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Nile tilapia, Oreochromis niloticus, is a widely distributed and economically most import fish species in Ethiopia. Effective management is essential to sustain their fisheries and the benefits for the local communities. However, little is known about the population dynamics of the Nile tilapia stock in Lake Langeno. Vital parameters of its population were determined using length frequency data collected from 5,949 specimens ranging from 8.5 to 35.7 cm total length (tl). These specimens were collected from August 2017to July 2018. The population parameters were determined using the elefan I routine in FiSAT software. Estimated von Bertalanffy growth parameters were asymptotic length (L∞) = 35.70 cm, growth curvature (K) = 0.32 yr-1, age at length zero (to) = -0.49, and growth performance index (Փ') = 2.61. Instantaneous total mortality (Z), natural mortality (M), and fishing mortality (F) rates were determined from length-converted catch curve and empirical models, respectively, and their values obtained were Z = 2.31 yr-1, M = 0.82 and F =1.56 yr-1, respectively. The exploitation rate (E) of Nile tilapia computed from its mortality rates was 0.67 yr-1, suggesting state of overfishing. The size at first capture (Lc) was estimated at 14.0 cm which is much lower than the size at first maturity (Lm=16.62 cm), which further substantiated the state of overfishing. The results of the study are very useful for fishery managers and scientists who wish to manage and further explore the Nile tilapia stock in Lake Langeno.

The study aimed to determine the diet composition, dietary change and diel feeding pattern of Oreochromis niloticus, in Lake Langeno. Fish specimens were collected monthly from July 2017 to June 2018 using a beach seine with 40 mm mesh size and scoop-net with small mesh size (1.0 mm). A total of 264 non-empty fish stomachs were examined for dietary analysis. The relative importance of different food items was analyzed using frequency of occurrence (%FO) and numerical abundance (%N). The most frequently encountered food items in the stomach of O. niloticus were phytoplankton, zooplankton, macrophytes and detritus. Among the phytoplankton groups, the genera Microcysts (%FO = 85.25), Cyclotella (65.51), Anabaena (%FO = 50.0), Cymbella (%FO = 65.15), and Chlorella (%FO = 54.17) contributed the most. Numerically, Cyanophyta, Chlorophyta, Bacillariophyta, and Zooplankton contributed 5.2%, 3.4%, 53.03% and 42.28%, respectively, to the total food items. Insect and fish remains occurred in rare occasions in the stomach content of all length groups of fishes. The dominance of animal-based food items progressively decreased as the fish grew to larger size while the importance of macrophytes and detritus increased. O. niloticus feeds mainly during the day time and stomach fullness peaked at 16:00 hr. The daily food consumption rate and gastric evacuation rate were 9.0% body weight and 48% per hr, respectively. Crude protein dominated nutrient composition of the fish and this could be attributed to the high percentage in frequency of nutritious diets like Rotifers (64.0%), Cladoceras (63.6%) and Microcystis (85.6%). It can be concluded that juvenile Oreochromis niloticus feeds mainly on animal food sources whereas adults feed primarily on macrophyte food items.