STOCK ASSESSMENT AND ESTIMATION OF OPTIMUM YIELD FOR THE TILAPIA STOCK (OREOCHROMIS NILOTICUS) OF LAKE HAWASSA, ETHIOPIA

This study was conducted on Lake Hawassa, one of the series of Ethiopian rift valley lakes. Data were collected on a daily basis for 514 days (from 27-12-2003 to 24-05-2005) from the area where fish were landed at one major landing site called fish market, locally known as Amora Gedel. The sampling regime falls into two periods, namely prior to reduction of the fishing efforts on the lake (i.e., 27-12-2003 to 08-04-2004) and after reduction of the fish efforts (i.e., 10-04-2004 to 24-05-2005). Length composition of Tilapia caught by the fishery, total Tilapia yield and fishing effort expanded were the basic information collected from the site. The aim of the study was to estimate the population number of Tilapia and fishing mortality coefficient by length group as well as predict the maximum sustainable fish yield and biologically optimum level of fishing pressure for the Tilapia stock of the lake. Jones length based cohort analysis model and length-based Thompson and Bell yield prediction model were employed to estimate the maximum sustainable yield with the corresponding biologically optimum fishing effort level.  The estimated current annual yields before and after reduction of fishing effort on the lake were 526.8 tons and 441.6 tons/year of Tilapia, respectively. Based on data collected before and after reduction of the fishing pressure on Lake Hawassa, the predicted values of Optimum Sustainable Yield were 514.5 tons/yr and 441.6 tons/yr, respectively, and these are obtained at fishing mortality factor of 0.5 and 1.0, respectively. Hence, the fishing effort expanded before reduction of the fishing effort (i.e., 1954 nets/day) was very high and it should be reduced by a factor of at least 0.5. Accordingly, the measure taken to reduce the fishing effort to below 800 nets/day was appropriate. The analysis on data collected after reduction of the fishing effort indicated that the recommended safe level of exploitation is at an F-factor of 1 and the effort can be maintained at its current level of 696 nets/day.