Dams in rivers modify the habitats and hinder dispersal and migration. Since moving around is an essential part of most organisms’ life histories, this represents a new regime for life in freshwater. This thesis addresses several issues that are either directly or indirectly related to fragmentation and aims to contribute to our understanding of living and coping in fragmented riverscapes. It contains studies conducted on different study species and several spatial, temporal, and ecological scales.

I first show that individuals in spawning migrating populations of cyprinid fish are phenotypically sorted along the length of a river with culverts. Results support the spatial sorting hypothesis, and are consistent between species, between sexes, and among individuals within sex; smaller and slimmer fish migrate further. I next show that eel ladders, which are passage solutions at dams aimed at increasing the distribution of European eel, did not remove the negative barrier effect of the dam.

Next, I show that the spatial configuration of distinct rapid-flowing habitats has significant impacts on the well-being of brown trout populations. Subpopulations in larger and closer located habitats were significantly denser and more stable, likely because of lower extinction rates and higher immigration rates. I further evaluated the effects of dams on spatial synchrony in populations of trout, Eurasian minnow, and northern pike; dams contributed to demographic isolation by decreasing synchrony in the two former species, but the effects of population synchrony on global population viability were weak.

Lastly, I show how the distribution and demography of the threatened freshwater pearl mussel is influenced by environmental heterogeneity and viability of host fish populations. Mussel populations residing in colder regions, and in locations with more viable host fish populations, had retained recruitment to a higher degree. The long-lived mussels exemplify how stress in aquatic environments can accumulate and manifest over time.

This thesis emphasizes in different ways that the spatial context in which individuals, populations and species move, distribute, and interact matters. Each study has important conservation implications regarding its study species, study system, or for the environmental aspect under scrutiny.