Elements and omega-3 fatty acids in fishes along a large, dammed river.

Damming of a river can trap and elevate levels of sediment-bound elements and alter food web dynamics in created reservoirs. It follows that dams may alter how elements and other nutrients, like the beneficial omega-3 fatty acids (n-3 FAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are accumulated in fish and thus the chemical composition of species above and below this barrier to migration. This study examined the spatial and species differences in contaminants and nutrients in fish from the Wolastoq | Saint John River (New Brunswick, Canada) in association with a large hydroelectric dam (Mactaquac Generating Station; MQGS), a river which supports both recreational fisheries and subsistence fishing by Indigenous communities. In 2020 and 2021, Smallmouth Bass, Yellow Perch, American Eel, and Striped Bass were collected from locations upstream (reservoir and river) and downstream of the MQGS and analyzed for mercury (Hg) and 30 other trace elements, n-3 FAs, δN, and δC. Fish from the reservoir were highest in the beneficial elements P, S, and K, while fish from upstream of the reservoir had lower levels of toxic elements, including Hg. The dam appeared to alter food web dynamics, as fish from the reservoir and immediately downstream of the dam had higher δN and reservoir fish were depleted in δC. DHA and Hg were positively corelated with δN, and EPA in Smallmouth Bass was higher in sites where fish had higher δC. Overall, this study suggests that the dam altered food web dynamics and the uptake of contaminants and nutrients by fish, and that location and species are important factors when examining the risks and benefits of consuming wild fish from a system impacted by a large dam.