Methylmercury Bioaccumulation Dependence on Northern Pike Age and Size in 20 Minnesota Lakes

Abstract

Mercury accumulation in northern pike muscle tissue (fillets) was found to be directly related to fish age and size. Measurements were made on 173 individual northern pike specimens from twenty lakes across Minnesota. Best fit regressions of mercury fillet concentration (wet wt.) vs. length, weight and age were found using power (log-log transforms) as compared with exponential or linear forms. The resulting equations (after averaging the individual equations for the 20 cases) are: [Hg], ppb fillet = 0.50·(length, cm)2.17 = 7.3·(weight, g)0.68 = 110·(age, yr)1.00, with r2 = 0.76, 0.72, and 0.76, respectively. Lengths, weights, and ages ranged over 22-110 cm, 60-7,500 g, and 0.3-11 yr, respectively, and [Hg] ranged from 50 to 2,000 ppb. Inter-lake comparisons were made using computed mercury levels for standard length, weight and fish age for each of the 20 lakes studied. Age based comparisons were found to correlate significantly with methylmercury concentrations in lake water, and with total mercury concentrations in lake plankton. Methylmercury bioaccumulation factors (BAFs, total Hg in fillets divided by total methylmercury in lake water) ranged from 0.3 - 14 x106 for individual specimens. Methylmercury BAFs showed a similar dependency on fish size and age as [Hg] in fillet, i.e., methylmercury BAFs = 3,030·(length, cm)2.17 = 47,100·(weight, g)0.68 = 752,000·(age, yr)1.00, where values represent the means of twenty different lake equations. Fish mercury levels indicate that each lake differs in its own characteristics controlling bioaccumulation processes and transport through the aquatic food chain. Assuming no size or age dependency of fish mercury concentrations and BAF's may result in impact overestimates for younger fish and underestimates for older fish from mercury contamination in aquatic ecosystems.