|1. Lates niloticus||2. O. niloticus||3. Roentgen. argentii|
|4. Haplochromis sp.||5. P. aethiopicus||6. Clarias sp.|
|7. Alestes sp.||8. Yards. frenetus||9. Synodontics sp.|
|10. Labeo sp.||11. Mormyrus sp.||several. Bivalves|
|13. Gastropods||14. Odonata nymphs||15. Chironomid larvae|
|sixteen. Ephemeropteran nymphs||17. Corixid grownups||18. Chaoborus larvae|
|19. Caridina nilotica||20. Crabs||21. Cladocerans|
|22. Ostracods||23. Copepods|
Results of the present investigations revealed that the dominant food items were, Lates niloticus juveniles and Rastrionebola argentii within the shallow inshore zones, whereas in the deeper zones the primary food item was Cardina nilotica.
Gee (1969) and Okedi (1970) stated that the primary food of Lates in Lake Victoria was Haplochromis spp. The general absence of Haplochromis from the stomachs in the present investigations, reflects their scarcity in the area studied. This point is illustrated by the fact that the bottom trawl hauls usually yielded less than step one.0kg/hr of Haplochromis at all depth zones (Table 1). The preponderance of juvenile Lates and Caridina niloticus in the stomachs of Lates is also indicative of lack of other suitable prey. Ogutu-Ohwayo (1984), while working in Lake Kioga, found that his observations differed from those of Hamblyn (1961), Gee (1964 1969) and Okedi (1970) and indicated that, as a result of the change in type of prey available, the Nile perch in Lake Kioga has shifted from one prey to another over the years. Earlier studies had shown that the main prey was Haplochromis and mormyrids which were later replaced by Rastrionebola. However, by 1984, Oreochromis niloticus had become the major prey of Lates in Lake Kioga. The changes in diet with time is probably related to the availability and relative abundance of the prey items. Royce (1972) states that when a newly introduced predator population starts to consume a prey population that has been in equilibrium with its competitors and other predators, the first consequence is an increase in the mortality rate of the prey population.
Macan (1977) describes how the introduction of Salmo trutta into an artificial pond was followed by a greater reduction in the numbers of tadpoles, certain beetles and Notanecta species mainly found in open water. He concluded that only small invertebrates can survive predation in open water.
In the present investigations the dominant fish prey were Lates juveniles and this confirmed that cannibalism is prevalent. Hopson (1972) considers that cannibalism probably results partly from a dearth of alternative food sources. He noted scarcity of alternative food among cannibalistic Lates on the open shore at Malanfatori in Lake Chad. Both Okedi (1970) and Ogutu amateurmatch bezplatnÃ¡ zkuÅ¡ebnÃ verze Ohwayo (1984) reported cannibalism in Lake Kioga. )
TABLE 1 Mean bottom-trawl catch rates (kg/hr) of the different organisms which form the food of Lates niloticus at different depth strata within the Nyanza Gulf.
In today’s studies, an identical scarcity of option victim are indexed (Table step 1
|No. off Hauls||21||72||21||24||13|
Young tilapiine cichlids of various species are capable of tolerating water with an oxygen content as low as one part per million in Lake Victoria (Welcomme, 1964). Greenwood (1966) noted that aerial breathers such as Protopterus aethiopicus and Clarias are able to live in the swamps because of their ability to utilise atmospheric oxygen. Hunter (1970) quoted the experimental work of Fish (1956) which proved that L. niloticus has a relatively high oxygen requirement compared with many other freshwater fishes. The low oxygen demand by the above species apparently enables them to escape predation by Lates niloticus. Thorpe (1977) referred to a suggestion by Regier et al. (1969), that, together with the smelt (Osmerus mordax), perch fry are probably protected from walleye predation due to hypolimnial oxygen depletion which excludes walleye from their foraging base in Lake Erie.