The influence of food web structure on the diet, mercury dynamics and bioenergetics of northern pike in reservoirs of the upper South Saskatchewan River Basin

This project investigated the influence of community structure in reservoirs on trophodynamics, growth and mercury biomagnification in the apex consumer and fisheries target species northern pike (Esox lucius). Where lake whitefish (Coregonus clupeaformis) were abundant in the prey community, pike were found to be piscivorous, consuming predominantly lake whitefish and other large-bodied fish species. Benthivorous feeding behavior was observed in reservoirs without abundant lake whitefish stocks, and amphipods formed the predominant prey consumed by pike. Growth rates of pike were found to be significantly lower under the benthivorous trophic regime. Benthivorous pike had lower mercury concentrations, consistent with lower dietary exposure, than piscivorous pike, but higher biomagnification consistent with low food conversion efficiency and slower growth. A bioenergetics analysis showed elevated feeding rates and activity levels in benthivorous pike on a diet of comparatively low energy density, relative to piscivorous pike, confirming a trophic bottleneck in response to trophic simplification and highlighting the importance of trophic coupling between pelagic resources and apex consumers in reservoirs.