• granulocytes;
  • immunity;
  • reproduction;
  • roach;
  • spawning;
  • spleen;
  • trade-off

Seasonal changes in immunocompetence are predicted by the hypothesized trade-off between reproduction and self-maintenance, whereby immune function is a measure of self-maintenance and reproductive effort is seasonally dependent. We examined seasonal patterns in immunological, haematological and body condition parameters for male and female freshwater fish. In two different populations, the relative size of the spleen and the chemotaxic migration activity of head kidney granulocytes decreased immediately before and after spawning, respectively. Those decreases were accompanied by an increase in haematocrit values shortly before and after spawning in both populations and a decrease in the relative body weight in one population, possibly due to physiological stress caused by increased activity during the spawning period. Breeding-related changes in other measures studied (phagocytosis activity of head kidney granulocytes, blood IgM concentration, white cell count and red cell count) were absent or inconsistent. Some of the results are in line with the idea of a trade-off between reproduction and immune defence. The present data suggest that the differential responses of the immune system may have different temporal patterns, which should be taken into account in immunoecological studies. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78, 117−127.