Length-weight relationship, fulton's condition factor and size at first maturity of Tilapia, Oreochromis Niloticus L. in lakes Koka, Ziway and Langano (Ethiopian rift valley)

Abstract

In Ethiopia, the tilapia, Oreochromis niloticus , is an extremely important fish from ecological, economic and social point of view. Information on biological parameters like the size at first maturity (L50), Fulton’s condition factor (K) and length – weight relationship are very crucial for the sustainable management of the fishery. Therefore, the present study aims at providing information on the length-weight relationship, condition factor and size at first maturity (L 50 ) of Oreochromis niloticus in Lakes Koka, Ziway and Langano. Fish samples were collected on monthly basis from October 2005 to February 2006. Length-weight relationships (total length (L) in cm; total weight (W) in g were calculated for O. niloticus in Lakes Koka ( W = 2.61*10 -2 L 2.89 ; n = 205; r 2 W = 1.04* 10 −2 L 3.19 ; n = 279; r 2 = 0.97) and Langano ( W = 1.64*10 −2 L 3.04 ; n = 525; r 2 = 0.88). The average K value of O. niloticus from Lake Koka (1.87) is slightly higher than Ziway (1.81) and Langano (1.84). The size at first maturity (L 50 ) of O. niloticus is smaller in Lake Ziway (Female =18.1cm, male=19.6cm) and Langano (both sexes 19.5cm) than Lake Koka (both sexes 23.5cm) which is contrary to the generally accepted principle that fish mature earlier in smaller water bodies than larger ones. Hence, the nutritional status and the level of fishing pressure appear to be important factors for the variations observed in the present study. Keywords/phrases: Condition factor; Lakes Koka, Ziway, Langano; L50; Length-weight relationship; O. niloticus Ethiop. J. Biol. Sci ., 7(2): 139-157, 2008