The feeding ecology of Ambassidae (Osteichthyes: Perciformes) in Natal estuaries

Abstract

The diets of three species of Ambassis in six estuaries of southern Africa were investigated using four methods of stomach content analysis. Diets varied from one estuary to another as the faunal composition changed according to physical conditions. Ecological separation among the species is achieved to a large extent through diet alone. While all Ambassis feed mainly on a wide variety of zooplankters, Ambassis productus and A. natalensis are also piscivorous and insectivorous respectively. Seasonal analysis of diet showed that A. gymnocephalus feeds mainly on fish eggs during winter (June-August). The possible impact of egg predation on fish numbers at St Lucia is discussed. Feeding periodicity was investigated using dry mass and volumetric methods. All Ambassisshowed similar bimodal feeding intensities during the night with peaks in early evening and early morning. Food selection at different levels in the water column showed that all Ambassis feed mainly on food in suspension. Additionally A. natalensis feeds on surface prey while A. productus takes ben-thic prey. It is postulated that interaction between Ambassis and filter-feeding planktivores in St Lucia is minimal due to feeding strategies and feeding periodicity. In addition the impact of the piscivorous predatory role played by Ambassis on estuarine fish communities is emphasized.