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Non-native trout species have been associated with many negative effects in receiving ecosystems. The first aim of this study was to determine the impact of non-native rainbow trout Oncorhynchus mykiss on distribution and abundance of native mountain catfish Amphilius uranoscopus within Afro-montane streams in Nyanga Mountains, eastern Zimbabwe. The second aim was to compare macroinvertebrate community responses to the presence of the trout and the catfish. We examined trout impact on catfish's habitat associations, whereas macro-invertebrate composition was compared using open fish and fish exclosure experiments in habitats with and without trout. Trout influenced both the distribution and abundance of the catfish that occupied shallow reaches possibly to avoid predation from trout that occurred in the deeper habitats. Within trout invaded reaches, most macro-invertebrate taxa were more abundant in exclosure than open treatments. By contrast, within trout-free reaches, most macro-invertebrates either did not differ between treatments or were generally more abundant in open than exclosure treatments. This suggests that the macro-invertebrate communities responded differently within invaded and non-invaded reaches. By influencing distribution and abundance of native biota, non-native rainbow trout may have wider ecological effects, such as influencing trophic interrelationships within invaded habitats.

The present study used a combination of stable isotope techniques and stomach content analyses to examine the diet and ontogenetic diet shifts of tigerfish, Hydrocynus vittatus, in the Sanyati basin of Lake Kariba, Zimbabwe. We compared tigerfish diet shifts in the presence of an invasive crayfish, Cherax quadricarinatus, with previous studies before invasion. Here, we assessed whether tigerfish now had a preference for C. quadricarinatus leading to a shift in its main dietary food sources from cichlids to crayfish. The dominance of cichlids has remained generally high when compared with historical assessments. Stable isotope analyses showed that tigerfish fed on a variety of food sources including those that may be temporarily unavailable. Results of stable isotope analyses also highlighted that C. quadricarinatus was now becoming an important food source across all tigerfish size classes. In conclusion, tigerfish in Lake Kariba showed remarkable dietary plasticity and ontogenetic shifts, identified by both stomach content and stable isotope analyses.

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