High levels of mercury in biota of a new Prairie irrigation reservoir with a simplified food web in Southern Alberta, Canada

This study examined mercury levels in northern pike (Esox lucius) from the Twin Valley Reservoir in southern Alberta, 2 years after construction in 2003. The hypothesis was tested that mercury concentrations in pike from the reservoir are significantly higher than in pike from the nearby Oldman River. Mercury concentrations in muscle tissue (0.37-1.54 ppm) generally exceeded the consumption guideline of 0.5 ppm total mercury (THg), and were significantly higher (3.5-fold) than northern pike mercury concentrations in the Oldman River. In addition, these levels exceeded (up to 2-fold) previously published data from other reservoirs of the northern hemisphere. Gill-netting followed by stomach contents and stable isotope analysis revealed a very simple food web. No zooplanktivorous fish species were present, and the benthic fish community contained only few white sucker (Catostomus commersoni). Thus, the reservoir almost completely lacked forage fish, and the pike fed primarily on amphipods (Gammarus lacustris and Hyalella azteca), whose average mercury concentrations were 0.21 ppm. An observed low growth trajectory of northern pike in this reservoir may reflect low growth efficiency as a result of their invertebrate diet.