OCCURRENCE OF NATURAL HYBRIDS BETWEEN NILE * TILAPIA, Oreochromis niloticus (L.) AND BLUE TILAPIA Oreochromis aureus ( Steind.) IN LAKE EDKU, EGYPT

Shnoudy Bakhoum
2002 Egyptian Journal of Aquatic Biology and Fisheries  
T he Nile tilapia {Oreochromis niloticus),b\vLt tilapia, {Oreochromis aureus) and specimens showing intermediate external appearance to both species were sampled from Lake Edku and examined for differences in morphological characteristics to determine the natural hybrids of these species. The results of univariate analyses indicated highly significant differences of biotnetric characters between the hybrid groups and parental species. Multivariate analysis of meristic and morphometric values
more » ... phometric values showed significant differences between all examined fish groups. The number of biometrics characters, which revealed significant differences between Nile tilapia and its hybrids, ranged from four to five out of seven meristic and twelve to fifteen out of twenty-one morphometric characters. Meanwhile, between blue tilapia and hybrids there were two to five out of seven and fourteen to sixteen out of twenty-one meristic and morphometric characters respectively. Also discriminate function analysis indicated that the biometric variables giving the most separation between the hybrid and parental species were dorsal fin spines, vertebrae number, pectoral fin length, postorbital length, head length and interorbital width. A classification to groups using the above functions derived from morphometric data yielded stronger separation than meristic counts for the examined groups. The results indicated the existence of natural hybridization between O. niloticus and 0. aureus in Lake Edku. NATURAL HYBRIDS BETWEE& NILE TILAPIA AND BLUE TILAPIA IN LAKE EDKI3 and Kolmorov-Simirnov test (Haddon and Willis, 1995) and multivariately using discriminant function analysis for the selection of the important variables (Henault and Fortin, 1989) . RESULTS Description of parental and hybrid specimens Straight or slightly convex head profile, black dorsal fin edge and black caudal fin edge with seven to twelve vertical bars characterize Nile tilapia (Boulenger, 1915) , Blue tilapia is distinguished by a straight head profile, pink or reddish dorsal fin edge and the caudal fin has a reddish edge but unmarked by any vertical bars (Trewaves, 1964). In the present study the hybrid specimens were differentiated morphologically to three forms: Hybrid 1 (HI) is characterized by a straight head profile, reddish dorsal fin edge and red caudal fin with one and few uncompleted vertical bars. Hybrid 2 (H2) is distinguished by a slightly convex head profile, reddish dorsal fin edge and red caudal fin with two or three complete and few uncompleted bars. Hybrid 3 (H3) is characterized by a straight head profile, black dorsal fin edges and a red caudal fin with two or three complete and few uncompleted vertical bars. Parental and hybrid specimens are sho.wn in Fig. 1 . For determination of the differences in fish abundance between parental and hybrid specimens, the study was based on random samples of 845 specimens (189, 77, 150, 267 and 162 for Oreochromis niloticus, Oreochromis aureus, HI, H2 and H3 respectively). The results indicated that hybrid specimens greatly dominated the tilapias {Oreochromis spp.,) catch-in lake Edku. The relative numerical abundance of parental and hybrid specimens revealed that hybrid 2 ranked at the top (31.60%) followed by 0. niloticus (22.37%), hybrid 3 (19.17%), hybrid 1 (17.75%) and O.
doi:10.21608/ejabf.2002.1754 fatcat:yjf2tzc2bzdknefrdzkf4xv35u