Luan Dinh Tran defended his PhD thesis on March 11th 2010

Ane Gro Siri Skjelfjord

Genetic studies of Nile tilapia (Oreochromis niloticus) for farming in Northern Vietnam: Growth, survival and cold tolerance in different farm environments

The present work explores genetic parameters for growth, survival and cold tolerance of tilapia in different farm environments in northern Vietnam. The results are important for further development of selective breeding programs for O. niloticus. It was found that the GIFT strain grew significantly faster than the Viet strain in freshwater, but a smaller strain difference was observed in brackish water. The estimated genetic parameters and genotype by environment interactions were based upon; firstly, harvest body weight and survival recorded in fresh and brackish water (Paper I); secondly, harvest body weight recorded in two tank environments with controlled temperatures (O=optimum and L=low temperatures) and in a natural pond environment (N=natural) located in the mountain province (Paper II); and thirdly, cold tolerance recorded in short-term cold challenge tests, harvest body weight and growth during overwintering (Paper III). Genetic parameters for weight gain (WG), relative weight gain (RWG) and specific growth rate (SGR) were also estimated in this study (Paper II). Their estimates were similar to those determined for harvest body weight with respect to magnitude and direction.

The results indicate some genotype by environment interaction for harvest body weight and survival recorded in brackish- and freshwater. The genetic correlations between records in the two environments were low for both harvest body weight (0.45±0.09) and survival (0.42±0.05). Furthermore, relatively high genetic correlations between the two traits within each test environment were estimated (0.73±0.05 and 0.67±0.07 for fresh and brackish water environments, respectively). The genetic correlations between harvest body weight records in the different temperature environments were higher, (0.88±0.19 between O and L, 0.78±0.10 between L and N, and 0.61±0.05 between O and N), indicating a low genotype by environment interaction (Paper II). The estimated genetic correlation between harvest body weight (HW) and growth during overwintering (OW) was high (0.94±0.05) (Paper III), while low but favourable genetic correlations were estimated between HW and cold tolerance measured in the challenge test; 0.17±0.18 with cooling degree hours (CDH), -14±0.17 with temperature at death (TAD), 0.05±0.07 with survival (SUR) and 0.07±0.10 with survival at 50% mortality (SUR50) respectively. Similarly, low and favourable genetic correlations were estimated between OW on one hand and CDH (0.23±0.26), TAD (0.26±0.18), SUR (0.12±0.07), and SUR50 (0.12±0.09) on the other. The results indicate that selection for harvest body weight may result in a high and favourably correlated response in growth during overwintering, and a low but favourably correlated response in cold tolerance.

The heritability estimates for harvest body weight and growth were 0.24±0.04 for freshwater, 0.19±0.06 for brackish water, 0.31±0.08 for controlled O tanks, 0.19±0.04 for controlled L tanks, 0.26±0.09 for N pond environment and 0.18±0.06 for growth during overwintering. The estimated heritability for pond survival was 0.27±0.04 and 0.20±0.06 for freshwater and brackish water environments, respectively. The heritability estimates for traits measured in the cold tolerance test varied from 0.07 to 0.26. This study indicates that substantial additive genetic variation exists for the growth traits and pond survival in different farm environments that can be exploited through selective breeding. The heritabilities estimated for cold tolerance were lower and less accurate. They do, however, indicate a potential for using selective breeding to improve cold tolerance. It can also be concluded that selection for higher harvest body weight does not have any negative consequences for survival at harvest, growth during overwintering or cold tolerance. Additionally, a focus on improved husbandry practices and management will be important to improve cold tolerance performance of O. niloticus.

It is recommended that the ongoing breeding program for tropical freshwater farming of tilapia in Vietnam should include harvest body weight and survival. In addition, it is suggested that a separate breeding program be implemented in order to improve growth and survival in brackish water aquaculture. In order to support tilapia farmers in the highlands of northern Vietnam, one should also consider to include cold tolerance in the selection criterion for freshwater farming. It is further recommended that a sample of each full-sib group may be tested for growth and survival in a mountain pond in addition to family testing in the on-going breeding program for freshwater farming. This would make it possible to select broodstock from the best families in the mountain pond to then be disseminated to hatcheries in the highland areas. Further research is recommended to validate and possibly improve the cold challenge tests for genetic improvement of cold tolerance.

Published: 12.04.10
Updated: 13.04.10
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