Do the dietary compositions of Acanthopagrus butcheri in four estuaries and a coastal lake vary with body size and season and within and amongst these water bodies?

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The dietary compositions were determined for Acanthopagrus butcheri in four estuaries and a saline, coastal lake, which vary in the extent, if any, of their connection to the sea and amongst which, in spring and summer, their salinities ranged from 2 to 7‰ in an intermittently open estuary to >40‰ in a normally closed estuary. The dietary compositions of A. butcheri in each of the five water bodies were significantly different, which reflected differences in the abundance of different components of the biota in those systems. Biotic differences amongst water bodies thus accounted for the far larger contributions made to the volume of the stomach contents by the macroalgae Cladophora sp. in the Moore River Estuary on the lower west coast of Australia, by amphipods and decapods in the Swan River Estuary 85 km further south and by polychaetes in the landlocked Lake Clifton a further 85 km further south. The diet of A. butcheri in the Nornalup/Walpole Estuary on the south coast of Western Australia contained atypically large volumes of the seagrass Ruppia megacarpa and teleosts, whereas that in the normally closed Wellstead Estuary, 260 km to the east was characterised by large volumes of the macroalgae Chaetomorpha sp. and a tube-dwelling amphipod. However, there is evidence that A. butcheri selects certain prey, when two or more of the typical prey of A. butcheri are present in the environment, and that it prefers to feed on or above the substratum, rather than within the substratum. The dietary composition of A. butcheri underwent pronounced ontogenetic changes in each water body, these being progressive in estuaries on the lower west coast and abrupt in those on the south coast. Within the upper Swan Estuary, the dietary composition changed in an upstream direction, reflecting changes in the relative abundance of certain benthic macroinvertebrate prey, but did not undergo conspicuous seasonal changes, which is consistent with the lack of any clear cut seasonal changes in the abundance of their major prey.