Reduction of the “ngege”, Oreochromis esculentus (Teleostei: Cichlidae) populations, and resultant population genetic status in the Lake Victoria region

Ngege, Oreochromis esculentus, originally formed the mainstay of the Lake Victoria Region (LVR) fisheries. Together with its indigenous congener O. variabilis, it was displaced from Lakes Victoria and Kyoga of LVR and was found to survive as isolated small populations within the peripheral minor lakes and reservoirs around the two lakes. Displacement of the two LVR indigenous tilapiines was thought to be principally driven by changed lake environment and predation by the introduced Nile perch, but also competition and genetic swamping by the closely related introduced and comparatively more ecologically versatile tilapine species. In a study carried out in the LVR between 1993 and 2003, micro satellites and RAPD markers were used to analyse the remnant populations so as to establish the population structure and extant genetic diversity of O. esculentus. Analyses indicated that the surviving O. esculentus retained a high proportion of genetic diversity with high differentiation between units an indication of genetic exchange between indigenous and introduced Nile tilapia where the two forms co-existed. While this heightened concern for genetic swamping of the remnant population units by the introduced tilapiines it was noteworthy that in a few of the satellite lakes where the O. esculentus was dominant evidence for introgression was weak.

The value of Nabugabo lakes in the conservation of Oreochromis esculentus

The wide-spread impact of exotic fishes especially Oreochromis niloticus and Lates niloticus together with over fishing in the Victoria and Kyoga lake basins during the 1950s and 1960s, caused endemic species such as the previously most important Oreochromis esculentus to become virtually extinct in the two lakes by the 1970s. Based on reports of presence of this native species in some satellite lakes within the two lake basins, a set of satellite lakes in the Victoria basin (Nabugabo lakes: Kayanja and Kayugi), were sampled between 1997-2002 with an objective of assessing their value as conservation sites for O. esculentus. Other satellite lakes (Mburo and Kachera) also in the Victoria basin, and Lemwa, Kawi and Nabisojjo, in the Kyoga basin, were sampled for comparison. Among the Nabugabo lakes, O. esculentus was more abundant in Lake Kayanja (20.1 %) ofthe total fish catch by weight compared to Lake Kayugi (1.4 %). The largest fish examined (38.7 cm TL) was caught in Lake Kayugi, (also the largest in all satellite lakes sampled), while the smallest (6.6 cm TL) was from Lake Kayanja. Fish from Lake Kayugi had a higher condition factor K (1.89± 0.02) than that from Lake Kayanja (1.53±0.0I), which was the second highest (compared with other satellite lakes) to Lake Kawi (1.92±0.2). Diatoms, especially Aulacoseira, which were previously known to be the best food for O. esculentus in Lake Victoria were mostly encountered (93.2 %) in fish stomachs from Lake Kayugi. In Lake Kayanja the dominant food item was the blue green algae (Planktolyngbya) while Microcystis was the most abundant diet item in fish from other satellite lakes. There were more male than female fish (ratio 1:0.91 and 1: 0.79 in lakes Kayugi and Kayanja respectively). This is comparable to the situation in Lake Victoria before the species got depleted. The highest mean fecundity was (771±218 eggs) recorded in Lake Kayugi compared to Lake Kayanja (399±143). Based on the results from Lake Kayugi, where diatoms dominated the diet of O. esculentus and where the largest, most fecund and healthy fish were found, this lake would be a most valuable site for the conservation of O. esculentus and the best source of fish, for restocking and for captive-propagation. This lake is therefore recommended for protection from over exploitation and misuse.