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.