Possible evidence for mortality induced by the parasite Apatemon gracilis in a population of brook sticklebacks (Culaea inconstans)

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Abstract
SUMMARY: Regular samples made on the 1978 cohort of brook sticklebacks (Culaea inconstans) from a swamp in Ile Perrot, Quebec, Canada were examined for Apatemon gracilis metacercariae. The prevalence of the parasite rapidly reached 100% in the fish population. The mean parasite burden increased from zero to a plateau of about 44 parasites/fish. Over-dispersion of the frequency distribution of parasites in the fish host, as measured by variance to mean ratios, increased to a peak and then decreased significantly while the mean parasite burden remained constant. The effects of parasite burden on the survival of the stickleback host are discussed, as well as the validity of the use of changes in over-dispersion for demonstrating parasite-induced mortality.