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