Like many global inland freshwater bodies, the lakes and rivers of Uganda are vital for economic development but have at the same time experienced alarming disturbances for virtually a century, since the 1920s, (1–6). The distressing challenges have been mainly mediated by anthropogenic activities as a result of exponential demographic growth that have triggered user conflicts and escalated demands for the fisheries' resources (7). For instance, the high demand for fishery products led to overfishing and a subsequent decline in fish stocks, which influenced the translocation of alien species (Nile tilapia and Nile perch) to various water bodies in Uganda (2, 8). Other than fish translocations, Uganda’s major water bodies have been invaded by aquatic weeds which have devastated the water quality resulting into negative effects on the fisheries (9). For instance, the water hyacinth (Eichornia crassipes), which was introduced in the Lakes Kyoga and Victoria as well as Victoria Nile in 1988 (6, 9, 10), and more recently (2013), the Kariba weed (Salvinia molesta) (11, 12) have caused challenges in these lakes. These weeds have proliferated in the water resources as a consequence of the massive influx of nutrients (mainly phosphates), propelled by various human-mediated activities (11, 13). The anthropogenic activities have dramatically upset fish biodiversity and ecology in nearly all the country’s water bodies either directly or indirectly.
Nile tilapia in Uganda is a popular species in both aquaculture as well as capture fisheries and is thus vital for the country’s economic development. The fish is the second most economically important species after Nile perch, but the most highly valuable under aquaculture (Balirwa et al. 2003). Traditionally, Nile tilapia is non-native to Lakes Victoria, Kyoga, Nabugabo, the upper Victoria Nile, and various satellite water bodies in Uganda (2, 7, 14–16). The species is native to Lakes Albert, Edward, George, Kazinga Channel, and the lower Victoria Nile (Uganda), and was introduced into Lakes Victoria, Kyoga, Nabugabo, and the upper Victoria Nile in the 1950s to augment the devastated capture fisheries (Balirwa 1992). Albeit the introduction of Nile tilapia triggered increased catches, it also coincided with the many negative impacts in these water bodies. These include the massive stock decline and in some cases, extinction of the native tilapiine species such as Oreochromis esculentus (Singinda tilapia, Ngege) and Oreochromis variabilis (2, 7, 14–16). These events coincided with the dramatic upsurge of the introduced Nile tilapia catches in the 1980s (Balirwa et al. 2003). As a result, the L. Victoria basin is now predominantly inhabited by three fish species notably, the native small cyprinid silver fish (Rastrineobola argentea) and the two non-native (introduced) species; Nile tilapia and Nile perch.
In Uganda, Lakes Victoria, Kyoga, Albert, George, and Edward are the major fish sources and generally the central focus of scientific investigations (6, 8, 17–21). The rather less studied Victoria Nile is also a salient fish source providing employment opportunities and livelihoods to the riparian communities (22). The river drains water from L. Victoria to L. Kyoga and then through Lake Albert to form the Albert Nile (10). The prevalent anthropogenic activities in Vitoria Nile might have compromised the genetic integrity of the fish populations. For instance, in the 1950s, the first hydroelectric power dam, previously referred to as Owen Falls dam and now Nalubale dam was constructed at the source of the Nile, Jinja-Uganda (10, 23). This dam is likely to have inhibited the geneflow of fish species between L. Victoria and other sections of Victoria Nile as well as L. Kyoga (10, 24). Later ( in the year 2003), another dam, Kiira, was constructed adjacent to Nalubale and perhaps exacerbated the segregation of the fish populations. In recent years, since 2007, several other hydroelectric power dams, including Bujagali and Karuma, have been constructed across the Victoria Nile, which in the long run, might pose further threats to fish biodiversity through impoundment (25, 26). Usually, the construction of hydroelectric power dams entails the structural reservation for fish-pass-ways as well as mimicking natural bays for fish spawning (26). It is not clear if these mitigation strategies were considered during the hydroelectric power dam construction in Uganda. Ecologically, power dams might promote fish population intraspecific divergent by blocking fish interface or, altering habitat conditions and existing niches. These may subsequently induce selection pressures that promote changes in the organism’s behaviour and morphological variations (27).
Recently, it was observed that the genetic diversity of Nile tilapia in Victoria Nile was lower than that of other habitats such as Lake Victoria (Tibihika et al. 2020). This was likely attributed to the consequence of the constructed hydroelectric power dams, loss of diversity through overfishing or water pollution, or founder effects, inter alia, consistent with the historical fish translocations (24). Nevertheless, in the study, the small sample size is likely to have limited the clear understanding of the state of Nile tilapia populations given that only one site in the Victoria Nile was sampled (21, 24). Therefore, in the current study, we sampled multiple sites in the Victoria Nile and stretched to adjacent main water bodies (Lakes). The major focus of this study was to investigate the extent of Nile tilapia morphological variation as a potential consequence of anthropogenic activities in the Victoria Nile. We approached this study by delineating the fish size and condition factor variations, and compared the morphological differences amongst fish from Lakes Victoria, Kyoga, and upper and lower Victoria Nile. Further, we singled out non-native and native fish strains and subsequently tested the underlying morphological differences between the populations. The current study is intended to unveil key information vital for management and sustainable conservation decisions regarding Nile tilapia species in Victoria Nile.