To study the infection dynamics of metacercariae of the digenean
Cryptocotyle lingua, wild living Atlantic cod, Gadus
morhua, were caged for 18 months close to the shore. Here they were
exposed to
naturally occurring transmission stages
of the parasite. First, both the abundance and the variance to mean ratio
of
metacercariae increased, but during the second
half of the study the abundance levelled out, and the variance to mean
ratio
showed a significant decrease. Host mortality
was negligible throughout the study. Based on the relationship between
pigment
spots and metacercariae observed by skin
digestion, there was no indication of density-dependent parasite mortality.
We conclude
that the infrapopulations of
metacercariae on the caged cod probably were regulated by density-dependent
host responses
acting against the cercariae.