COMPARATIVE ECOLOGY OF THE AFRICAN PIKE, HEPSETUS-ODOE, AND TIGERFISH, HYDROCYNUS-FORSKAHLII, IN THE ZAMBEZI RIVER FLOODPLAIN

The ecology of sympatric African pike, Hepsetus odoe (Hepsetidae), and tiger fish, Hydrocynus forskahlii (Characidae), were compared during high (May-August) and falling water (September-December) conditions in the Upper Zambezi R. drainage of Zambia. Both species were common in the central and southern regions of the river and associated floodplai n, and Hydrocynus was common in the northern region in swift flowing t ributaries where Hepsetus was rare. Hepselus inhabited vegetated envir onments of river backwaters, lagoons, and sluggish tributaries, wherea s Hydrocynus occupied the open water of the main river channel almost exclusively. During the period of annual flooding, juveniles of both s pecies coexist in flooded savanna regions. Size distributions of adult Hydrocynus and Hepsetus changed relatively little between high and lo w water conditions. Stomach contents analysis indicated that adult siz e classes of both species arc almost entirely piscivorous, and both sh ow diet shifts with changes in size. Only very small seasonal diet shi fts were noted. Approximately 50% of the diet of Hepsetus consisted of haplochromine cichlid fishes, but also included large numbers of tila pine cichlids and mormyrids. Hydrocynus consumed primarily cichlid fis hes, but also consumed large percentages of Hepsetus and small characi d fishes. Small size classes of Hepsetus fed heavily on small mochokid catfishes (Synodontis spp.) and later shifted to a diet of cichlids a nd mormyrids, and small Hydrocynus preyed heavily on Barhus spp. (Cypr inidae), small characids, and mormyiids. Ratios of prey length-predato r length averaged approximately 0.26 for Hydrocynus and nearly 0.40 fo r Hepsetus. The large potential for food resource competition appears not to be realized due to a very high degree of habitat partitioning b etween larger size classes of the two species. At the interface betwee n river backwaters and channel habitats, Hydrocynus is a significant p redator of Hepsetus, a factor that should further restrict the occurre nce of the latter in open areas of the main channel.