Assessment of sustainable yield and optimum fishing effort for the tilapia ( Oreochromis niloticus L. 1758) stock of Lake Hawassa, Ethiopia

The tilapia ( Oreochromis niloticus , L. 1758) stock of Lake Hawassa, Ethiopia, was assessed to estimate sustainable yield (MSY) and optimum fishing effort (fopt) using length-based analytical models (Jone's cohort analysis and Thompson and Bell). Pertinent data (length, weight, catch, effort, etc.) were collected on a daily basis for 514 days during 27-12-2003 to 24-05-2005 at the major landing site known as Fish market or AmoraGedel . The sampling days fell into two periods demarcated by the date (08-04-2004) when a management action was implemented, which reduced fishing effort by half (from1954 gillnets/day to below 800 nets/day). Thus, data obtained during 27-12-2003 to 08-04-2004 pertained to the period prior to effort reduction whereas the rest pertained to the period after the reduction. The two data sets were analyzed separately to get a basis to evaluate the effect of the management action of effort reduction. The assessment gave an estimate of current yield of 526.8 t/year for the period before effort reduction whereas 441.6 t/year afterward. The predicted MSY was 514.5 t/year and 441.6 t/year for the period before and after effort reduction, respectively. The respective F-factor is estimated to be 0.5 and 1.0.This suggested that the fishing effort before the reduction of effort (1954nets/day) was very high and, as already implemented, should have been reduced by half(i.e., an F-factor of 0.5).Therefore, the implemented management measure to reduce effort below 800 nets/day is appropriate. Likewise, since the estimated MSY of441.6 t/year for the period after the reduction would be obtained at an F-factor of 1.0, it was concluded that the current level of fishing effort of 696 gillnets/day can be maintained as fopt for sustainable exploitation of the stock. Keywords : Stock assessment, MSY, Optimum fishing effort, Analytical yield prediction model, Lake Hawassa, O.niloticus , Ethiopia