We examined the effect of a dam on concentrations of metals (Mn, Cu, Cd, Zn, and Hg) in white sucker (Catostomus commersoni) and in northern pike (Esox lucius). Fish were caught upstream and downstream of the Moses-Saunders Power dam on the St. Lawrence River near Cornwall, Ontario, and Massena, New York. Upstream of the dam, Lake St. Lawrence is a 136-km(2) reservoir resulting from the flooding of 90 km(2) of agricultural land in 1958, and downstream is Lake St. Francis. In northern pike, no difference in the mean concentrations of metals was found between upstream and downstream fish. Hg concentrations increased with length in upstream and downstream pike, whereas Zn showed an inverse relationship with length downstream. For a particular length, Hg concentration was higher in downstream northern pike. On average, white sucker were five times more contaminated by Hg upstream than downstream. The levels of Mn and Cu were also found to be higher upstream. No bioaccumulation of metals was observed in white sucker. Based on the results obtained for the northern pike, after 37 years, it seems that the high metal concentrations observed in Lake St. Lawrence after the creation of the reservoir have now disappeared. We believe that the high level of contamination of the white suckers in this lake is linked with this species' habit of being in constant contact with the sediments, which could be locally highly contaminated.
