Effects of Hydroelectric Dam Operation on Downstream Fish Populations
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 result of system-specific food increases resulting from impoundment, hydropeaking, or a combination of both, while the varying responses in condition and survival were likely driven by species and life-stage specific differences in behaviour, access to food and increased energetic costs associated with daily hydropeaking. Thermal habitat differed among the two coldwater species evaluated and is likely related to species-specific temperature preferences and behaviour. Hydrologic and thermal indices explained little of the variation in fish growth, likely as a result of both the indirect and interacting effects associated with altering river discharge and temperature. In a regulated system in northern Norway, the availability of different thermal habitats influenced the success of the invasive C. albula. Stable isotope evidence suggested that thermal habitat partitioning was occurring in a site where C. albula and C. lavaretus coexist, while dietary resource partitioning was occurring in a site where C. albula were outcompeting C. lavaretus, relegating them to the littoral zone. This thesis highlights the variation in biological responses to river regulation amongst species and within systems, providing evidence for the species-specific effects of hydroelectric dam operation. The potential for both direct and indirect impacts, and the complexity of biological responses within the forage fish community, the coldwater fish guild, and between native and invasive species, necessitates the use of multiple species and multiple indicators of fish health to thoroughly characterize the effects of river regulation on fish species. Given the different habitat and temperature preferences and behavioural patterns exhibited within the fish community, it is important to manage river environments not just for specific thresholds, but to ensure the availability of a variety of different flow and thermal habitats. Maintaining the availability of a variety of habitats within the riverine environment should be considered as an important component of river management strategies.