Habitat utilization and reproductive biology of Nile Tilapia (Oreochromis Niloticus) in Albert Nile, Nebbi District
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Some aspects of habitat utilization (distribution, abundance, size classes, condition factor) and reproductive biology (sex ratio, size at first maturity, fecundity) of O. niloticus were examined in Albert Nile in Nebbi District between January and July 2007 with an aim to understand the extent of habitat utilization and reproductive potential of the species among habitat types of Albert Nile. A total of 596 fish samples were collected from four major habitat types namely Vossia cuspidata, Eichhornia crassipes, Phragmites mauritianus dominated shores and open water areas using experimental gill nets of mesh size range 37.5mm to 125mm and small seine net operated from a boat or beach. The study established that the inshore areas of Albert Nile were characterized by diverse habitat types ranging from open water areas to various vegetation types (V. cuspidata, E. crassipes, and P. mauritianus) that provided suitable habitats for fish. The habitats were further characterized by shallow, slow flowing, warm water of average temperature 28.4ºC, and high dissolved oxygen with mean concentration of 6.4 mg/l, and pH oscillating around neutral value (7.04) which makes it suitable environment for fish habitation. Oreochromis niloticus was found in all habitat types but the relative abundance varied among habitats with highest number recorded in V. cuspidata (35.1%) followed by open water (27.5%) and the least in P. mauritianus habitat (14.9%). The mean length of O. niloticus captured in all the habitat types in Albert Nile was 16.4 cm TL. High abundance of juveniles of sizes ≤ 20 cm TL were found in V. cuspidata and E. crassipes habitats while mature individuals were more abundant in the open waters, indicating the importance of macrophytes as critical habitats for fish breeding, nursery grounds and providing shelter for protection against enemies. The condition factor (K) for O. niloticus in all the habitat types varied from 1.56 to 2.48 and the differences among habitats were significant (F= 6.4312, p < 0.05) with highest mean value among fish from open water (K= 2.19) and least in V. cuspidata (K= 1.92) which indicates that the habitats occupied by O. niloticus were not the same in terms of food availability, seasonality and other environmental factors that support fish life. Sex ratios were different among habitat types but in V. cuspidata and E. crassipes the differences from the hypothetical 1:1 were significant (χ2 = 4.36, p < 0.05 & χ2 = 3.18, p < 0.05 respectively). The overall sex ratio (male: female) of the pooled population of O. niloticus in this part of Albert Nile was 1:1.29 which was not significantly different from the hypothetical 1:1 (χ2 = 7.42, p < 0.05) but generally females were more abundant than males in the study area which is a good indication of high potential for replacement of stocks and sustainability of the fisheries resources in the river. The mean length at first maturity was 22.4 cm and 23.0 cm TL in females and males respectively while the number of eggs produced per female ranged from 412 to 2380 and the overall mean fecundity was 854 eggs per fish. The relationships of fecundity with body length and body weight were found to be linear and increasing with increasing fish length and body weight. Fecundity correlated more closely with body length (r = 0.88, p < 0.01) than body weight (r = 0.73, p < 0.01) but in both cases the relationship was significant (p < 0.01).