Absence of noticeable mercury effects on fish populations in boreal reservoirs despite threefold to sevenfold increases in mercury concentrations

At the La Grande hydroelectric complex (Quebec, Canada), total mercury concentrations were measured in more than 25 000 fish over a 20-year period. Fish population characteristics, such as fishing yield, growth rate, condition factor, and recruitment, were also monitored. In reservoirs, total mercury concentrations in all species increased rapidly after impoundment, peaking after 4 to 9 years in nonpiscivorous fish and after 9 to 11 years in piscivorous species, at levels three to seven times those measured in surrounding natural lakes, then declined gradually and significantly. Despite this increase, most species showed increases in fishing yields (by factors ranging from 2 to 8), growth rates, and condition factors (for more than a decade). The percentage of small specimens of the main species was generally maintained or increased in the first years after flooding, indicating good recruitment.