To better understand factors related to Hg concentration in fish and the potential impacts on walleye populations in South Dakota, this study addressed the following objectives: (1) develop empirical models for predicting Hg concentrations in South Dakota fishes; (2) quantify seasonal and gender-related in Hg concentration of walleyes Sander vitreus; (3) evaluate relationships between Hg concentration and reproductive condition of walleye; and (4) model effects of different stocking strategies on Hg accumulation in walleye.
Using data on watershed characteristics, water quality attributes, and state-wide Hg sampling, I explored relationships between physicochemical attributes of lakes and Hg concentrations in walleye. I found lakes that experienced the greatest change in surface area between wet (2000) and dry (1987) years contained walleye with the highest Hg concentrations. The results suggest that Hg contamination of walleyes and other sport fishes in Prairie Pothole lakes should be monitored regularly, particularly after water levels increase. Surface area change may prove to be a reliable predictor for identifying lakes with a potential risk of Hg contamination.
Walleye were collected seasonally (May-October) from Bitter and Twin Lakes in eastern South Dakota to assess seasonal variation in muscle tissue Hg concentrations. Mercury concentration of walleye, adjusted for length, was significantly higher in the spring for both Bitter (P<0.008) and Twin Lakes (P<0.017); average Hg concentrations were 43 to 68% higher in spring than in summer or fall months. Comparisons between male and female fish showed no differences in Hg concentration when evaluated by length (mm), weight (g) or age. However, confidence intervals (95% CI) plotted around regression lines for male and female walleye did not overlap for fish 425-510 mm and 850-1200 g, suggesting that larger male walleye contain more Hg than females.
To evaluate the influence of Hg concentration on walleye reproduction, I compared seasonal reproductive characteristics between two walleye populations—one that exhibits low Hg concentration (Pelican Lake, mean Hg = 0.05 μg/g -1) and one that has high Hg concentration (Bitter Lake, mean Hg = 0.99 μg/g-1). Mean monthly concentrations of estradiol-17β and testosterone for both male and female walleye were suppressed in fish from Bitter lake (high Hg) compared to Pelican Lake because waterborne Hg concentration in lakes with elevated fish Hg were within the range where fertilization success declined, Hg concentrations could negatively impact fertilization and reproduction in walleye.
To evaluate the influence of different stocking strategies, I tracked the 2005 cohort of walleyes, stocked as fry, in Bitter Lake, South Dakota for 2.5 years and modeled Hg accumulation using a bioenergetics model. After calibrating the model to empirical field data, I simulated the effects of different stocking strategies on Hg accumulation in adult fish. Stocking walleyes the following spring did not prolong differences in the recovery of Hg concentrations. Simulating age-1 walleye with 0 μg/g-1 Hg stocked the following spring (simulation day 274) revealed faster Hg accumulation than observed from an October stocking, converging with observed Hg concentration (0.19 µg/g-1 Hg) on the same date (simulation day 343). Stocking options appear to provide little-to-no benefit at reducing Hg burdens. Rather, Hg concentration of prey, seasonal variation in prey use, and activity level appeared to strongly affect Hg burdens in walleye. Future studies exploring the relationship between prey types and activity costs, and potential effects on Hg accumulation are necessary to better understand Hg dynamics in contaminated populations. (Abstract shortened by UMI.)