3. RESULTS
A total of 202 exotic O. niloticus and 166 native C. rendalli were caught in the upper Kabompo River in the range of 20 to 150 mm TL and placed in size classes <50, 51−100, and 101−150 mm during the survey period. The length of the two species in each size class were similar, but number of catches of O. niloticus of native C. rendalli examined for SCA in the 3 size classes was 48% low from the overall catch (Table 1). Both species were caught in different time period (06:00hrs, 12:00hrs. and 17:00hrs) during the survey and the species were found evenly abundant in all sampling points, except for 06:00hrs that had 68% less catches (Table 1).
The native C. rendalli in size class 51-100 mm and 101-151 mm fed on algae, diatoms and detritus (Table 2). The diet for both species in the <50 mm size class consisted mainly of algae, whileO. niloticus diet shifted in the 51−100 mm size class, with detritus, zooplankton, insects and microfauna becoming the most important food items, indicating a slight ontogenetic diet shift (Table 2). For both species fewer stomachs were empty and food items found were grouped into food items categories. The feeding patterns of C. rendalli were discontinuous and cephalus than of exotic O. niloticus (Table 2). In general, O. niloticus had a more diverse diet than C. rendalli resulting from foraging on algae, chlorophytes, diatoms, microfauna, and detritus in all size classes except for size class <50 mm. Coptodon rendalli fed selectively on similar food item in different size classes, indicating a no ontogenetic diet shift. The mean number of food items observed in size class <50 mm did not differ significantly among stomachs of the two species, but exotic O. niloticus showed a larger volume than that of C. rendalli in size class 51-100 mm and 101-150 mm with significant peaks at 17:00hrs and higher feeding intensity at 12:00 hrs.
The mean SF was relatively high in O. niloticus than C. rendalli in size class <50 mm, 51-100 mm and 101-150 mm (U =129, n =89, p =0012; U =153, n =81,p =0165 and U =144, n =129, p =0283, respectively), while VI was low in the largest size classes (51–100 mm and 101 to 150 mm) (U =338, n =133, p =0.001;U =319, n =127, p =0128 and U =387,n =141, p =0172, respectively) (Table 3). The Levin’s index mean values for O. niloticus and C. rendalli showed dietary segregation partially in the size classes, except in size class <50 mm (U = 216, n = 91, p = 01397) (Table 3). In the size class 51–100 mm and 101-150 mm, C. rendallishowed a significantly higher degree of individual dietary specialization than O. niloticus (U =291, n =87,p =0.001 and U= 288, n= 101, p< 0001, respectively) (Table 3). However, a partial dietary segregation was also observed between O. niloticus and native C. rendallispecies, particularly in size class 51-100 mm (U =291,n =87, p =0.001), as O. nilotius also consumed other food items such the zooplankton and other micro fauna (Table 3).Coptodon rendalli also showed a significantly higher degree of individual dietary specialization compared with O. niloticus in size class 101-150 mm. The mean volume of the food items consumed by both species observed in their stomach contents did not significantly differ, O. niloticus showed a larger number of the consumed food items than C. rendalli (Tables 2 and 3).
Dietary overlap between exotic O. niloticus and C. rendalli was significantly high in size classes <50 mm and 51-100 mm (niche breath coefficient 0.637and 0.667, respectively) (Table 4). Both species in the <50 mm size class consumed mainly diatoms and algae, while O. niloticus alone in the 51−100 mm and 101−150 mm size classes consumed mostly zooplankton, chlorophytes, and algae, dominated by Microcystis, Gonium, and Phacus, with significantly low dietary overlaps (niche breath coefficient 0.422and 0.223, respectively) (Table 4). Oreochromis niloticus is predominantly omnivorous, but its diet consisted of zooplankton, microfauna and insects in 101-150 mm size class, and there significantly low dietary overlap with C. rendalli diet comprised mainly as diatom, cyanophytes, chlorophytes and macrophytes (niche breath coefficient 0.118)(Table 4).