Open access publications in the SLU publication database

Abstract

The 80,000 km² Lake Tumba Landscape (LTL) is the only one of the 12 UN Priority
Conservation Areas in Africa with a focus on freshwater ecosystems. Surprisingly little
though is known about fish in this area. The objective of this thesis is to increase
knowledge of fish occurrence in the LTL.

The first paper uses catch statistics to quantify the fish populations in three LTL
streams against the background of their physical and chemical environments (Paper I).
All the species caught represented 23% of the known Congo Basin fish species. The
Lebomo stream had higher catches during the dry than wet season, but this was not the
case for the Bambou and Bongo streams. Significant differences in fish species
composition between the streams were found, but this probably related more to pH and
nutrient status than land use. These streams yield too little fish to be a regular part of
the local diet, but the streams may be important reservoirs of biodiversity in the
landscape, and also serve as ”sentinels” of change for larger downstream aquatic
ecosystems.

The aim of the second paper was to characterize the breeding sites of Tilapia congica
in order to guide the zoning process for fishery management in Lake Tumba. For this
purpose, the nest depths, distances from the shoreline, exposure to sunlight and habitat
types were measured. Four extensive groups of breeding sites were located near where
tributary rivers entered the lake. All nest sites were exposed to the sun and 90% of sites
were within 51-250m of the shoreline. The breeding substrates for T. congica are sandy
and under hippo grass (Vossia cuspidata) (48%) or water lilies (Nymphaea stellata)
(39%). Tilapia congica shared 42% of its nesting sites with other fish species, so
protecting the breeding habitat of T. congica would likely help other species.

The major overall findings of this thesis are that there is much diversity in the
running waters of the LTL, and that knowledge of the location as well as the
seasonality of fish breeding patterns can inform better fish management. At this point,
it is not clear what effect land use/land cover has on the fishery, but better monitoring
can establish a baseline against which eventual changes can be defined in relation to
changes in land use and even climate.