Risk of parasitism and microhabitat selection in juvenile sticklebacks

Abstract

In laboratory experiments, we tested the following hypotheses: (i) that juvenile threespine (Gasterosteus aculeatus) and blackspotted (Gasterosteus wheatlandi) sticklebacks face higher risks of parasitism by the ectoparasite Argulus canadensis in shallow, vegetated microhabitats and (ii) that they can alter their microhabitat distribution in response to parasites. Reducing water depth increased the number of parasites per infected fish in G. aculeatus and increased the percentage of G. wheatlandi infected. For both species, intensity of infection and percentage of fish infected were greater among fish inhabiting vegetated microhabitats than among fish living in open microhabitats. In the absence of fish, A. canadensis swam near the bottom and in the vegetation. Fish of both species swam near the bottom in the absence of the parasites, but swam near the surface in the presence of parasites. Without parasites, both species of fish preferred to swim in vegetation. With parasites present, the spatial distribution of G. aculeatus changed only slightly, whereas G. wheatlandi became evenly distributed between open and vegetated microhabitats. These results suggest that microhabitat selection by juvenile sticklebacks may serve to reduce the risk of parasitism.