Field studies of the diet of Rhabdosargus holubi (Pisces: Teleostei: Sparidae)

Rhabdosargus holubi is a marine teleost endemic to south east Africa. The juveniles occur mainly in estuaries but the adults are largely confined to the sea. Stomach contents of over 2000 juvenile R. holubi were examined from fish captured in the open Kowie and Msikaba estuaries, and from the closed West Kleinemond and Kasouga estuaries. Juvenile R. holubi feed mainly on aquatic vegetation. The fish cannot, however, digest the vegetation eaten due to the absence of a cellulase or a method of breaking up the plant tissue. Therefore, the plant material is passed out in an undigested state. Epiphytic diatoms and sessile ectoprocts (Bryozoa) are, however, removed from the macrophytes or multicellular algae in the stomach, and are subject to digestion. Diatoms may form up to 50% of the dry weight of the plants consumed. A variety of animals was also eaten. Maximum total food consumption was estimated at 1·9% of body weight per day although this is probably an underestimate. Feeding only took place during daylight hours. The teeth of juvenile R. holubi are specialized for grazing aquatic plants and differ from those of adult R. holubi. Adults consume mainly bivalves and large crustaceans. It is suggested that the different food requirements of the adults and juveniles determine at what stage this species enters and leaves estuaries.