Conservation and improvement of the stocks of the originally most important Lake Victoria tilapiine, Oreochromis esculentus, Graham 1929
The tilapiine fish species Oreochromis esculentus was endemic to only lakes Victoria and Kyoga, and was the most important commercial fish species in these two lakes. This species started to decline steadily after the introduction of modern fishing technology until 1960 when the last reserves were depleted and is now apparently extinct from these two lakes. This situation prompted a search of water bodies in the two basin lakes for evidence of its possible survival in some localities. Therefore a study of four satellite lakes in the Lake Victoria basin (Mburo, Kachera, Kayanja and Kayugi) and three (Nabisojjo, Kawi and Lemwa) in the Kyoga basin were investigated for the presence of O. esculentus using an experimental fleet of gill nets to try and assess the populations that would be useful in conservation efforts. O. esculentus were relatively most abundant in Lake Nabisojjo (58.1 %), followed by Kayanja (22.6 %), Lemwa (14.2 %), Mburo (6.3 %), Kachera (5.9 %), Kawi (1.7 %) and Kayugi (1.4 %). The fish examined was 7.5- 38.7 cm total length (tl) with a modal size of 16-21 cm tl compared to records of 30-32 cm in Lake Victoria. The largest fish was from Lake Kayugi 38.7 cm, followed by Kachera (28.5 cm), Kayanja (28 cm), Nabisojjo (26.7 cm), Mburo (26 cm) , Kawi (25 cm) and Lemwa (24.5 cm) tl. The fish had an overall mean condition factor k of 1.77±0.02 as compared to historical records of 2 in Lake Victoria. Fish from Lakes Kawi and Kachera had the highest k (1.92± 0.02) followed by Kayugi (1.90 ± 0.03), Lemwa (1.78 ± 0.03), Mburo (1.71 ± 0.01), Nabisojjo (1.65±0.02 and Kayanja (1.50 ± 0.02). Diatoms were previously recorded as the best food for O. esculentus. Blue green algae especially microcysts, were the most dominant item ingested in Lakes Kachera, Mburo and Lemwa. Planktolyngbya was most important in Kayanja, while the diatom aulacosira was most important in Kayugi and Kawi and detritus in Nabisojjo. Size at first maturity was 20.5 cm tl in Kayugi, 19.4 cm in Nabisojjo, 17.2 cm in Lemwa, 16.8 cm in Mburo and Kachera, 15.6 cm in Kayanja and 12.5 cm in Kawi. Originally, O. esculentus matured at 26-27 cm in Lake Victoria and 21 cm tl in Kyoga. There were more males than females (1:0.83) as was the case in historical records. Sex ratios were in Lake Mburo 1:1.1, Kachera 1:0.9, Kayanja 1:0.85, Kayugi 1:0.97, Nabisojjo 1:0.69, Kawi 1:0.52, and 1:0.75 in Lemwa. Fecundity was directly proportional to size of O. esculentus and the highest was (963±148 eggs) in Lake Kayugi, followed by Lemwa (532 ± 18), Kachera (518 ± 24), Kawi (507± 32), Kayanja (468 ± 184), Nabisojjo (429±11) and Mburo (341±19). O. esculentus from Lake Kayugi where diatoms (aulacosira) dominated in their diet were the largest and had the highest condition factor. This indicates that diatoms were important food and is valuable in the survival of O. esculentus. On this basis, Lake Kayugi is the best source of O. esculentus either for restocking or for captive propagation. However, with the shift of algal communities to blue greens the capacity for O. esculentus to assimilate blue greens should be investigated further.
- Conservation and improvement of ...
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