DECREASE IN MERCURY CONTENT OF FISHES IN BALL LAKE, ONTARIO, SINCE IMPOSITION OF CONTROLS ON MERCURY DISCHARGES

Weight, fork length, and white muscle mercury concentrations of walleye (Stizostedion vitreum vitreum), northern pike (Esox lucius), and lake whitefish (Coregonus clupeaformis) from Ball Lake, northwestern Ontario, were compared for 1971, 1972, and 1976. The lake had received mercury from 1962 to 1970; since then there have been increasingly rigorous controls. Adjusted mean white muscle mercury concentrations for walleye at the group mean length of 444 mm were 1.99, 2.71 and 1.39 mg/kg, respectively, for the three sample years. Corresponding values for pike 594 mm long were 5.05, 5.72 and 1.80 mg/kg. Whitefish values for 1972 and 1976 were 0.62 and 0.42 at 441 mm fork length. The changes are discussed in relation to the decreased input of mercury to the lake and it is suggested that mercury in suspension may be a controlling factor.