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  • WATERLINE - August, 2023

    Ecology reports on fifteen years of monitoring bass mercury levels in Washington lakes

    by Callie Mathieu, Washington State Department of Ecology

    Since 2005, the Washington State Department of Ecology (Ecology) has been monitoring mercury concentrations in edible fillets of freshwater fish collected from lakes and reservoirs across the state. Each year we analyze fillet tissue from ten individual largemouth or smallmouth bass from six waterbodies and then re-sample those sites every five years to evaluate trends in mercury levels over time. A recent Ecology publication reported trends observed by the monitoring program between 2005 and 2019.

    As of 2019, Ecology has sampled all 30 sites in the program three times, five years apart. Statewide trends over 15 years of sampling revealed no change in bass mercury concentrations for the majority of sites over a five-year and ten-year period (65% and 63% of sites, respectively). Mercury concentrations decreased in one-third of the sites over a five-year period and in 19% of sites over a ten-year period. Few waterbodies saw increases in bass mercury concentrations, with increases in 7% and 15% of the monitoring sites over the five-year and ten-year period, respectively. Monitoring programs in other areas of the United States have found similar stability in fish mercury levels over the past two decades.

    Figure 1. Standard-length (350mm) mercury concentrations estimated for bass fillets collected on Ecology’s first sampling visit (2005-2009), second visit (2010-2014), and third visit (2015-2019).

    Over the 15 years of monitoring, the general spatial pattern of mercury contamination in bass across Washington state has remained consistent. Figure 1 displays estimated mercury concentrations for a 350mm bass calculated using log10 mercury:length linear regressions for each waterbody. In general, standard-length 350mm mercury concentrations have remained elevated in western Washington compared to eastern, central, and southwestern areas of the state. After the first round of sampling (2005-2009) at all study locations, we found that mercury bioaccumulation closely followed rainfall patterns in the state, with sites receiving the most rain containing the highest levels of mercury in bass. Most of the spatial variance in the statewide dataset can be explained by annual watershed precipitation (increase in bass mercury) and lake alkalinity (decrease in bass mercury). This pattern remained consistent through all three study periods, with the wet forested sites containing the highest bass mercury concentrations and the alkaline lakes in agricultural or arid open spaces containing the least.

    More information on the monitoring program – as well as reports, data, and information on methods — can be found on Ecology’s website. If you have questions, contact Callie Mathieu at callie.mathieu@ecy.wa.gov