Stock Assessment of Nile Tilapia (Oreochromisniloticus, L.) in Gilgel Gibe Reservoir, Omo-Turkana Basin, Southwest Ethiopia

Stocks assessment is the first step to determine the level of tolerable exploitation for arriving at maximum sustainable yields from fish resources. Assessment of stocks and study of impact of present level of exploitation on exploited stocks are necessary for maintenance of stocks at maximum sustainable level. A stock assessment of Gilgel Gibe fisheries has never been conducted despitethe reservoir’sfishery importance to the local people. In the presentstudy, length based stock assessment approach was used in order to estimate vital parameters such as fish growth and mortality. A length-based Thompson and Bell model (Thompson and Bell, 1934) was used for analysis of yields and long-term yieldpredictions. Nile Tilapia, Oreochromisniloticus, is one of important commercial fish in Africa and other tropical regions. InEthiopia, it accounts for more than 60% of the total annual landings.A total number of 25,994 specimens of Nile tilapia werecollected from March, 2020 to July, 2021. Stock assessment software FISAT II was used to compute and analyse all the basic parameters and functions. The estimated current annual yields were 251.07 tons /year. The average annual recruitment in the reservoir was 121,955 fish. The mean value of growth parameters The L∞ and K values were determined to be 46.67cm and 0.280, respectively, at Rn value of 0.174. The fish growth performance index (Φ'), computed from the fish growth rate and the asymptotic length, was 2.79. Estimation of current rate of exploitation of yield and biomass was 6.48 tons and 28.12 tons respectively. Therefore harvest level should be decrease from maximum sustainable yield to maintain and recover highly depleted biomass.The stock biomass at the current level of exploitation has already declined to 26.32% of the biomass at the unexploited stock. Therefore, there is an urgent management need to regulate fishing effort in order to safeguard the resource and thereby ensure its sustainability and the livelihood of the community that depends on it for its livelihood.