Growth rates of alien Oreochromis niloticus and indigenous Oreochromis mortimeri in Lake Kariba, Zimbabwe

Abstract

Growth rates of indigenous Oreochromis mortimeri and alien Oreochromis niloticus from Lake Kariba were estimated from samples collected in 1997–2000, 2003–2005 and 2010–2011. Growth zones on scales and otoliths of O. niloticus and on the otoliths and opercula of O. mortimeri were deposited annually. Age estimates obtained from otoliths, scales and opercula were similar for both species. Length at age was described by the von Bertalanffy equations L(t) = 32.4(1 - exp(-0.25(t + 2.3))) and L(t) = 30.2(1 - exp(-0.23(t + 3.73))) for O. niloticus and O. mortimeri, respectively, in 1997–2000, and by L(t) = 44.6(1 - exp(-0.29(t - 0.05))) and L(t) = 36.8(1 - exp(-0.64(t - 0.73))), respectively, in 2003–2005. There were more older (≥10 years) age classes and lower L_∞ in the 1997–2000 dataset for both species than in the later sampling periods. Mean length at age of O. niloticus was higher than that of O. mortimeri for the 1997–2000 dataset, but similar for the 2003–2005 dataset. This study supports the hypothesis that, in Lake Kariba, the alien O. niloticus has a higher growth rate than O. mortimeri, and infers its competitive advantage over the indigenous species.