Ecosystem Engineering by Mussels ( <scp> <i>Margaritifera margaritifera</i> </scp> ) Influences the Behaviour of Their Host Fish Brown Trout ( <scp> <i>Salmo trutta</i> </scp> ) Under Various Flows

ABSTRACT Ecosystem engineers modify habitats in many ways with consequences for other species. Aggregations of autogenic ecosystem engineers in lotic systems, such as unionid mussels, can increase habitat complexity and change water flow, affecting the diversity and abundance of other benthic species. The effects of unionid mussel aggregations on mobile species, such as their host fishes, are less well‐studied, but the increase in habitat complexity caused by mussels may provide refuges from water flow and may change fish behaviour. Using stream flumes, we examined how the presence and density (zero/low density/high density) of the freshwater pearl mussel ( Margaritifera margaritifera ) and water flow (low/high) affected behaviour of three different size classes of young‐of‐the‐year brown trout ( Salmo trutta ) in May, June and August 2023, respectively. We found that small‐sized trout were more likely to reside on the bottom at high than at low water flow but increased the likelihood to swim when mussel density increased. Medium‐sized trout were more likely to reside on the bottom during high flow at zero to low mussel densities, while the proportions of swimming individuals increased regardless of flow at high mussel density. Large‐sized trout were instead more likely to reside on the bottom when mussel densities were high, and in contrast to the small‐ and medium‐sized trout, positioned themselves within the mussel zone more often. Our results demonstrate that freshwater pearl mussels can influence juvenile trout behaviour, probably by modifying habitat structure and flow conditions in the downstream area nearest to the mussels, particularly under hydrological stress. These findings underscore the role of mussels as ecosystem engineers and highlight potential feedback loops between freshwater pearl mussel presence and the behaviour of juvenile brown trout, which may be highly relevant for conservation strategies targeting both species.