Investigating immune status of the Mediterranean sardine in response to seasonal fluctuations in food resources

Since 2008, Mediterranean small pelagic fish populations, particularly sardines Sardina pilchardus, have shown shrinking and adult overmortality, likely driven by bottom-up processes. Although energy availability has declined, adults still invest heavily in reproduction, possibly at the expense of other traits such as maintenance, growth and survival. Such trade-offs may alter their immune status, making individuals more prone to diseases. Here, we experimentally investigated the effects of seasonal feeding conditions (winter vs. summer) on sardine immune status. We provided the first description of leucocyte populations in sardines, including both non-specific and adaptive immune cells. Feeding conditions did not highly affect proportions of immune cells. However, the oxidative status of leucocytes was modified over time, likely reflecting seasonal changes, including energy re-allocation mechanisms during reproduction. These findings suggest that poor feeding conditions do not necessarily affect negatively immune status, but this requires confirmation through immune challenges.