الفهرس 
HybridizationOnly 14 pages are availabe for public view
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Abstract
Tilapias are constantly gaining importance in aquaculture. Today tilapias are found in virtually every country of the world. They are cultured as a food source in mono-or poly-fish culture practices with other species. Intensive studies have been carried out to improve the quality of the tilapias for domestic consumption and export. Hybridization between different species of tilapias is carried out mainly with the aim of achieving high yield at low cost, and to produce more attractive fish for the consumer. The aim of the present study is to evaluate six different combination crosses between three important tilapia species. The pure line species employed in these studies were: Nile tilapia (Oreochromis niloticus). N Blue tilapia (0. aureus). A Red tilapia (0. species). R Evaluation of the pure line species and the produced hybrids was achieved through two critical developmental stages; fry and fingerling stages. 1- Hybridization: During the present study two trails were conducted. The first trail was undertaken in indoor aquaria. All interspecific and intraspecific crosses were made in 100-liter glass aquaria. Fry obtained in aquaria were only from the pure bred species. Crossbreeding R x N, R x A, A x N and their reciprocal crosses failed to spawn in aquaria. The second trail was carried out in outdoors-concrete ponds. Ripe females and males of different tilapia species were selected and stocked at a rate of 4 breeders/m2, the sex ratio was 3 females to 1 male. Six different hybrids were tested and evaluated during the present study: 1-Female 0. niloticus x male red tilapia 2-Female red tilapia x male 0. nilotims 3-Female 0. aurezis x male red tilapia 4-Female red tilapia x male 0. aurezrs 5-Female 0. niloticzrs x male 0. azirem 6-Female 0. alirelis x male 0. niloticzrs For control: 1- Female 0. i~iloticzis x male 0. niloticus 2- Female 0. alrrezis x male 0. alrrerrs 3- Female red tilapia x male red tilapia. 2- Fry Production: The mean number of fry produced showed some variability among species. The mean number of fry produced by Nile tilapia females was 1.2 and 1.3 times as high as that of 0. azrrezis and red tilapia females, respectively. However, the differences in fry production of 0. aureus and red tilapia females when spawned with males of different species are insignificant. 3- Rearing of Larvae during Nursing Period: Groups of tilapia species and hybrids were grown for 90 days in concrete ponds. Stocking rates were 1000 Wpond. 0 Progeny of the intraspecific spawns and young fry of interspecific combinations showed that: (l)F1 hybrids produced by crossing female red tilapia x male Nile tilapia showed a superior growth in weight, length, specific growth rate and higher survival percentage than the pure line species. (2) Red tilapia females and blue tilapia males crosses showed similar trends. Growth of F1-progeny was much faster than that of pure blue tilapia (0. aweus). Comparing the hybrids with both pure line species, it has the highest survival rate, best-feed conversion ratio and high condition factor. (3) In contrast, when male red tilapia crossed with female Nile or blue tilapias survival percentage and feed conversion ratio were improved in the FI progeny. Meanwhile, it showed very poor growth in weight, length and specific growth rate when compared with those of pure line species, the differences were significant (PC 0.05). (4) Crossing female Nile tilapia and males blue tilapia produced the superior hybrid. F1 progeny possessed the highest gains in weight and length, highest survival percentage as well as best feed conversion ratio than their parental species. (5) The F1 progeny of the reciprocal hybrid (blue tilapia female and Nile tilapia male) exhibited intermediate inheritance for growth in weight and length, but it showed superiority in survival rate and good feed conversion ratio when compared with the pure line species, although having less values in condition factor. 4- Rearing of Fingerlings: At the end of fingerlings rearing period (90 days) the different hybrid combinations were compared for their growth performance. Growth of hybrids (female red tilapia x male 0. niloticus) was faster than of (female red tilapia x male 0. nureus). While that of (male red tilapia with female 0. ~~iloticzis or 0. alrrezrs) were the , poorest growth rates. The best of all was the hybrids resulted from crossing Nile tilapia female with blue tilapia male. It showed the highest gains in weight and length. Individual weight gains for the six groups were: 65.69, 55.52, 42.34, 34.22, 87.38 and43.36 gforRxN, Rx A, NxR, AxR, Nx A and A x N, respectively (female comes first). Survival percentage of the six hybrids were: 90.6%, 91.8%, 89.4%, 98%, 88.04%and97.4% forRxN, Rx A, NxR, AxR, Nx A and A x N, respectively. 5- Heterosis: (a) All hybrids exhibited positive heterosis values for all the investigated parameter except those of the cross breeding of females Nile or blue tilapia with males of red tilapia. Hybrids of female red tilapia and male Nile tilapia were significantly heavier and exhibited higher positive heterosis in total individual weight and length gains as well as specific growth rate. than those of female red tilapia and male 0. azireus The best of all was the hybrid produced from crossing female 0. niloticzrs with male 0. arirezis. This superior hybrid was significantly larger than its reciprocal, but growth of both was faster than that of the pure line species. Hybrids of female 0. azirezis with 0. niloticzis or red tilapia male showed the highest positive heterosis values for survival compared with those of the pure line species or other crosses (35.39 and 26.17 for A x N and A x R, respectively). This proves that these hybrids were the most cold tolerance (maternal effect). The hybrid appeared to inherit at least partly, the specific resistance of one of the parental species.