Abstract
It is of great importance to increase the natural disease resistance of farmed fish. Response to direct selection for increased survival in commercial environments has been low, basically due to inaccurate records of the trait. However, challenge tests with family groups which were infected with specific pathogens showed significant genetic variation in mortality, indicating that improvement by selection is possible. Several studies have shown a significant positive genetic correlation between growth rate and survival. Therefore survival would be expected to increase as a correlated response when selecting for growth rate. Significant genetic variation has been found in immunological parameters, but more knowledge is needed about the genetic correlations between these parameters and survival before indirect selection for increased survival can be applied. Heterosis for survival has not shown large effects. However, it may be still of interest to develop resistant lines to specific diseases and cross them to produce hybrids. Disease resistance genes have not been identified in fish. However, the production of transgenic fish with enhanced resistance to specific diseases remains as a possibility for the future.
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