Induction of experimental furunculosis in heterogenous test populations of Atlantic salmon (Salmo salar L.) by use of a cohabitation method

The results described in this article are based on historical data. A total of 57945 Atlantic salmon (Salmo salar L.) smelts were tested in 12 cohabitation challenge tests with Aeromonas salmonicida, over a period of 5 years. Analyses of the 12 tests demonstrated that acute furunculosis with high mortality numbers was induced in all tests. However, the time from the first mortality among cohabitant fish to the first mortality among test fish varied by 5 days between different challenge tests, and the time from introduction of cohabitant fish to a level of 50% cumulative mortality among test fish varied by 19 days between the same tests. The reason for these variations in mortality pattern could be caused by variation in infectious load between test tanks but also due to the fact that the test groups compared were heterogenous both with respect to genetic origin and rearing conditions prior to challenge. As all families from one breeding farm were challenged in one large test tank, the applied challenge test was regarded as suitable to rank families of fish within one generation. The test system was regarded as less suitable to measure improvements in genetic resistance to furunculosis from one generation to the next due to a possible variation in challenge pressure between challenge tanks from different years. (C) 1998 Elsevier Science B.V.