National Agricultural Research Organisation (NARO), Fisheries Resources Research Institute (FIRRI), P.O. Box 343, Jinja, Uganda. E-mail:firi@infocom.co.ug

Conservation of Oreochromis esculentus in the Lake Victoria and Lake Kyoga basins

The tilapiine, Oreochromis esculentus is the most important commercial fish species in lakes Victoria and Kyoga. By the 1960s, its stock was depleted through over-fishing. Some population survived in satellite lakes within the two lake basins. The relative abundance, population structure, condition factor, food, and fecundity of populations in seven satellite lakes (Mburo, Kachera, Kayanja, Kayugi Nabisojjo, Kawi and Lemwa) were examined to guide conservation of the species. O.esculentus were most abundant in Lake Nabisojjo (58.1%), followed by Kayanja (22.6%), Lemwa (14.2%), Mburo (6.3%), Kachera (5.9%), Kawi (1.7%) and Kayugi (1.4%). The largest specimen (38.7 cm) was from Lake Kayugi, followed by Kachera (28.5 cm), Kayanja (28 cm), Nabisojjo (26.7) cm, Mburo (26 cm), Kawi (25 cm) and Lemwa (24.5 cm). The mean condition factor (K) was 1.77±0.02 with the highest of 1.92±0.02 coming from lakes Kawi and Kachera, followed by Kayugi (1.90±0.03), Lemwa (1.78±0.03), Mburo (1.71±0.01), Nabisojjo (1.65±0.02) and Kayanja (1.50±0.02). Relative fecundity was highest (963±148 eggs) in Lake Kayugi, followed by Lemwa (532±18), Kachera (518±24), Kawi (507±32), Kayanja (468±184), Nabisojjo (429±11) and Mburo (341±19). Blue-green algae were ingested mostly in lakes Kachera, Mburo, Kayanja and Lemwa and detritus in Nabisojjo while diatoms were mostly ingested in lakes Kayugi and Kawi. Lake Kayugi had the largest fish, the highest K and relative fecundity and where diatoms were the main food, contains the healthiest stock of O. esculentus. However, with increased shift of algal communities to blue-greens as a result of increased eutrophication of aquatic systems, adaptation to ingest and assimilate blue-greens my have survival advantage and should be investigated.