1. INTRODUCTION
Introduction of the exotic species have become a major threat to many native species and further affecting community structuring and ecosystem functioning globally (Agostinho et al., 2015; Jere et al., 2021; Zengaya et al., 2020). Exotic species in the fresh waters can cause serious disruption in the community assemblage of native species through competition of food resources (Jere et al., 2021; Zengaya et al., 2020). Thus, examining the invasion of exotic species in understanding their impacts on native fish species is critical in fisheries management and conservation biology of lotic environments, which are seen to have dynamic environmental changes along a fluvial gradient (Bwanika et al., 2004; Eloranta et al, 2015; Figueredo & Giani, 2005; Kıran kaya & Ekmekçi, 2013).
Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), is the most widely distributed exotic fish species in Sub Saharan Africa. In Zambia, it was introduced in the late 1980’s by Department of Fisheries for research and development in aquaculture (Bbole at al., 2014; DoF, 2018; FAO, 2012). However, the introduction of O. niloticus poses a major threat to native fishes as it is known to cause trophic cascades with potential impacts on the entire fish community, particularly in highly diverse regions such as the North-western Province of Zambia (AES, 2014; Bok & Bills, 2012; Jere et al, 2021).
Tropic interactions between introduced and native species are suggested as one of the processes responsible for structuring populations and, consequently, communities (Figueredo & Giani, 2005; Kıran kaya & Ekmekçi, 2013). For example, in the Kafue River many populations of the native cichlids currently are threatened by the introduction of exoticO. niloticus and this has resulted in competition in the food resources leading to the decline in population of native species (Bbole at al., 2014). In addition, studies conducted by Gozlan et al. (2010) showed that, in contrast to terrestrial organisms which partition resources mainly through habitat segregation, for assemblages of fishes and other aquatic animals trophic separation is more important. Dietary segregation of food resources is one of the main drivers in the assemblage of fish communities in nearly all aquatic habitats (Bwanika et al., 2004; Canonico et al., 2005; Eloranta et al., 2015). Therefore, increasing resource availability reduces competition since resources are no longer limiting, and if resource diversity and availability increases, species will better partition resources (Agostinho et al., 2015; Kırankaya & Ekmekçi, 2013; Miller & Crowl, 2006; Pilger et al., 2010). For instance, direct competition for food resources between exotic and native species may result in diet shifts and affect native species’ fitness (Eloranta et al, 2015). In certain cases, native species can also be affected indirectly by exotic species. For example, in the northern most of North America where exotic Lake trout is generalist feeder and overlapping the natural distribution of the native Arctic charr through dietary competition, this seems to have displaced the native species from several habitats (Ke et al., 2008; Kırankaya & Ekmekçi, 2013; Philippsen et al., 2015). Such negative impacts on the population of native species may become more common if other effects such as climate change and other anthropogenic activates that may limit availability of food resources (Agostinho et al., 2015; Figueredo & Giani, 2005; Scheffer et al., 2006; Zengeya et al., 2015).
Competition in the food resources in natural environment is one way in exotic species impact native fish species. Therefore, our study used stomach content analysis to evaluated the feeding interaction of exotic O. niloticus and most abundant and aquaculture preferred native fish species Coptodon rendalli(Boulenger, 1896) in the upper Kabompo River, Zambia from 2019 to 2020.Oreochromis niloticus is an omnivorous species and exhibiting a generalist feeding behaviour, while C. rendalli is an herbivorous species (Kenzo & Mazingaliwa, 2002; Zengeya et al., 2015), but both species in early stages of their growing periods they show similar foraging behaviour in lower size classes (Zengeya et al., 2015). However, as they grow further in size Oreochromis niloticusspecies assumed to show somewhat more generalist feeding behaviour thanC. rendalli , this renders a greater feeding competition between them (Marshall, 2011; Zengeya et al., 2020). The aim of this study was to investigate the feeding interaction between O. niloticus andC. rendalli in different size classes both local and regional level in natural distribution to understand the food web structuring and help to clarify the impact of exotic species on native species. Although several studies have investigated the diet of each species separately, no studies have investigated resource segregation and overlap in the size classes of their natural distribution, which may vary with presence or absence of an exotic species due to possible competitions. We hypothesised that O. niloticus and C. rendalli in upper Kabompo River should show significant overlap in their feeding, thus competing for similar food resources in their size classes. To determine this, we used Stomach Content Analysis (SCA) from fish samples collected in sampling points of the upper Kabompo River, northwest of Zambia. We expect both species to occupy different dietary niche in different size classes The results of the study provides new insight regarding coexistence of species in the natural environment and help practitioners and ecologists in Zambia and across the globe to develop strategic management plans for the conservation and management of native species.