Morphological variation and differentiation in GIFT strains Nile tilapia, Oreochromis niloticus

Morphological characteristics of fish can be used as important breeding indices.A population or a strain is often characterized with a uniform morphological shape while morphological variance and differentiation within the population or strain tends to be ignored.In every selectied generation of the GIFT(Genetically Improved Farmed Tilapias)strain Nile tilapia,Oreochromis niloticus,120-200 families are maintained and 20 thousandfish from these families are selected as the population for next generation selection.Due to so many different families in GIFT tilapia population,study on the morphological variance in the population is necessary so as to put the morphological variance into consideration when selective breeding is carried out.GIFT tilapia from sixty families were introduced to the Freshwater Fisheries Research Center of Chinese Academy of Fishery Sciences from the WorldFish Center in August,2006.In this study,the conventional morphological measurement was adopted to measure body weight,body length,body depth,body width,head length,caudal peduncle length and caudal peduncle depth of GIFT tilapia individuals from the 60 families.The morphological difference within GIFT tilapia population was surveyed by principle component analysis and correlation analysis.The results showed the highest correlation(R2=0.900 2)was observed between body depth and body weight in the GIFT tilapia population. Two bands were presented in the scattered plot of body length-body weight distribution in total population, female population,male population and every family population.Difference between sub-sums of body weight of individuals in the two bands was not significant(P0.05).The ratio of individual number in the two bands appromimate a constant with the value of 0.7.The scattered plots of body depth-body weight,body width-body weight,head length-body weight,caudal peduncle length-body weight and caudal peduncle depth-body weight distribution all present only one band with different varation range.The research results indicate morphological difference exists in the GIFT tilapias which implied genetic variance exists in the GIFT strain Nile tilapia population.The multi-trait selection program can be carried out in the GIFT strain Nile tilapia population.