Length-based growth and stock characteristics of the Nile Tilapia Oreochromis Niloticus (linnaeus, 1758) in Lake Tana, Ethiopia

Abstract

Age, growth and stock characteristics of the Lake Tana Nile tilapia (Oreochromis niloticus) were studied from 2000 to 2006. Fishery independent data from the Bahir Dar Fisheries and Other Aquatic Life Research Centre (BFALRC) and 12 months commercial data of the motorized boat from Fish Production and Marketing Enterprise (FPME), Bahir Dar branch, were analysed. Commercial length frequency and catch data were recorded and more than 5000 samples were taken from experimental fishery. Individual fish in the experimental fishery were caught monthly during February 2000 to December 2004 and bimonthly during January 2005 to December 2006. The size of O. niloticus in the commercial landings ranged from 13 to 41 cm total length (TL). The length at first capture (LC) was 26.95 cm and about 53% of the fish in the commercial catches measured were <27 cm. Length-based stock assessment using the FISAT software package showed an asymptotic length (L∞) of 44.2 cm TL, growth coefficient of 0.43 yr-1 and t₀ = ˗0.33296 yrs. The estimated maximum age (longevity) was 6.64 years. These results gave a growth performance index (ø) of 2.92. The total mortality coefficient was estimated to 1.53 yr^-1, a natural mortality of 0.63 yr-1 and fishing mortality of 0.9 yr^-1. The estimated total mortality which is in relative terms, was considered average (1.53 yr^-1), coupled with the currently observed exploitation rate of 0.59 for O. niloticus, estimated from the mortality rates, suggested that the species was maximally exploited. The estimates of MSY stood at 155.11 MT by Cadima’s formula.

Keywords/phrases: Growth, Lake Tana, Oreochromis niloticus, Stock assessment.