Acquiring immunity through vaccination is energetically efficient in rainbow trout

<b>Background</b>Vaccination temporarily inhibits appetite in fish, but the physiological mediators of long-lasting growth impairments after vaccination are largely unexplored. Here, we hypothesized that immunization elevates basal energetic costs leaving less energy available for growth. In addition, we hypothesized that this energetic trade-off between immunity and growth would be associated with endocrine downregulation of growth mediated through the somatotropic axis. These effects were predicted to be long-lasting due to the antigen depot-properties of the mineral oil adjuvant present in the vaccine. To test these hypotheses, we compared whole-animal aerobic energetics and endocrine growth regulation in vaccinated and unvaccinated rainbow trout (<i>Oncorhynchus mykiss</i>) at 14-, 54-, and 89 days post injection.<b>Results</b>Contrary to our initial hypothesis, vaccination did not affect aerobic energetics at any point in the experiment. However, vaccination did reduce the gene expression of insulin-like growth factor I, which was reflected in a lower concentration of circulating hormone.<b>Conclusion</b>Together, these observations suggest that the putative energetic cost of immunological upregulation following vaccination is paid in part through endocrine downregulation of growth.