| Summary: | Naturally flowing rivers form a longitudinal gradient of physical conditions to which fish communities are adapted. Hydroelectric dams disrupt the river continuum, resulting in alterations to downstream hydrologic and thermal characteristics. Changes in physical conditions downstream from hydroelectric dams can have a variety of effects on local fish populations. However, the tendency for biological responses to be species- and system-specific complicates the development of broadly applicable management strategies. Therefore, it is necessary to conduct long-term, large-scale studies on the impacts of river regulation under different hydroelectric dam operating regimes, and to investigate the impacts on multiple species within a given system. In this thesis I report data from two long-term, large-scale field studies (in northern Ontario and northern Norway), and investigate the impacts of river regulation on downstream fish. Specifically, the effects of river regulation on an important recreational fish, Salvelinus fontinalis, the forage fish community (Cottus cognatus, Rhinichthys cataractae, and Percopsis omiscomaycus), the coldwater fish guild, and native-invasive species interactions (Coregonus lavaretus and Coregonus albula, respectively) are studied. Indicators of fish health used to assess the effects include growth, condition, survival, thermal habitat and field metabolism. Potential driving forces such as changes to river discharge and water temperature are investigated to identify the causal mechanisms behind the effects on fish health. Fish growth was higher in a northern Ontario river with a 15 MW hydropeaking dam, relative to a nearby naturally flowing river, regardless of the dam operating regime. Condition and survival varied between and among species, and between the regulated and naturally flowing river. S. alpinus exhibited a higher field metabolic rate in the regulated river, which was positively correlated with time spent hydropeaking. The higher growth in the regulated river was likely a ...
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