Variation in the intraspecific relationship between fish length and intensity of parasitic infection: biological and statistical causes

In a meta‐analysis, the overall mean correlation between fish length and the intensity of parasitic infections derived from 76 different host–parasite species was positive but weak and non‐significant, following corrections for sample size. Whether the parasites were acquired by ingestion or by skin contact had no influence on the strength of the relationship. For cestodes, larval digeneans, and gnathiid isopods, however, the mean correlation between fish length and intensity of infection was significant. Some statistical parameters influenced the strength of the raw correlations computed within samples and thus led to over‐ or under‐estimation of the true relationship. Sample size correlated negatively with the value of the correlation coefficients, whereas range in both fish lengths and intensities of infection correlated positively with the value of the correlation coefficients. Distinguishing between statistical noise and the biological processes shaping the size v. intensity relationship will be important if this relationship is to be incorporated into fish population models.