Abstract:

An ecological study of wetlands was undertaken in northern Lake Victoria (East Africa)
between 1993 and 1996 with a major aim of characterising shallow vegetationdominated 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), Phragmitesmauritianus 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. Inspite
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, N03-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.,
0. niloticus and Tilapia zil/i) 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 t