International Institute for Infrastructural, Hydraulic and Environmental Engineering, Wageningen University and Research Centre, 6708 PB Wageningen, THE NETHERLANDS.
An ecological study of wetlands was undertaken in northern Lake Victoria (East Africa) between 1993 and 1996 with a major aim of characterising shallow vegetation dominated interface habitats, and evaluating their importance for fish, in particular, for the stocked and socio-economically important Oreochromis niloticus LINNÉ (the Nile tilapia). From field and laboratory experiments, five major habitat types could be defined by the type of the dominant emergent macrophyte at the shore from the more than 40 identified plant species along a 110 km shoreline. These were: Cyperus papyrus L. (papyrus), Phragmites mauritianus Kunth (reeds), Typha domingensis Pers. (bulrush), Vossia cuspidata (Roxb.) (hippo grass), and the alien floating Eichhornia crassipes (Martius) Solms-Laubach (water hyacinth). From digital data, considerable long term changes in the shoreline wetland landscape of the lake were discerned and appeared to be primarily associated with increasing human activity (e.g., agriculture, biomass harvests) which had resulted into a 5 % reduction of wetland cover. In spite of the absence of a well developed euhydrophyte community (e.g., Potamogeton and Ceratophyllum), and increasing infestations with E. crassipes mats, the width of the littoral zone was established by secchi transparency as being about 50 - 70 m away from the shallow (less than 1 m deep) vegetation fringe sloping to between 2 and 4 m in depth at its outer fringe. Hydrological influences associated with seasonal changes (the alternation of rainy with dry periods) explained most of the observed variation in abiotic (e.g., Si, tot.-P, soluble reactive-P, NO3-N, pH, temperature) and biotic (phytoplankton, macrofauna, fish) factors, but there was also significant (p < 0.05) variation due to vegetation, distance from the shore out towards open water and interaction effects between these factors. At least 30 species of fish were identified from the shallower (2.5 m) vegetated habitats in contrast to 10 species from the deeper (4 - 8 m) open water habitats. There were other significant (p < 0.05) spatial and temporal differences in habitat use by fish. Species diversity was dominated by haplochromine species but three stocked species (the Nile perch, Lates niloticus L., O. niloticus and Tilapia zilli) contributed at least 90% of the estimated numerical and biomass densities of which, the Nile tilapia was the most important component making up 45 - 65 % of the biomass of all fish. Season was a major factor in size-related abundance patterns but generally, most of the Nile tilapia biomass was associated with Phragmites- Vossia- Typha-dominated habitats which were also important for small (-1) and had a higher condition index than populations of the species in Lake Kyoga (also stocked) and Lake Albert (a native habitat). It was inferred that these differences could be a result of a better nutritional base in Lake Victoria where the species was shown to be omnivorous (with detrital and animal foods as major dietary items) contrary to the previously believed herbivorous (phytoplankton) habits. Successional patterns associated with water hyacinth and the strong hydrological influences on shallow vegetated habitats imply that basin disturbances could therefore be a major threat to water quality and the fisheries.