2.5 Stomach content analyses
Stomach content analysis (SCA) were performed to examine taxonomic composition and frequency of occurrence of recently ingested food items and subsequently the difference in both the intraspecific and interspecific differences in diets of native C. rendalli and the exotic O. niloticus . Stomach emptiness was achieved using vacuity index (VI) to indicate the quantity of unfilled as number of empty stomachs , while the Stomach fullness (SF) that refers to the average number of relatively full stomachs, and both were estimated on a scale of 0 for empty to 4 for fully distended according to Hyslop (1980). A measuring ruler was used to measure the total length of each stomach (accuracy). The frequency of occurrence method was used to determine the percentage of stomachs containing a particular food item relative to the total number of stomachs containing food (Hyslop 1980). The fish were first grouped into 3 major feeding groups: microphages, macrophages and carnivores, and omnivores, and then later grouped into lower taxonomic or functional categories for quantitative comparisons. Diet categories were chlorophytes, cyanophytes, diatoms, microfauna (zooplankton, insects and protozoa), fish, macrophytes, detritus, and unidentified items. The area occupied by each food item in the stomach of a specimen was transformed into a percentage of each food category. Numerical analysis was not used to categorize broader food items because (1) lumping resource states often inflates niche overlap values, and (2) broader categories generally represent one or more renewable resources (Winemiller 1989).
The dietary niche breath was estimated using Levin’s index (1968), whereas a symmetric niche overlap coefficient of Pianka (1973) was used to estimate individual dietary specialisation within a populations of both the exotic and native species. The Levin’s index values near 0 signify specialized diets with almost no overlap, whilst 1 is similar use of food resources or complete overlap. Overlap using symmetric niche breath was categorized as low (0.0−0.29), moderate (0.30−0.59), or high (0.6−1.00), with high overlap indicating biological significance (Langton 1982).