PATTERNS OF SELECTIVE PREDATION BY JUVENILE, BENTHIVOROUS FISH ON ESTUARINE MACROFAUNA

Benthic feeding on macrofauna was studied in juveniles of the sparids Lithognathus lithognathus and Rhabdosargus holubi in the upper reaches of the Gamtoos Estuary, South Africa. Fish and benthic macrofauna wer e sampled simultaneously, and the selection of invertebrate prey asses sed. Both fish species strongly selected for corophioid amphipods and consumed other benthic taxa in low numbers. R. holubi also exploited a quatic autotrophs, while L. lithognathus had a narrow prey-spectrum, f eeding almost exclusively on the tube-dwelling amphipod Grandidierella lignorum. G. lignorum was the most abundant prey species, both in the benthos and the fish's diet. This species also showed prominent behav ioural differences between the sexes; males were markedly more active on the sediment surface than females, who rarely left their tubes duri ng the day. Males switched from an infaunal to epifaunal microhabitat in search of receptive females, concurrently increasing their exposure to fish predators. Consequently, L. lithognathus selected significant ly more males than female amphipods, causing a marked bias towards fem ales in the sex ratio and age-structure of the amphipod population. Ju venile amphipods were less preyed upon, presumably as a result of lowe r prey-detection or capture efficiency by the predators. Accepting cur rent notions about predation as an important structuring element for b enthic communities, our data also stress the prominence of size-and se x-selective predation in structuring individual prey populations.