Analysis of the gut contents of the needlefish, Hyporhamphus knysnaensis (Smith), from Rondevlei, southern cape

Abstract

The gut contents of 313 specimens of Hyporhamphus knysnaensis from Rondevlei were analysed. The total lengths of the collected fish varied between 5, 1 and 21, 8 cm. It was found that the annual cycle in the occurrence of submerged macrophytes in Rondevlei appears to directly influence the feeding of H. knysnaensis of 9 to 17 cm long. This lengthgroup fed mainly on animal material (mostly amphipods and isopods) in winter and the first half of spring while submerged macrophytes were scarce. After the submerged macrophytes became more abundant, however, the 9 to 17-cm length-group changed to a predominantly plant diet (consisting of Ruppia spiralis and Potamogeton pectinatus). H. knysnaensis between approximately 5 and 9 cm long appeared in summer and fed mainly on animal material (mostly amphipods and isopods). Of the 47 specimens collected in this length-group between December 1978 and March 1979, all had animal material in their gut contents. It was the dominant food in 94% of the specimens and formed 90% of the gut contents. In general, therefore, H. knysnaensis can be considered an omnivore.