African Journal of Marine Science

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Influence of prey availability on the distribution of Dusky Kob argyrosomus japonicus (sciaenidae) in the Great Fish River Estuary, with notes on the diet of early juveniles from three other estuarine systems

MH Griffiths


The stomach contents of 391 juvenile dusky kob Argyrosomus japonicus (23–805 mm total length, TL) from the Great Fish River estuary were analysed. Early juveniles (<50 mm TL) fed predominantly on mysids
and calanoid copepods. As the juveniles increased in size, copepods were replaced by teleost prey, but mysids remained an important dietary component of all dusky kob <600 mm TL. Teleosts were the principal
prey of dusky kob >100 mm TL, but the dominant species varied with predator size: early juveniles of 100–149 mm TL preyed mostly on the late larvae and early juveniles of other fish species, particularly the
Cape stumpnose Rhabdosargus holubi; juveniles of 150–400 mm TL fed largely on early juvenile mugilids and on the small, pelagic estuarine round herring Gilchristella aestuaria; juvenile dusky kob of >400 mm TL
fed on several teleost species, including early juvenile conspecifics, but their most important prey item was G. aestuaria. Based on the distribution patterns of A. japonicus and their predators and prey in the Great Fish
River estuary, it is concluded that predator avoidance and prey availability determine the spatial distribution of early juveniles in estuaries, and that the distribution patterns of larger juveniles (>400 mm TL) depend on
those of their principal prey. Important prey items of early juvenile from three other estuarine systems included calanoid copepods, mysids, insects, amphipods and swimming prawns. The impact of reduced
freshwater inflow on the abundance of the principal prey items of A. japonicus, and on the nursery potential of estuaries, is discussed.

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