Synergistic effects of dietary iron and omega-3 fatty acid levels on survival of farmed Atlantic salmon, Salmo salar L., during natural outbreaks of furunculosis and cold water vibriosis

The present study demonstrates that farmed Atlantic salmon, Salmo salar, health is positively and significantly affected by synergistic effects between very long‐chain polyunsaturated fatty acids of the n‐3 family eicosapentaenoic acid/docosahexaenoic acid (EPA/DHA) and iron, where positive effects of high dietary levels of EPA/DHA are enhanced when combined with low levels of iron. Based on cumulative mortalities in the different experimental groups, relative percentage of survival (RPS) for the high EPA/DHA‐low iron group was 70% during an outbreak of furunculosis and 96% during an outbreak of cold water vibriosis compared with the controls. A non‐additive effect between EPA/DHA and iron was confirmed by statistical analyses that revealed a significant effect of EPA/DHA alone and an interaction of iron with EPA/DHA. Liver cell cultures treated with EPA/DHA revealed that the synergistic effect could be related to an EPA/DHA dependent regulation of mRNA for proteins important for transport (transferrin) and storage (ferritin) of iron in the salmon. In keeping with this finding, the transcriptional down‐regulation of iron metabolism in vitro was reflected in decreased in vivo iron stores with increasing levels of dietary EPA/DHA. Hence, to avoid overloading of the iron transport/storage‐systems resulting in increased susceptibility to bacterial infections, high levels of dietary EPA/DHA should be accompanied by low levels of dietary iron.