Does glochidia infestation of the freshwater pearl mussel (Margaritifera margaritifera) affect foraging behavior and growth in brown trout (Salmo trutta)?

Fish serve as hosts to a wide range of parasites, which can affect them in many ways, including their survival, growth, and foraging behavior. The highly threatened unionid mussels have an ectoparasitic life stage on the gills, skin, or fins of fish. Here, we compared prey handling time and growth between groups of brown trout (Salmo trutta) that were infested and non-infested with glochidia larvae of the freshwater pearl mussel (Margaritifera margaritifera). In a prey handling test, individual trout were presented with one large prey (maggot) and one small prey (Gammarus pulex) daily over eight days. Compared to non-infested trout, prey handling time was longer for infested trout feeding on larger but not on smaller prey. In a growth test, where trout were fed daily with minced calf liver over nine weeks, the growth rate was lower for infested than for non-infested trout. Moreover, the growth rate was higher for infested trout from which glochidia were expelled compared to trout from which glochidia were not expelled. We suggest that glochidia infestation can increase prey handling time of fish, ultimately reducing fish growth, but that slow prey handling may be transitory for fish losing glochidia, and result in compensatory growth of the fish. Slow prey handling, in combination with a negative effect of glochidia infestation on food competition between infested and non-infested trout, may be reasons behind the lower growth of infested fish.