We examined relations between mercury concentrations in walleyes Stizostedion vitreum and the characteristics ofclear‐water Wisconsin lakes, which spanned a broad range of pH values (5.0–8.1) and acid‐neutralizing capacities (–9 to 1,017 μeq/L). Total concentrations of mercury in axial muscle tissue of walleyes (total length, 25–56 cm) varied from 0.12 to 1.74 μg/g wet weight. Concentrations were greatest in fish from the eight lakes with pH less than 7.0; concentrations in these fish equaled or exceeded 0.5 μg/g in 88% of the samples analyzed and 1.0 μg/g in 44%. In the five lakes with pH of 7.0 and above, concentrations exceeded 0.5 μg/g in only 1 of 21 walleyes. Multiple regression revealed that lake pH and total length offish accounted for 69% of the variation in mercury concentration in walleyes. Regression models with total length and either waterborne calcium or acid‐neutralizing capacity as independent variables accounted for 67% of the variation in concentration. The observed differences in fish mercury concentration between the low‐pH and high‐pH lakes could not be logically attributed to differences in growth rate or diet among the walleye populations. Moreover, it is improbable that mercury influxes to the low‐pH lakes were greater than those to the high‐pH lakes, because of the close proximity and spatial interspersion of low‐ and high‐pH lakes. We attributed the observed pH‐related trend in mercury concentration in walleyes to variation among lakes in within‐lake processes that affected the production and bioavailability of methylmercury.
