CHANGES IN FISH MERCURY CONCENTRATIONS IN AN INTENSIVELY FISHED LAKE

A small polyhumic Finnish lake inhabitated 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 Hg levels in the top predator, northern pike. A total catch of 29.5 kg.ha-1 (74% roach) was removed which represented c50% 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. -from Author