Food resource sharing and partitioning among some fishes of the Pongolo River floodplain

Abstract

The diets of 12 fish species which coexist in the Pongolo floodplain lakes were determined from an analysis of the contents of 3 691 stomachs collected over period of 12 months. Although almost a species were found to feed on variety of foods, with consequent dietary overlaps, quantitative analysis of the results showed that distinct preferences for specific food types considerably ameliorated the effects of such overlaps. Partitioning within the major food categories, differences in relative population sizes, spatial and temporal differences in feeding and shifts in diet following inputs of seasonally occurring foods, all tended further to reduce effective competition between species. The results are discussed in the broader context of colonization of the Pongolo system by tropical fish community in circumstances where the principles of island biogeography may be expected to apply. Die diete van 12 visspesies wat in die vloedpleinmere van die Pongolorivier saamleef is deur ontleding van 3 691 maaginhoude, wat oor 'n tydperk van 12 maande ingesamel is, vasgestel. Hoewel byna al die spesies 'n verskeidenheid voedselsoorte vreet, met gevolglike oorvleuelings van hulle diete, dui kwantitatiewe ontleding op besliste voorkeure vir spesifieke voedselsoorte, wat die gevolge van die oorvleuelings tot 'n groat mate versag. Verdeling binne die vernaamste voedselsoorte, verskille in relatiewe populasiegroottes, verskille in tye en plek van voeding, en verskuiwing in dieet na seisoenale insette, dra almal by tot 'n verdere vermindering in effektiewe kompetisie tussen spesies. Die resultate word bespreek in die breer verband van die kolonisasie van die Pongolo-sisteem deur 'n tropiese visgemeenskap onder omstandighede waar dit verwag kan word dat die beginsels van eilandbiogeografie stellig van toepassing is. • To whom all correspondence should be addressed •• Present address: do Hewlett-Packard S.A. (Pty) Ltd., Linton Arcade, Cape Road, Port Elizabeth, 6000 The Pongolo River floodplain (Figure 1), located between latitude 27°15'S and the confluence of the Pongolo and Usutu Rivers, comprises some 13 000 ha of low-lying land adjacent to the Pongolo River. It includes numerous shallow depressions, the floodplain lakes, which are filled with water during floods. These lakes occupy an area estimated at 2 600 ha at their maximum retention level. The larger, deeper lakes retain water from one flood season to the next except under extreme drought conditions, thus providing permanent refuge for diverse aquatic flora and fauna. The likelihood of detrimental downstream effects resulting from the impending impoundment of the Pongolo River by the 10zini Dam, then under construction, prompted the initiation, in 1974, of programme of wide-ranging studies aimed at establishing the extent to which the floodplain ecosystem depended on the natural flooding regime of the Pongolo River, particularly where this affected the floodplain resources important in the existing local socio-economic structure.