Hydroelectric power, mercury, energetics, and stress: Biological and toxicological implications for fish and fisheries

Abstract

Hydroelectric power is a critical component of the global energy budget and its capacity is projected to increase by 73% over the next 20 years, with much of this new capacity installed in the developing world. It is therefore important that we understand the impacts of these developments and how they might affect human and ecological health. Mercury accumulation in fish in reservoirs after dam construction is a well-researched and established phenomenon. While increased mercury concentrations have also been observed in fish downstream of dams , less is known about the dynamics of mercury in downstream sites, meaning there is unassessed historical risk to downstream wildlife, fisheries, and consumers. Further, hydropeaking that leads to fish strandings may represent a contemporary environmental stressor to downstream fishes. This could exacerbate mercury exposure in downstream fish by reallocating their energy from growth to addressing the stressor, resulting in higher mercury concentrations due to decreased growth dilution. This thesis explores the potential relationships between mercury and energy stores in fish up and downstream of a hydroelectric dam to determine historical relationships of fish mercury between these locations, and whether ongoing dam operations may exacerbate mercury concentrations in downstream fish. It also introduces a novel stress challenge protocol using a common minnow species in order to assess potential chronic environmental stress. Using historical records of commercial fishes from a reservoir and downstream fishery, I found rates of mercury decline were similar in fish populations within both sites since the 1970s. Yet where differences were noted, mercury consistently took longer to decline from downstream populations; mercury concentrations were also greater in fish immediately downstream of the dam relative to those from farther downstream despite minimal mercury concentrations (1-5 ng/L) in the water column. Higher mercury concentrations were also found downstream of the dam in a common minnow species (spottail shiner; “shiner”: Notropis hudsonius), and the same populations showed reduced energy stores relative to upstream fish in both August and September of 2014. Despite this connection, I noted minimal effects on fish condition, and there were no direct predictive relationships between fish mercury concentrations, energy stores, and condition. A newly developed acute stress challenge protocol also provided mixed evidence for the effects of hydropeaking on shiner stress responses. Glycogen concentrations were slightly higher iii in downstream fish from a site of concern in October compared with September, though minimal differences were found across time points, months or sites. Where patterns were observed, concentrations were highest within the first five minutes of capture and ultimately reached basal levels after 15 minutes. Minimal differences were noted in triglyceride concentrations across site, month, or time point. Cortisol secretion was successfully induced by the stress challenge as measured by whole-body cortisol concentrations, and the two upstream sites showed nearly identical patterns of cortisol concentration increase over time across months, with concentrations peaking ≈45-minutes post-challenge at both sites in both months. Minimal differences were noted across time points and sites within months, though cortisol concentrations in fish at a downstream site of concern were slightly elevated compared to upstream sites in September, and dropped significantly from September to October of 2015. While it is not possible to draw definitive conclusions about hydropeaking as a stressor based solely on these data, these results suggest that mercury concentrations may take longer to decline in fish populations downstream of dams, and that ongoing dam activities may be imparting direct or indirect effects on downstream fish that exacerbate long-term mercury concentrations. Finally, these results suggest that cortisol concentrations in shiner in response to an acute stressor may be successfully developed as a biomarker of chronic environmental stress.