Is genetic resistance to AGD from a bath challenge-test a good predictor of genetic resistance from a field-test?

Summary

Estimates of genetic parameters for resistance to AGD were obtained based on gill-scores from a bath challenge test and a field test. In the field test, the amount of P. perurans on the gills was obtained by CT-qPCR. In the field test, the body weights were recorded at the time of gill-scoring. The body weight of a sib group reared in an AGD-free environment were recorded at harvest. The genetic correlation between gill-score in the bath challenge test and the field was close to zero. A bath challenge test for resistance to AGD can therefore not replace a field test in a selective breeding program. Efforts should be taken to develop a challenge test more similar to that the fish experience in a field test, e.g. a cohabitation test. In the AGD-affected environment the high genetic correlation of gill-score with CT-qPCR for P. perurans (rg=0.81 ± 0.16) and body weight (rg=-0.88 ± 0.09) indicate that CT-qPCR and growth may be used as indirect trait measures of resistance to AGD. The high genetic correlation between the body weights in the AGD-affected and the AGD-free test environment (rg=0.86 ± 0.05) indicate a true favourable genetic correlation between resistance to AGD and growth in Atlantic salmon. Consequently, selection for increased growth rate will result in a favourable genetic correlated response in resistance to AGD. These genetic correlation need to be verified in a similar experiment.