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).