14 Août Another significant protective size is parental look after the egg and more youthful
Some cichlids are mouth-brooders, and in such cases the males, females or both carry the eggs to special brooding grounds. The parents eventually leave their young in nursery areas which are situated inshore. Welcomme (1964) has shown that juvenile Oreochromis variabilis migrate away from the sheltered nursery beaches when they are around 5.0cm total length. In both Lake Albert and Lake Turkana the young of prey-species use the cover of the sublittoral Ceratophyllum zone and the shallow lagoons in order to escape predation (Gee, 1969). Greenwood (1965) found that Protopterus aethiopicus spends much of its first year in the cover provided by the matted root systems of papyrus in Lake Victoria. Macan (1977) reported that Asellus on mud is at risk from predation by Sialis intaria (L) but amongst Elodea it is relatively safe from all predators.
Another important adaptation developed by prey-species is migration. Some species such as Barbus altianalis, Labeo victorianus and Schilbe mystus are potamodromous, moving up rivers to spawn. The young often remain upstream in shallow water where they are more likely to escape predation by large fish (Lowe McConnell, 1975). The majority of the fishes in Lake Victoria appear to be more demersal than pelagic (Kudhongania and Cordone, 1974), however, in order to partially segregate themselves from predators during periods of peak susceptibility, some Haplochromis species migrate vertically on a diel basis. This may also be a feeding strategy since the zooplankton upon which they feed also shows such diurnal migrations. Kudhongania and Cordone (op. cit.) observed higher catch rates for Haplochromis and Bagrus docmac in bottom-trawl hauls made during the day than during the night, whereas the mid-water trawl catches were higher during the night than during the day. This suggests that the prey fish may be gaining some protection by descending to dimmer lit zones during the day. In Lake Victoria there are probably two periods of peak feeding activity by Bagrus on Haplochromis, one in the morning the other in the evening (EAFFRO – Progress Report 1969). Ochieng (1982) confirmed two peak feeding periods in Bagrus docmac, one at sunset and the other at sunrise. Such crepuscular feeding activity by Bagrus may be due to low optimal light conditions required by the predator for feeding or may reflect diel behavoural changes in the haplochromine prey species. Elliot (1970) reported an increase in invertebrate drift at night and proposed that trout utilise some of this readily available drift food especially in the early hours of the night. The Lake Tanganyika clupeids (Limnothrissa and Stolothrissa) undergo vertical migration movements to escape large, diurnal predators such as Lates (Coulter, 1966).
3. LATES NILOTICUS
was introduced into Lake Victoria between 1959 and 1963 at Jinja in Uganda. The specimens introduced were from Lake Albert, Uganda. The species has established itself mainly in the shallow, inshore areas but is now colonising the deeper waters of https://datingranking.net/cs/chatango-recenze/ the Lake. Goudswaard and Witte (1984) reported catches of 80 to 150 kg/hr of Nile perch at depths of 5060m in Tanzanian waters. In Kenya, the present commercial fishery is based mainly on Lates which, in 1984, contributed 57% of the total landing. Similar figures have been noted in Uganda (Acere, 1984).
is the largest carnivorous fish in Lake Victoria and is appreciably larger than the indiginous Bagrus docmac. The species is endowed with speed, large size and a massive mouth enabling it to capture large prey. The largest specimen caught in the gulf was 204cm TL (pers. obs.). needs to make short bursts of high speed swimming to capture its prey (Holden, 1967).