46 

 6.3 Mercury in Cookson Fish 



Mercury levels in fish in the Cookson Reservoir have been measured on five occasions, 

 In 1979, 1983 and 1991 by Environment Canada (Water Quality Branch 1980, Munro 

 1985, Shaw 1991, in prep) and in 1980 (Waite et al.) and 1984 by Saskatchewan 

 Environment and Public Safety. Samples were collected to assess the safety of the fish 

 in the reservoir for human consumption. These populations are confined to Cookson 

 Reservoir and as such could only travel downstream as a result of a high runoff event. 

 As suggested by Waite etaj. (1 980), based on observations from other reservoirs flooding 

 throughout the world, high initial mercury levels in fish would be expected as a 

 consequence of organic decay and methylation of sediment-borne metal. Elevated 

 mercury concentrations in fish tissues would decline to pre-flooding background levels as 

 organic matter was assimilated. 



Mercury concentrations in edible tissue of two size-classes of walleye ( Stizostedion 

 vitreum) from 1979 to 1991 (Figure 6.9) followed the expected pattern. Mercury burden 

 in edible tissue of smaller classes of walleye (25-35 cm) declined rapidly from a median 

 concentration of 0.85 mg/Kg in 1979 to 0.29 mg/Kg in 1983. Likewise, the mercury 

 concentration in larger fish (35-50 cm), while remaining higher than in the smaller fish, 

 declined from 1 .32 mg/Kg in 1 979 to 0.42 mg/Kg in 1 984. Mercury concentration of white 

 suckers (Catastomus commersoni) from Cookson reservoir was low (median 0.25-0.29 

 mg/Kg Hg) and showed a slight but significant decline from 1979 to 1991 (Kruskal-Wallis 

 2-sample test, p=0.61). 



