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Predatory fishes are able to consume cylindrical-shaped fishes which are of a larger size than the deep bodied fish which they may also prey upon. An effective combination of strong fin spines and a deep body probably protects from most of the predators in Lake Victoria. In Lake Kioga, hledÃ¡nÃ profilu caffmos fishermen have reported the appearance of Nile perch floating dead on the surface and in many cases a large has been found stuck inside the buccal cavity of the predator (Okedi, 1970). The shape and movements of long posterolateral spines in the rotifer Brachionus calyciflorus significantly decreases predation by Asplanchna (Wetzel, 1975). Wetzel (Op. cit.) found that adult Asplanchna sieboldi could capture nearly 100% of adult spineless B. calyciflorus contacted, but only about 78% of the long spined forms. Synodontis species which occur in Lake Victoria have a boney head region provided with stout spines on both pectoral and dorsal fins. When opened, these spines effectively increase the size of the fish thus making them more difficult to swallow and restricting their availability to larger predators. Hopson (1972) observed that Synodontis preyed upon by Lates niloticus in Lake Chad rarely exceeded 18% of the predators length. He also reported finding a dead Nile perch with a Synodontis jammed in its throat.
Figure 1. Diet of L. niloticus in 10 cm size groups assessed by percentage occurrence method. Figures in parenthesis are sample sizes.
Percentage occurrence of food items in the stomach of L. niloticus from five depth strata. Figures in parenthesis are sample sizes.
Kudhongania and Cordone (1974) noted that the best catches of both Haplochromis and Bagrus were within a 1060m depth range, and, from these results, concluded that Bagrus follows Haplochromis more closely than any other species, in terms of relative abundance, depth preference and diel vertical movements. Lock (1975) pointed out that vertical migration appeared to be an adaptation to maintain Bagrus bayad in contact with its prey in Lake Turkana. Echosounding in Lake Kivu has indicated that regani lives in shoals that disperse at dusk and reform at dawn (Fryer and Iles, 1972). Bagrus docmac seems to be most active during the transition between day and night when prey shoals are either forming or breaking up. Most of the prey items in Lake Victoria are probably either diurnal (pelagic species) or nocturnal (mainly demersal species) and are ill-equipped for rapidly changing conditions that prevail during the transition periods both in the evening and morning.
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