Oreochromis Esculentus Research Articles

Changes in the fish and fisheries ecology of a large man‐made lake in Tanzania, 1965–94

Abstract Nyumba ya Mungu Reservoir in northern Tanzania has a maximum surface area of 180 km2, an average depth of 6 m, and contains moderately saline, fertile water. The postimpoundment decade, 1965–74, was characterised by a spectacular rise and fall of a tilapiabased fishery. Since then a gill‐net fishery catching large numbers of large, periphyton‐grazing tilapias has given way to a mainly beach‐seine fishery producing large numbers of small, phytoplankton‐feeding Oreochromis esculentus (Graham). Yields remain higher than preimpoundment predictions but the size at maturity of all tilapia species has fallen and few larger than 15 cm TL are caught. These changes, and others in the fish community trophic structure, are chiefly explained by changes in the littoral environment and fishing pressures. Nyumba ya Mungu provides a case history of developments in the tilapia fishery of a man‐made river lake conducted on an essentially laissez‐faire basis.

Diet and food utilization in Oreochromis esculentus (Graham) in Lake Kanyaboli, Kenya

Abstract Blue‐green algae (Cyanophyta) and green algae (Chlorophyta) were the principal food items ingested by Oreochromis esculentus (Graham) in Lake Kanyaboli. Diatoms (Bacillariophyta), euglenoids (Euglenophyta) and zooplankton were also consistently present in the diet in small quantities. O. esculentus of different sizes fed on similar food items in the lake. The species fed actively during the day, from 0700 h until 1700 h when peak feeding activity occurred. Thereafter, digestion took place steadily until 0600 h, when 90% of the fish had empty stomachs. The diet composition remained the same throughout the year, as did the composition of phytoplankton in the lake. O. esculentus tended to select and digest the diatoms and zooplankton and, to some extent, the euglenoids. Blue‐green and green algae were the least utilized food items and passed through the gut undigested.

The Effects of Predation by Nile Perch, Lates niloticus L., on the Fish of Lake Nabugabo, with Suggestions for Conservation of Endangered Endemic Cichlids

As in Lake Victoria and Lake Kyoga, most of the native fish species of Lake Nabugabo have either been depleted or have disappeared since Lates niloticus, Nile perch, and Oreochromis niloticus, a tilapiine, were introduced. L. niloticus, O. niloticus, Schilbe intermedius, Brycinus jacksonii, and Rastrineobola argentea are the only species still abundant and widespread in the lake. Of the five haplochromine species endemic to Lake Nabugabo, at least one has vanished and the remaining ones are very rare. The prey of the Nile perch in Lake Nabugabo, with the exception of the prawn Caridina nilotica, which is absent from the lake, are similar to those the predator switched to in Lakes Victoria and Kyoga after haplochromines had been depleted. These consist of ephemeropterans—especially Povilla, the anisopteran nymphs R. argentea, tilapiines, and juvenile Nile perch. Three small lakes adjacent to Lake Nabugabo—Lake Kayanja, Lake Kayugi, and Lake Manywa—contain large numbers of Oreochromis esculentus and two haplochromine species previously known only in Lake Nabugabo. These three lakes should be designated conservation areas for the cichlids and other native species that were previously abundant in Lakes Victoria, Kyoga, and Nabugabo. The swamps around these lakes should be protected so that the Nile perch does not spread into them. Further research should be carried out to ensure the preservation of the endemic species.

A Salmonella Strain Isolated from Camels in Palestine

Investigation on accumulation of selected heavy metals of Zinc, Mercury, Copper, Lead, Chromium and Nickel in sediment, water and muscle tissues of Clarias gariepinus (African catfish) and Oreochromis esculentus (Singida tilapia) fish was done in Lake Rukwa, Tanzania. Samples were obtained from transects of 100 m long extending from Luika and Songwe River mouths to offshore. Water and sediment samples were collected directly from the study sites while fish were obtained from fisherfolk operating in the Lake. Sampling was done in dry and wet seasons. Heavy metals analysis was done using the Atomic Absorption Spectrophotometry. Concentration of heavy metals was higher in catfish than in tilapia (p < 0.05). There were no significant differences in metal concentration between seasons except for Zn (p < 0.05). In this study only Zn was above standard WHO concentrations in fish muscles. Likewise, the concentrations of heavy metals were within recommended limits in water except Pb. The detected metals in sediment were above recommended limits. Other heavy metals in particular Hg, Ni and Cr were not detected in all samples. Therefore, studied fish from Lake Rukwa may threaten human health upon consumption. The detected heavy metals in water were within the maximum residual levels (MRLs) permitted by WHO. Sustainable Lake Rukwa's fish, ecosystem management and conservation are recommended to discourage heavy metals discharge from elevating beyond permissible limits and thus prevent harmful health effects to fish consumers and water users.