Stock assessment of Nile tilapia Oreochromis niloticus (Linnaeus 1758) in Lake Tana, Ethiopia

Nile tilapia Oreochromis niloticus is a commercially important fish species in the Lake Tana fishery and contributes to 65% of the total annual catch. This study estimated the maximum sustainable yield (MSY) and effort at maximum sustainable yield (f(MSY)) for the O. niloticus fishery using the Thompson and Bell yield prediction model. Catch and effort data from four representative landing sites were collected daily from June 2016 to May 2017 and used to estimate the total mortality coefficient (Z) using catch curve analysis, natural mortality (M) using Pauly's empirical formula, and fishing mortality (F) as F = Z - M. Population abundance was evaluated using the Jones length-based cohort analysis model. The estimated Z, M and F values were 1.1, 0.52, and 0.49 per year, respectively. In total, 5 077 tons of O. niloticus were produced, which exceeds the MSY of 4 904 tons per year that could be obtained from the fishery at an F-0.1. Consequently, the recommendation is to reduce the fishing effort by 15%. Collection of catch-and- effort data from more landing sites, over an extended period, could improve the yield estimates and this should be considered in conjunction with a bioeconomic analysis in future.