Adaptability, growth and reproductive success of the Nile tilapia, <i>Oreochromis niloticus</i> L. (Pisces: Cichlidae) stocked in Lake Small Abaya, South Ethiopia

Abstract

Despite the high contribution of the introduced Oreochromis niloticus as source of food and additional income to the local community, little has been documented on the adaptability and reproductive success of stocked fish in Ethiopia. This study was therefore, conducted to examine the adaptability and reproductive success of O. niloticus stocked in Lake Small Abaya in 1997. Monthly fish samples were collected from two sampling sites (open water and littoral area) between September, 2002 and August, 2003 using gillnets of different mesh size (60 mm, 80 mm, 100 mm and 120 mm stretched mesh size) and beach seine of 20 mm mesh size. The size of O. niloticus captured ranged between 4 and 33 cm TL (mean ± s.d., 17. 9±6.99 cm TL) and from 1 to 659 g TW (mean ± s.d.. 148.3±134.9 g TW). Males were more numerous than females (χ² = 5.61, p< 0.05) in total catch as well as in samples taken in February (χ² = 7.45, p< 0.05) and August (χ² = 6.64, p< 0.05), whereas females significantly dominated the catch in January (χ² = 4.13, p< 0.05). Individual O. niloticus with ripe gonads were caught throughout the year however; their frequency in the catch varied a great deal with season. Based on the proportion of ripe gonads and mean gonado-somatic indices (GSI) the main breeding season of O. niloticus in Lake Small Abaya occurred between December and May peaking in January, February and March. A minor peak was also observed in June, July and August. The mean monthly Fulton´s condition factor (FCF) of O. niloticus in Small Abaya also varied significantly between months (ANOVA, p<0.05). However there was no significant difference in FCF between sexes (ANOVA, p>0.05). Calculated length-weight relationship of the fish was found to be curvilinear (TW = 0.0174TL^2.99, R² = 0.99, n = 1676). Generally, O. niloticus in Lake Small Abaya has established a breeding stock which can serve as source of inexpensive fish protein and income for the local community.

Keywords/phrases: Breeding season, Fulton’s condition factor, Lake Small Abaya, Length-weight relationship, O. niloticus, Sex ratio

Ethiop. J. Biol. Sci., 10(2): 153-166, 2011