First Selection for IPN-resistance in Rainbow Trout

Nr. 1 / 2009

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Eyed eggs from the Aqua Gen population of Rainbow Trout stripped in 2009 are the first generation with parents selected for increased resistance for IPN. Using selective breeding to reduce the extend of the disease is new within commercial breeding programs.

The prevalence of IPN in Rainbow Trout

Systematic breeding for improved resistance to IPN (Infectious Pancreatic Necrosis) in Atlantic salmon has been performed by Aqua Gen since 2001. After three generations a considerable selection response has been achieved for this trait. In Norway, a total of 14 IPN outbreaks were reported in Rainbow Trout stocks in 2008. Twelve of these were reported to be in fresh water (National Veterinary Institute). From August 2008 IPN is no longer a notifiable disease, and this may affect the number of outbreaks reported. For Chile, no statistics are available that shows the prevalence of the disease, but we do know that the virus is most problematic from the first feeding up to about five grams.

Increased resources invested in breeding 

Healthy yolk sack fry of Rainbow Trout

Healthy yolk sack fry of Rainbow Trout

Since 2006 Aqua Gen has increased the use of resources in the Rainbow Trout breeding program. Number of traits recorded has increased and three separately bred populations have been crossed into one breeding population (breeding nucleus) using the same model as for Atlantic salmon. The advantages of this amalgamation of three populations to one are increased selection intensity and reduced variation for the customer from year to year. Together with the crossing of the three existing populations an additional strain (Strain B) has also been included in the testing.

IPN challenge test

200 families of first fed fry (0.2 grams) were challenged with IPN virus in separate tanks (Fig. 1). At the end of the test the accumulated mortality was 26.5 %. The average mortality between families ranged from 0 to 87 %.
Three families were randomly chosen to be tested in four replicates each (Table 1). As shown in the Table 1 different families showed stable results, indicating a good repeatability of the test.

 

Replicate Family 1 Family 2 Family3
1 2 43 15
2 1 28 17
3 1 35 15
4 1 40 13
Average 1.25 36.5 15

Genetic variation 

Figure 1. IPN challenge testing of 200 families in separate tanks.

Figure 1. IPN challenge testing of 200 families in separate tanks.

The data from the IPN challenge test were analysed to see if there was genetic variation between families for IPN resistance. The results from the analyses showed a heritability of 0.29. This means that 29 % of the variation in resistance to IPN is genetic. Based on this result it is possible to achieve helpful improvements for IPN resistance through genetic selection. Families used for breeding in 2009 were selected by using the IPN challenge result at the fry stage, including only those with an average mortality of 4 % or less.

Differences between strains

The fish tested originated in three different year classes of Rainbow Trout from Aqua Gen and two different year classes of strain B. None of the year classes had previously been selected for IPN resistance. Figure 2 presents the results for the crossings of the different year classes. As can be seen from the Figure, the crosses Aqua Gen x Aqua Gen had less mortality than the other crosses. Differences in mortality between AG03 and strain B were found to be significant.

 

Figure 2. Variation in mortality between the different crossings in the IPN challenge test. AG03= Aqua Gen population first fed 2003, AG02= Aqua Gen population first fed 2002, AG01= Aqua Gen population first fed 2001, AG00= population first fed 2000, Strain B= strain from an external breeding program.

Figure 2. Variation in mortality between the different crossings in the IPN challenge test. AG03= Aqua Gen population first fed 2003, AG02= Aqua Gen population first fed 2002, AG01= Aqua Gen population first fed 2001, AG00= population first fed 2000, Strain B= strain from an external breeding program.