorous fishes. Observations during several years of fieldwork in northeastern Australia indicate that the assemblage of large (>= 100 mm) piscivorous fishes within shallow tropical estuarine nurseries may have been considerably underestimated by previous sampling efforts. This study utilised visual surveys of shallow sandy shorelines in the lower reaches of estuaries to estimate the abundance of large piscivores. Flathead (Platycephalus spp., Platycephalidae) were the only large piscivores sighted within transects. A total of 296 flathead between 100 and 600 mm TL were observed in waters between 0.02 and 0.62 m deep. The density of flathead observed during the present study (0.04 ind. m(-2)) equated to 1 piscivore >= 100 mm TL for every 10.5 m of shoreline surveyed, and far exceeds density estimates for large piscivores in shallow estuarine habitats elsewhere in the world. Furthermore, the estimated biomass of flathead (11.56 g m(-2)) was equivalent to comparable biomass estimates of entire fish assemblages from shallow estuarine habitats in other parts of the world. The densities and depth distribution of these large piscivores suggests that shallow water nurseries may not provide small fishes with the level of refuge from predation previously assumed.