Do Mouth‐Infecting Parasites Alter the Foraging Ecology of Host Fish? A Test With the Parasitic Copepod <i>Salmincola markewitschi</i> and White‐Spotted Charr <i>Salvelinus leucomaenis</i>

ABSTRACT Many parasites infect host mouth cavities and are expected to physically impede host foraging. Despite these potential impacts, few studies have evaluated host foraging activities and their findings are equivocal. We examined the effects of the mouth‐infecting copepod, Salmincola markewitschi , on wild white‐spotted charr Salvelinus leucomaenis foraging using stomach contents analysis. Because the mouth‐infecting copepod occupies a large space in the host mouth cavity, we hypothesized that infected fish exhibit reduced stomach fullness and total prey consumed compared to uninfected fish. In addition, we also hypothesized that infected fish shift their main diets from terrestrial to aquatic invertebrates because parasites reduce host activity, body condition and competitive ability. Contrary to our predictions, stomach fullness and total prey consumed were not significantly different between infected and uninfected fish: smaller infected fish consumed more prey. We also found that smaller infected hosts foraged on a lower proportion of terrestrial invertebrates than their uninfected counterparts. Our results suggest that small infected fish increased foraging activities to compensate for energetic loss induced by infection. They also shifted their diet from large terrestrial to small aquatic invertebrates, possibly due to physical impediments and reduced competitive abilities associated with copepod infection. Our study indicates that mouth‐infecting parasites affect host foraging ecology, but the impacts are weaker than expected and depend on host body size. Nonetheless, behavioural shifts could have important ecological consequences such as loss of host fitness and strengthening trophic cascades in aquatic ecosystems.