Naturally found in Lake Victoria, the Kyoga Basin and a few regional satellite lakes, Oreochromis esculentus has been introduced into other equatorial waterways. Together with the other native tilapine, Oreochromis variabilis, before the 1980's, was the major ingredient to a pre Lates niloticus commercial fishery. Native fishing habits of using pulled gill nets (the British settlers introduced flax gill nets to native Ugandan villagers in 1905) was the preferred harvest method. Over fishing soon led to the use of smaller and smaller loop sizes as stocks dwindled. Smaller and younger fish led to immature stocks being harvested as a result few adult fish were able to fill the population fissure. Other tilapine species were introduced to sustain a fishery. These fish included O. leucostictus, O. niloticus, Tilapia zillii and T. melanopleura (Gee, 1964). As these alien species competed for habitat and food sources, both species were lost from many areas. Perhaps the most incredible incident to fathom is the extinction of Oreochromis esculentus from the massive Lake Victoria.
Reported sizes of 50cm have been reported from Lake Victoria however 20cm is large in captivity. Interestingly, the Lake Kyoga population reached 26cm and smaller lakes yielded a smaller adult size for segregated populations in smaller waterways. The main food source was phytoplankton but the individual diatom species differed from lake to lake (Kalule, 2004). Specialized gill rakers filter minute food particles from the open waters. O. esculentus secretes mucus in the mouth which traps small food particles. These form into small bulbs which are ingested. Open waters are the preferred habitat of adults. All historical data suggests that the male to female ratio always showed a predominance of males over females by a factor of almost 2:1 which additionally occurs in captive populations.
This is a long lived species and although sexual maturity can occur within six months, full growth is reached after 9-10 years. Adults school together in open waters following plankton blooms while the young inhabit inshore waters in areas of dense aquatic vegetation. Brooding females also raise their young for up to two weeks in these planted areas. After this time, the young are left to fend for themselves. Growth is rapid.
Most species of Tilapia are hardy, enduring many differing conditions. O. esculentus is able to withstand temperatures of 10°C for short periods suffering no ill effects. Higher temperatures in excess of 40°C are also tolerated along with the low oxygen levels these conditions produce (Borstein, 2007). It is not recommended to house O. esculentus in these circumstances for prolonged periods but this ability to adapt to rogue conditions are quite possibly an evolutionary advantage to survival. The ability to tolerate and flourish in many different ecosystems has put this cichlid and many other Tilapine on prohibited species lists in most temperate and semi-tropical regions.
Continue to next page »