The hypothesis that interindividual differences in the activity of brown trout alter the exposure to parasitic freshwater pearl mussel glochidia was tested in a Swedish stream. Wild yearling brown trout (N = 103) were caught, individually tagged for identification and scored for open‐field activity during standardized laboratory tests in June. Fifty gravid freshwater pearl mussels were relocated to the stream, where after the trout were released back into the stream. The fish were recaptured in October (N = 35), checked for glochidia encystment (infested individuals: n = 6) and re‐scored for open‐field activity traits. Swimming velocity during the test was higher in fish infected with glochidia, suggesting that high activity could increase their exposure to glochidia. Potentially, as metabolism rate and ventilation rate typically increase with activity, elevated activity may lead to an increased likelihood of glochidia passing over the gills. This novel finding suggests that glochidia infestation is non‐random and that the behaviour of the host fish can influence the likelihood of glochidia infestation.

Movement activity levels of wild animals often differ consistently among individuals, reflecting different behavioral types. Previous studies have shown that laboratory-scored activity can predict several ecologically relevant characteristics. In an experiment on wild brown trout Salmo trutta, spanning from June to October, we investigated how spring swimming activity, measured in a standardized laboratory test, related to relative recapture probability in autumn. Based on laboratory activity scores, individuals clustered into 2 groups, which showed contrasting patterns in the size-dependency of their recapture probability. Size had a slightly positive effect on recapture probability for passive fish but a clear negative effect on active fish. Our results show that the population structure in a cohort, in terms of relative proportions of behavioral types in different size classes, can vary over time. The results of this study could depend on either selective mortality or migration. However, selective disappearance of individuals with specific phenotypes, regardless of the mechanism, will have implications for trout population management, such as stocking efficiency of hatchery fish with high growth rates or maintenance of fishways past migration barriers.