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The introduction of invasive alien species in an ecosystem poses a greater risk to cause ecological disruption in stable environments

Investigating the determinants of reproductive biology of fishes is an essential component of biological research. Breeding pattern was investigated to determine the impact of exotic Oreochromis niloticus on the native congeneric Oreochromis macrochir in the upper Kabompo River. Gonado-somatic index and sex ratio was used to investigate the breeding pattern in both invaded (where O. niloticus is present) and uninvaded (where O. niloticus is absent) sections of the river. Oreochromis macrochir was the only native congeneric species found in both sections. Results showed that the overall gonado-somatic index means for both sexes of O. macrochir in both sections were similar. For O. niloticus in invaded section indicated all year reproduction though reduced spawning in cold season (May-June), but with increased spawning activity in wet season (February-March). In O. macrochir, males and females were found breeding in both dry and wet seasons only, as for cold season no reproduction was recorded. Sex ratio (females: males) was 1:1.3 and 1:1.7 for O. niloticus and O. macrochir respectively, and both significantly deviated from the sex ratio of 1:1 (ꭓ2=8.42 and 9.37; p<0.05). Oreochromis niloticus formed the most abundant fish caught 221(63.5%) than O. macrochir 127(36.5%). Our study has revealed that O. niloticus was able to spawn in all seasons with 23% higher breeding population than O. macrochir, which explains the suppression in the abundance. We expect O. niloticus to invade further downstream of the Kabompo River due to natural dispersion.

Evaluating the feeding interaction of fishes is fundamental in understanding the impacts of introduced species on the community structure and ecosystem function of native species. In the Zambian Rivers, studies have shown that native species has been impacted upon by exotic Oreochromis niloticus, but few studies have investigated feeding interactions between exotic Oreochromis niloticus which is omnivorous and aquaculture preferred herbivorous native species (Coptodon rendalli) in the natural distribution. We analysed the stomach contents of 368 specimens of the fishes to examine the feeding interactions between O. niloticus and C. rendalli in the upper Kabompo River, northwest of Zambia. Both species were grouped into size classes of <50, 51−100 and 101−150 mm total length (TL). However, in the size classes 51-100 mm and 151-100 mm O. niloticus had a larger dietary niche because of its generalist diet than the native species, whose diet mainly comprised of diatoms and algae. There was no significant ontogenetic diet shift for both species, except for O. niloticus in size class 101-150 mm, whose diet consisted mainly of zooplanktons and micro fauna. The results showed an interspecific competition in their size classes, except for 101-150 mm size class, that indicated low dietary overlap. This poses major impacts on food web structure and may explain population decreases of some native species in the upper Kabompo River. Future studies are required to further understand feeding interaction between the exotic and other native fishes in the Kabompo River and other rivers across African where the species coexist.

Evaluating the food and feeding habits of fish is fundamental in fisheries and conservation biology research. In this study, the diet of exotic Oreochromis niloticus was compared with the 2 most abundant and aquaculture preferred native cichlids of native species (Orochromis macrochir and Coptodon rendalli) in the upper Kabompo River, Zambia. We hypothesized that exotic and native cichlids would show no dietary niche overlap. We analyzed the stomach contents of 114 specimens of the fishes sampled. Fishes were grouped into 3 major feeding groups: microphages, macrophages and carnivores, and omnivores. They were also grouped into size classes of <50, 51−100, 101−150, and 151−302 mm total length (TL). O. niloticus had a larger dietary niche than two native species (71% and 22%, respectively). The dietary niche overlap between O. niloticus and native C. rendalli species in size classes <50 was significant (F (2, 45) = 0.084, p < 0.05). Dietary niche overlap between the native O. macrochir species in size class <50 mm was low (F (2, 33) = 2.13, p > 0.05), while as in size classes 51−100 mm and 101−150 mm was high (F (2, 35) = 0.27, p < 0.05) for C. rendalli. There was no clear evidence of ontogenetic diet shift of native cichlids, with the exception of O. macrochir, which showed ontogenetic diet shifts within the 51−100 mm size class. The dietary overlap results indicate interspecific competition between exotic O. niloticus and native O. macrochir, which may have major impacts on food web structure in the upper Kabompo River and may explain population decreases of some native species.

Invasive alien species are one of the greatest threats to biodiversity and ecosystem globally, affecting delivery of ecosystem goods and services, and consequently human well-being. Oreochromis niloticus is one of the most highly successful invaders of aquatic ecosystems with competitive characteristic advantages over indigenous species. The aim of this study was to investigate the potential presence and impact of O. niloticus on Mitukutuku fishery. The study was undertaken using fisheries independent surveys for the period of 2 months. Fish identification key and gel electrophoresis identification methods were also used to confirm presence of O. niloticus and also possible hybridization with indigenous congeneric species. Fisheries dependent survey was conducted with fishers and traders to further assess presence of the invasive species and its impacts on their catches and socioeconomic well-being. A total of 32 Oreochromis hybrids species and 19 of O. niloticus were found in the reservoir. This confirmed a higher proportion of O. niloticus in the reservoir. Results from interviews of fishers and traders revealed catch per unit effort of O. hybrids and O. niloticus to be higher compared to that of indigenous congeneric species. Similarly, fish traders confirmed that income for the fish sales increased from 5% in March to 7% in August. Overall, these results suggested that O. niloticus does not only exist in Mitukutuku reservoir, but also hybridizes with native species and dominates the entire stretch of the reservoir threatening loss of biodiversity and aquaculture breeding programme. There is need for restricting culture of O. niloticus in Zambia.

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