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Document Type

Thesis - University Access Only

Award Date

2008

Degree Name

Doctor of Philosophy (PhD)

Department

Wildlife and Fisheries Science

First Advisor

Steven R. Chipps

Keywords

south dakota, mercury, fishes, walleye

Abstract

Mercury (Hg) contamination in South Dakota fishes has become an important issue. In recent years, several lakes in South Dakota have been found to contain fish with elevated Hg concentrations and fish consumption advisories have been issued for Bitter Lake (Day County), Twin Lake complex (Highway 81; Brookings/Kingsbury Counties), Lake Isabel (Dewey County), Lake Hurley (Potter County), and most recently North Island Lake (McCook/Minnehaha counties). Presently, factors contributing to recent elevated Hg levels in fish are not well understood, although the combination of recent high water levels and increased atmospheric Hg deposition likely contribute to elevated Hg concentration in fishes. Thus, 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. Following several consecutive years of high precipitation, several fish populations in eastern South Dakota were found to contain fish with elevated (>1 μg/g-1) Hg concentrations. The discovery of elevated Hg concentrations in fishes was surprising because there were no apparent point-source inputs of Hg, and the limnological conditions of these lakes generally do not favor Hg methylation or bioaccumulation. 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. Diet analysis from Bitter Lake walleyes showed that consumption of fish prey (primarily fathead minnows Pimephales promelas) increased from June through October, and was congruent with increased Hg concentrations in walleyes entering the over-winter period. Ontogenetic patterns of Hg accumulation showed that mean Hg concentrations for the 2005 year class were elevated in May 2006 (0.2 μg/g-1) and 2007 (0.74 μg/g-1) compared to the previous fall (October, 2005 = 0.15 μg/g-1 ; October, 2006 = 0.46 μg/g-1), likely due to increased piscivory in early autumn and negligible growth over the winter 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. These findings indicate that seasonal and gender-related variability in Hg concentration are important and should be considered in standardized monitoring programs and when posting Hg-related fish consumption advisory notices. Cyclical climate patterns in the Northern Great Plains influence water availability, and hence surface area of many glacial lakes. Since the mid-1990s, the surface area of many natural lakes in eastern South Dakota has expanded, resulting in newly flooded vegetation, increased Hg methylation and elevated fish Hg concentration. Adult walleye in many of these lakes can have Hg concentrations exceeding 1 μg/g-1 and often exhibit limited natural reproduction. 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 (low Hg). Gonadal development, as measured by gonadosomatic index (GSI), was significantly lower in male fish from Bitter Lake than in Pelican Lake fish. Laboratory bioassays showed that fertilization success decreased significantly with increased waterborne MeHg concentration (extrinsic factor) and ranged from 65.1 % at 0 mg/l-1 to 27.6 % at 1 mg/l-1. Moreover, eggs fertilized with sperm from males with high Hg concentrations (mean Hg = 0.99 μg/g-1) had significantly lower fertilization success (30.1 %) than those from low Hg males (mean Hg = 0.05; 37.7 %). 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. Fish stocking has been proposed as one mechanism to reduce Hg levels in fish harvested from contaminated lakes. Sub-adult and/or adult fishes that are stocked into Hg-contaminated lakes generally have lower Hg concentrations than resident fish. In South Dakota, Hg-contaminated lakes are often stocked with walleye fry in an effort to augment the populations. However, other stocking strategies are available (age-0 fall fingerlings, age-1 fish) that might reduce contaminant burdens in fish once they attain harvestable size. 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. Simulated stocking strategies include fall stockings of age-0 fish and spring stocking of age-1 fish. Neither initial Hg concentration nor timing of fish stockings (i.e., age-0 fall fingerlings or age-1 spring stocking) affected Hg concentration in walleyes by the time they recruited to the fishery. Simulating a fall fingerling stocking with 0 μg/g-1 Hg (simulation day 65) revealed fast Hg accumulation through the overwinter period, ultimately matching our observed values (0.19 μg/g-1 Hg) by the following summer (simulation day 343). 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). Hence, 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.

Library of Congress Subject Headings

Fishes -- Mercury content -- South Dakota
Walleye (Fish) -- Mercury content -- South Dakota

Format

application/pdf

Number of Pages

141

Publisher

South Dakota State University

Rights

Copyright © 2008 Trevor M. Selch. All rights reserved.

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