Changes in Fish Mercury Concentrations in an Intensively Fished Lake

A small polyhumic lake inhabited by northern pike (Esox lucius), burbot (Lota lota), perch (Perca fluviatilis), roach (Rutilus rutilus), and eel (Anguilla anguilla) was fished intensively during spring 1984 and 1985 to lower the high mercury levels in the top predator, northern pike. A total catch of 29.5 kg∙ha−1 (74% roach) was removed which represented about 50% of the total fish biomass. Mercury concentrations in burbot, large pike, and small roach had decreased by 1987 and 1988, but increased in young pike and some perch. Northern pike had the clearest increase in growth rate, while that of perch and roach levelled off within 3 yr at the prefishing level. Only a small increase in growth rate of burbot occurred. The yearly accumulation of mercury in northern pike remained constant, and the decrease in [Hg] was apparently due to growth dilution. The decrease in [Hg] of burbot and roach is explained by a switch to a diet with lower [Hg] or a decrease in water methylmercury concentrations. The amount of methylmercury removed from the lake by fishing was equivalent to several years of calculated mercury methylation and accumulation in the fish. Overfishing may be a feasible means of lowering methylmercury levels in this type of oligotrophic lake.