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).