Impact of river regulation and hydropeaking on the growth, condition and field metabolism of Brook Trout (Salvelinus fontinalis)

Brook Trout (Salvelinus fontinalis) is an important fish species in Ontario, Canada, supporting recreational fisheries that contribute significantly to local economies. Hydroelectric dams disrupt the river continuum, altering downstream conditions and impacting riverine fish populations. Specifically, Brook Trout activity has been found to increase during hydropeaking periods, when dam operators rapidly increase river discharge to meet electricity demands. Higher energetic outputs driven by hydropeaking may decrease the energy available to allocate towards fish growth and condition, negatively impacting Brook Trout. We investigated the impact of two different hydropeaking regimes on resident Brook Trout populations downstream from a 15-MW dam used for hydropeaking, compared to a population in a nearby naturally flowing river. Length-at-age as determined by otolith back-calculations was higher in the regulated river relative to the naturally flowing river. Muscle tissue caloric content and weight-length relationships did not differ between rivers. Field metabolism, as inferred from fish otolith C-13 values, was higher in the regulated river relative to the naturally flowing river and was significantly positively related to time spent hydropeaking. Higher metabolic outputs in the regulated river were likely offset by an increased food supply, allowing for higher Brook Trout length-at-age. The opposing and complicated impacts of river regulation on Brook Trout highlight the need for studies to consider multiple indicators of fish health when characterising the response of fish populations to river regulation.