Age and Growth of Nile Tilapia, Oreochromis niloticus (Linnaeus, 1758), From Koka Reservoir, Ethiopia

The age and growth rate of the fish species must be known since they are essential input variables that compose stock assessment models to assess the stock status and estimate the exploitable potential of the stock. The age and growth of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), in Koka Reservoir, Ethiopia, were studied based on 981 sagittal otoliths collected monthly from December 2017 to November 2018. Otoliths were grounded and examined under a stereoscopic microscope at 10–40× magnification for macrozones analysis using a reflected light source. The age of the fish was determined based on the number of translucent zones counted in the otoliths of O. niloticus by considering the time of translucent zone formation and median hatch date. Seasonal records on the macrozones at the edge of otoliths and relative marginal increments suggested that two translucent macrozones associated with biannuli were formed each year, one from January to February, and another one from June to July. The observed ages of O. niloticus from Koka Reservoir ranged from 1 to 6.5 years, and the most dominant classes were 2 and 3 years. Growth was determined using the von Bertalanffy growth function. The von Bertalanffy growth parameters (sexes combined) were; asymptotic length (total length), L∞ = 35.6 cm, growth rate constant, K = 0.37 year-1, theoretical age at zero-length, t0 = -0.42 years and the growth performance length, φ’ = 2.67. It could be concluded that the growth of O. niloticus in Koka Reservoir is within the range mentioned in Ethiopian lakes.