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

Abstract

The 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 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.