THE CATFISH FAUNA OF LAKE-VICTORIA AFTER THE NILE PERCH UPSURGE

Stocks of the indigenous catfish species of Lake Victoria have decreas ed dramatically since the beginning of the 1980s. This decline coincid ed with the Nile perch boom and concomitant ecological changes in the lake. In deep water, where Nile perch densities were higher, the decli ne proceeded more quickly than in shallow water. In the former all cat fishes eventually vanished. Of the two largest species, Clarias gariep inus and Bagrus docmak, juveniles disappeared faster than adults. This indicates that predation by Nile perch may have played an important r ole in their decline. Other possible impacts were the deoxygenation of deepwater areas and the decline of haplochromine cichlids which were an important food source for B. docmak, C. gariepinus and Schilbe inte rmedius. The various catfish species were not all affected to the same extent. The endemic Xenoclarias eupogon, which lived predominantly in deep water, may have become extinct. B. docmak currently seems to be mainly restricted to refugia in rocky habitats. Synodontis victoriae a nd S. afrofischeri are still present in small numbers in shallow litto ral areas. Schilbe intermedius and C. gariepinus seem to be the least affected of the catfishes in littoral and sublittoral areas. This may be caused, among other reasons, by their smaller habitat overlap with Nile perch than the other species. S. intermedius is partly pelagic, a nd a considerable part of the C. gariepinus stock lives in bodies of w ater surrounding the lake. The patterns of decline of the catfishes ar e very similar to those; observed for haplochromine cichlids in the la ke. The importance of catfishes for the fisheries in the lake is curre ntly negligible.