Seasonal changes in the immune activities of common carp (Cyprinus carpio)

Various endocrinological and immunological parameters involved with the seasonal changes of immunocompetence in common carp, Cyprinus carpio reared under natural conditions were investigated. Plasma concentrations of steroid hormones, IgM levels, IgM-secreting cell (IgMSC) numbers, and water temperature were considered as major parameters. Testosterone, 11-ketotestosterone in males and estradiol-17β in females detected by RIA, increased simultaneously with gonadal maturation, but no significant correlation was found with IgM production measured by ELISA. IgM levels were very high during the spawning season, but did not fluctuate much during the other seasons even at high water temperatures. These changes of IgM are certainly related to sexual maturation. In addition, IgMSC numbers were also high during the spawning season. This positive interaction between endocrine and immune parameters could contribute to the immune systems of common carp being more effective during spawning season. Interestingly, cortisol levels increased with the water temperature and were high during the spawning season, but had no suppressive effect on the IgM levels and IgMSC numbers.