Age and growth of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) from Lake Tana, Ethiopia

Abstract

Age and growth of Nile tilapia, Oreochromis niloticus, from Lake Tana were studied from June 2016 to May 2017. In total, 1 597 otoliths were collected from three representative sampling sites for macrozone analysis. Otoliths were ground and examined under a dissecting microscope at 7–40× for the presence of translucent and opaque macrozones using a reflected light source. The age of the fish was determined from the total number of translucent macrozones counted in the otoliths considering the time of translucent zone formation and median hatch date. Two peaks of translucent macrozones were observed in January and February and in June and July. The mean marginal increment was found to be highest (73–85%) in November, December and May, and lowest (0–9.3%) in January, February, June and July. The von Bertalanffy Growth Function was fitted to observed length and age, with L_∞ = 45.1 cm, K = 0.21 y⁻¹ and t₀ = −0.514 years. The growth rate for O. niloticus from Lake Tana was lower than that previously reported for Ethiopia, warranting additional studies to determine causative factors that would explain the differences.

Keywords: biannuli, growth rate, Lake Tana fishery, macrozone, otolith