Impaired growth after immunization with a bivalent oil-adjuvanted vaccine occurs without detectable changes to aerobic capacity in juvenile rainbow trout

The trade-off between growth and immunity in fish has been assumed to relate to energetic costs associated with immune functions. Here, we hypothesized that immunization with a bivalent mineral oil-adjuvanted vaccine elevates standard metabolic rate and inhibits growth through changes in the growth hormone-insulin-like growth factor I system in rainbow trout (Oncorhynchus mykiss). These effects were predicted to be long-lasting due to antigen deposition in the adjuvant. To test these hypotheses, we compared whole-animal oxygen uptake and endocrine growth regulation in vaccinated and unvaccinated rainbow trout at 14- and 54 days post injection. Contrary to our initial hypothesis, vaccination did not affect aerobic metabolism at any point in the experiment. However, our data indicate that vaccination inhibited length growth without affecting feed intake, which appeared to be mediated through reduced production of insulin-like growth factor I. In conclusion, our data agree with previous research showing that immune function is energetically cheap in rainbow trout and indicate that reduced growth after vaccination is decoupled from aerobic capacity.