The aging Mactaquac Hydroelectricity Generating Station (MGS) on the Wolastoq | Saint John River (SJR) is one of Canada's largest dams and it is reaching the end of its service life. My research focused on quantifying existing and future fish habitat downstream of the MGS, considering current management options to renew the infrastructure in the short-term with a longer-term solution of rebuilding or removing. In detail, my project applied a hydraulic-habitat model to assess habitat change and predicted effects on fish communities for future regulated and climate-induced flow regimes. Fish communities were surveyed and related to habitat characteristics both up- and downstream of the facility, and habitat requirements were defined for three distinct fish assemblages based on meso-scale habitat use and expert opinion. Modelling predictions suggest that dam operation and flow regulation resulted in a general decrease in habitat availability for each fish assemblage when being compared to the historic flow regime prior to the construction of the MGS. Furthermore, under the current dam operation scheme, rheophilic species were predicted to be limited in habitat conditions during the critical summer low flow period, with habitat availability averaging below 20% and never exceeding 30% of the wetted channel area at any day in the time record (1968 to 2015). The implementation of environmental flows ranging around the proposed Q50 flow rate was predicted to minimize the duration of stress events and increase the availability of suitable habitats on a community scale. Similarly, a future climate induced flow regime under a dam removal scenario was predicted to result in improved conditions for fish species. Recommendations informed by this thesis aim to improve habitat conditions for multiple species and are given to hydropower and fisheries managers. Proposed strategies include: i) implementing environmental flows ranging around a Q50 flow rate for the SJR, and ii) continuing fish community monitoring in the SJR and extending surveys to seasons other than the summer period to gain better insights into flow-ecology relationships of imperiled species.
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