Size-selective harvesting affects the immunocompetence of guppies exposed to the parasite Gyrodactylus

Harvesting is typically positively size-selective, targeting large
individuals. This is expected to lead to reduced average body size and
earlier maturation, i.e., faster life histories. Such changes can also
affect traits seemingly unrelated to harvesting, including
immunocompetence. The pace-of-life syndrome predicts that faster life
histories are correlated with decreased immunocompetence, i.e., a negative
association between positively size-selective harvesting and
immunocompetence. However, the energetic trade-off between early growth
and immunocompetence suggests the opposite pattern. Here, we empirically
evaluated these predictions using an experimental system consisting of the
ectoparasite Gyrodactylus turnbulli and lines of guppies Poecilia
reticulata that had been subjected to either positively, randomly, or
negatively size-selective harvest. We followed the infection progression
of individually infected fish for 15 days. We found significant
differences between the harvested lines: fish from the negative
size-selection lines had the highest parasite loads. During the early
phase of the infection, parasite loads were the lowest in the
positive-harvested lines, whereas the terminal loads were the lowest for
the randomly harvested lines. These results agree with the predictions
from the energetic trade-off hypothesis but contradict with the
pace-of-life syndrome ones. To our knowledge, this is the first
demonstration of the consequences of size-selective harvesting on
immunocompetence.