To study the infection dynamics of metacercariae of the digenean Crypt
ocotyle lingua, wild living Atlantic cod, Gadus morhua, were caged for
18 months close to the shore. Here they were exposed to naturally occ
urring 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 varia
nce to mean ratio showed a significant decrease. Host mortality was ne
gligible throughout the study. Based on the relationship between pigme
nt spots and metacercariae observed by skin digestion, there was no in
dication of density-dependent parasite mortality. We conclude that the
infrapopulations of metacercariae on the caged cod probably were regu
lated by density-dependent host responses acting against the cercariae
.