Spatial and Temporal Variability of Mercury in Upper and Lower Red Lake Walleye

Author

Tyler Orgon

Date of Award

1-2022

Document Type

Thesis

Department

Biology

First Advisor

Andrew Hafs

Abstract

Mercury is a global pollutant that is released into our environment by natural and anthropogenic processes resulting in extensive studies conducted on mercury cycling in aquatic ecosystems which has led to the issuance of human-health-based fish-consumption advisories. We examined total mercury concentrations in Walleye Sander vitreus from Upper and Lower Red Lakes, located in north central Minnesota, between 2019 and 2020. Upper and Lower Red Lake form a contiguous water body consisting of two large (483.1 and 665.6 km2, respectively) basins that are naturally connected by a 1.4 km-wide strait. Both basins are important fisheries for recreation, subsistence, and commercial fishing for the Red Lake Band of Chippewa Indians. The eastern half of Upper Red Lake is also an important sport fishery for non-Tribal anglers in Minnesota. Sampled Walleye (n = 265) ranged from 158 to 610 mm in total length from an age range of 0 to 16 years. Mercury concentrations within the Red Lakes’ Walleye ranged from 0.030 mg/kg to 0.564 mg/kg (x̄ = 0.179 ± 0.105 mg/kg; x̄ = mean ± sd, all fish-mercury concentrations expressed on wet-weight basis). The best supported model for predicting mercury concentrations in Red Lake Walleye included the independent variables: length, age, sex, and lake basin. This model indicated that basin was an important predictor variable for estimating Hg in Walleye from Upper and Lower Red Lake. This model also suggests that individuals who rely on fish for subsistence should target Walleye that are ≤ 400 mm from Lower Red Lake. With no physical barriers between the Lakes to prohibit migration, observed differences in mercury concentrations could be linked to the differences in wetland area, fish growth rates, and physicochemical parameters between the two basins. Spatial variability of mercury showed that Upper and Lower Red Lake exhibit fish-mercury concentrations comparable to other large lakes within the region after adjusting for length as the covariate. Given that basin was an important predictor variable for estimating fish-Hg concentrations, future pollutant monitoring efforts should treat Upper and Lower Red Lake as separate lakes and not assume that data from one basin can apply to the other. This will be important over a longer time scale as ecosystems respond to changes in mercury emissions and other environmental changes.

Recommended Citation

Orgon, Tyler, "Spatial and Temporal Variability of Mercury in Upper and Lower Red Lake Walleye" (2022). Biology Graduate Theses. 3.
https://pines.bemidjistate.edu/thesis-biol/3