Measured Immunocompetence Relates to the Proportion of Dead Parasites in a Wild Roach Population

1. Although various methods are used to measure immunocompetence, their relationship with the actual parasite clearance or parasite load is seldom demonstrated in natural systems. 2. We combined nine measures of immune function using principal component analysis (PCA), and examined the relationship of the collective measures with (i) the proportion of parasites killed by the host, (ii) the burden of several parasite species and (iii) a viral disease in a wild population of the roach, Rutilus rutilus. We also studied if these variables were associated with the concentration of steroids (testosterone and oestradiol). 3. Most significant correlations between the loads of ecto-and gill parasites and the measures of immunity were positive, suggesting either a parasite-induced immunoactivation or temporal covariation. When the temporal covariation was statistically removed, the functional aspects (respiratory burst, chemotaxis) of phagocytotic cells and the total concentration of immunoglobulin M correlated positively with the proportion of dead Rhipidocotyle campanula (Digenea) parasites. However, no relationships remained between the parasite loads and the measures of immunity or concentrations of the hormones. 4. The present results suggest that the measures of phagocytotic activity and IgM concentrations relate to an observable competence of roach to eliminate parasites. However, new experimental approaches are needed to reveal the causal mechanisms behind the sometimes ambiguous correlations.