Fish communities in the perennial wetland of the Sudd, southern Sudan

SUMMARY. 1. Cyperus papyrus and Typha domingensis swamps were recognized as the major ecological zones in the perennial wetland of the Sudd. 2. Fish sampling, chiefly by gill‐netting and electric fishing, was concentrated in the open‐waters of channels and lakes together with submerged, fringing and marginal vegetation in the accessible papyrus zone. Sixty‐two species were recorded including fifteen catfishes. thirteen characoids, seven cyprinids, seven mormyrids and seven cichlids. 3. The highest diversity and abundance of tishes was obtained in the shallow Sudd lakes which revealed considerable uniformity in population structures. The most numerous species in gill‐nets were Alestes dentex, Synodontis frontosus and S. schall , whereas Heterotis niloticus, Mormyrus cashive and Distichodus rostratus assumed importance in terms of biomass. Micralestes avutidens and cichlids were abundant among the small fishes inhabiting fringes of Eichhornia crussipes , and air‐breathing species occurred beneath the vegetation mat. 4. Flowing‐water habitats exhibited a similar but restricted fauna. Catches were dominated by Synodontis frontosus in open‐water and by Micralestes acutidens and Chelaethiops bibie in the margins. The latter species was shown to have a clear preference for running water. 5. Peak catches in a representative river‐lake could be interpreted as the arrival of fish for breeding or their migration through the lake for reproduction elsewhere. The capture of fry or juveniles of many species in fringing or marginal vegetation indicates that the permanent floodplain of the Sudd provides suitable spawning and rearing grounds. 6. Access to the extensive Typha zone was limited but from available evidence it was inferred that air‐breathing fishes, notably Heterotis and Gymnarchus , may penetrate the swamps to reach areas remote from perennial transmission channels.