Population Dynamics of Two Cichlids, Oreochromis aureus and Tilapia zilIi, from Wadi EI-Raiyan Lakes, Egypt

Abstract

Age, growth, mortality, relative yield-per-recruit and relative biomass-per-recruit of Oreochromis aureus and Tilapia zilli collected from Wadi EI-Raiyan lakes during the period from August 2000 until July 2001 have been estimated. Age was determined from scale readings and the results showed that the maximum life spans of O. aureus and T. zilli were three and four years, respectively. The von Bertalanffy growth equations were L1= 27.15 (l-e'0.56(t+0.32) for 0. aureus and L1= 33.5 (l-e -0.49(t+0.15) for T. zilli. The instantaneous annual rates of total, natural and fishing mortality were calculated as 1.69, 0.25 and 1.44, respectively for 0. aureus and 1.1, 0.2 and 0.9, respectively for T. zilli. Exploitation rate was estimated as 0.85 and 0.82 for 0. aureus and T. zilli, respectively. Relative yield-per- recruit and relative biomass-per-recruit analysis indicated that the stocks of O. aureus and T. zilli were overexploited and that the current exploitation rate should be reduced by about 53% for 0. aureus and 55% for T. zilli to ensure that at least 50% of the individuals could be maintained for spawning and recruitment.