Mercury in fish tissue

Mercury Trends in Freshwater Fish is a long–term, statewide study on mercury levels in edible fish in 30 Washington waterbodies. We collect freshwater fish from six waterbodies every year for mercury analysis and use the data to assess changes and estimate trends in mercury across the state. We provide data to the Department of Health to update state fish consumption advisories.

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Why we monitor mercury

Methylmercury cycle in freshwater lakes infogram — How mercury enters fish as methylmercury in freshwater systems.

Mercury is toxic in any form and can persist in the environment. In Washington, most people are exposed to mercury through eating contaminated fish.

Our long-term monitoring study was spurred by the Chemical Action Plan (CAP) for mercury (2003), a collaboration between our agency and the Department of Health to reduce mercury in the state.

The Mercury Trends in Freshwater Fish Quality Assurance Project Plan (QAPP) and QAPP addendum define our project objectives and the procedures for monitoring mercury trends in freshwater fish.

Mercury trends

Since 2005, we've collected mercury data from freshwater fish to assess whether concentrations are decreasing or increasing. We use the data from annual sampling events to track trends in a waterbody over time. As we finalize results, we update our online StoryMap with the latest mercury trends.

Every five years we complete a full sampling cycle of all 30 waterbodies. Data from each cycle is used collectively to compare trends across the state. Measuring Mercury Trends in Freshwater Fish in Washington State: 2017-2019 Sampling Results (2023) is our latest cumulative report.

Where we monitor mercury

Today, we monitor lakes and rivers across the state in a variety of landscapes and land uses. We select waterbodies that can support a stable community of largemouth or smallmouth bass and for their site accesssbility, the fish collection potential, any history of mercury contamination, and their distance to urban areas and local point sources of mercury.

Waterbodies	Previous sample years	Next sample year
Banks, Failor, Kitsap, Pierre, Snake, Whatcom	2009, 2014, 2019	2024
Liberty, Loon, Potholes, Silver, Spokane, Yakima	2005, 2010, 2015, 2020	2025
Mason, Meridian, Moses, Newman, Offut, Sammamish	2006, 2011, 2016, 2021	2026
Deer, Fazon, Lower Goose, Ozette, Samish, St. Clair	2007, 2012, 2017, 2022	2027
Goodwin, Horsethief, Leland, Loomis, McIntosh, Nahwatzel	2008, 2013, 2018, 2023	2028

Monitoring locations

Use this map to find study IDs for all studies in each waterbody. Use these study IDs to search for data in our EIM database. To explore data further, view our StoryMap: Mercury trends in freshwater fish.

Our field crew in action

Two scientists check their gear before fishing.

Gear check!

Scientist dip netting for fish off a boat at night

Electrofishing is easier at night with bright lights.

This smallmouth bass looks like a keeper.

An electrofishing boat sitting on a trailer at a boat launch.

Good eletrofishing conditions at Leland Lake.

Landscape view of Horsethief Lake facing the shore.

Another beautiful morning at the office (Horsethief Lake 2023).

Our work day is just beginning at Lake McIntosh (2023).

Not every fish is a keeper. This largemouth bass was too big and was released.

Getting stuck in the weeds at Lake McIntosh.

A scientist sets up the anode boom on an electrofishing boat.

This might look like a broken umbrella, but the anode array is an important piece of our electrofishing equipment.

Scientist collecting a rainbow trout from a dip net.

Knocking out a rainbow trout.

How we monitor mercury in fish

Collecting fish

Our collection period is in the fall, and we are often limited to one or two sampling events at each waterbody. To make sure we meet the goals for the study we collaborate with Washington Department of Fish and Wildlife and employ a few different collection methods on our own.

This is our primary collection method. A generator-powered electric system creates an electric field at controlled levels at the front of the boat. This temporarily stuns the fish so we can scoop them out with dip nets.

This method is efficient, allowing us to collect multiple fish at once, ensuring we reach our 60+ collection goal within our timeframe. It's also our best method for fish recovery. We optimize our settings and exposure time so stunned fish recover quickly before release.

Scientists remove a yellow perch from a gill net on the bow of the electrofishing boat.

We use gill nets when we need additional species of fish or if the lake or river is too large or too deep for electrofishing. Electrofishing is only effective in shallow waters less than five feet deep, limiting us to shorelines.

Fishing larger lakes or long sections of rivers takes a longer time, and sometimes adequate fish habitat on the shoreline is hard to find. In these cases, we use gill netting and angling in deeper water to reach our collection goals.

Our target fish

Every year, we collect 10 largemouth bass or smallmouth bass from six of the 30 monitored waterbodies. We generally look for bass that range between 200-450 mm, which accounts for the mercury-size relationship that is characteristic of bioaccumulative toxics. Flip through the photos at the bottom of the page to see the fish that we catch for this study. We end each fishing period with 60 individual bass samples that are analyzed for mercury.

Additional fish sampling

We try to collect up to two additional, or ancillary, species per waterbody that represent popular sport or subsistence catch. We collect up to 15 fish for each ancillary species, combine them by size into several composite samples, and analyze these composites for mercury.

We identify the fish we observe by species and record a general estimate of their numbers. We process fish that meet our project needs according to our standard operating procedure (SOP) for field fish collection.