Comparative ecology of the African pike, Hepsetus odoe, and tigerfish, Hydrocynus forskahlii, in the Zambezi River floodplain.

Journal of Fish Biology 45 (2): 211–225, August 1994. DOI: 10.1111/j.1095-8649.1994.tb01301.x

Abstract:

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 floodplain, and Hydrocynus was common in the northern region in swift flowing tributaries where Hepsetus was rare. Hepsetus inhabited vegetated environments of river backwaters, lagoons, and sluggish tributaries, whereas Hydrocynus occupied the open water of the main river channel almost exclusively. During the period of annual flooding, juveniles of both species coexist in flooded savanna regions. Size distributions of adult Hydrocynus and Hepsetus changed relatively little between high and low water conditions. Stomach contents analysis indicated that adult size classes of both species are almost entirely piscivorous, and both show diet shifts with changes in size. Only very small seasonal diet shifts were noted. Approximately 50% of the diet of Hepsetus consisted of haplochromine cichlid fishes, but also included large numbers of tilapine cichlids and mormyrids, Hydrocynus consumed primarily cichlid fishes, but also consumed large percentages of Hepsetus and small characid fishes. Small size classes of Hepsetus fed heavily on small machokid catfishes (Synadontis spp.) and later shifted to a diet of cichlids and mormyrids, and small Hydrocynus preyed heavily on Barbus spp. (Cyprinidae), small characids, and mormyrids. Ratios of prey length-predator length averaged approximately 0·26 for Hydrocynus and nearly 0·40 for Hepsetus. The large potential for food resource competition appears not to be realized due to a very high degree of habitat partitioning between larger size classes of the two species. At the interface between river backwaters and channel habitats, Hydrocynus is a significant predator of Hepsetus, a factor that should further restrict the occurrence of the latter in open areas of the main channel.