Influence of Season, Sex, Age and Diet Composition on Mercury Concentration in Walleye Sander vitreus

We collected Walleye Sander vitreus (May-October) from Bitter and Twin lakes, South Dakota to assess seasonal- and diet-related variation in tissue mercury (Hg) concentration. The average Hg concentration in Walleye was 43-68% higher in the spring for Bitter (p < 0.008) and Twin Lakes (p < 0.017) compared with summer or autumn months. Bioenergetics analysis of Bitter Lake Walleye showed that consumption of fish prey (primarily Fathead Minnow Pimephales promelas) increased from late summer through winter and was linked to increased Hg accumulation in Walleye the following spring. Mercury concentration varied significantly with Walleye age but was similar for comparably-aged male (0.62 μg/g) and female fish (0.62 μg/g). However, after adjusting for Walleye size (total length, mm), mean Hg concentration was greater in male (0.66 μg/g) compared with female (0.50 μg/g) fish, likely due to slower growth rate of male Walleye. At 425 mm, male Walleye in Bitter Lake were approximately 1 year older than female fish. These findings show that diet, age, and gender-related growth affect Hg concentration in Walleye and are important factors to consider in fish contaminant monitoring programs.