Is Port Susan Seafood Safe? Contaminants in Stillaguamish Salmon and Clams

By Patrick Moran, Franchesca Perez and Dave McBride. USGS Fact Sheet 2020–3043, prepared with the Stillaguamish Tribe of Indians and the Washington State Department of Health.

The greater Port Susan area of central Puget Sound has been part of the Stillaguamish Tribe's fishing, hunting and gathering grounds since time immemorial, and is a popular sport and commercial fishing area. Its large shellfish beds and yearly salmon runs feed Tribal members and the public, and are woven into Tribal culture and history. As western Washington keeps urbanizing, worry about mercury, PCBs (polychlorinated biphenyls) and organochlorine pesticides in seafood has grown. From 2016 to 2019, the Tribe, the U.S. Geological Survey and the Washington State Department of Health (DOH) reviewed the data to judge what risk, if any, comes from eating locally caught Chinook salmon and Eastern softshell clams from Port Susan Bay.

Where the clams and Chinook salmon were collected. The Stillaguamish Tribe is based in Arlington. Map: USGS.

The Stillaguamish watershed is fairly undeveloped — mostly forest and farmland — and has low but measurable contaminants. But pollutants, often historical, build up in the Pacific and can reach local seafood. The fish and clams were collected from 2016 to 2018.

Where the pollutants come from

• Organochlorine pesticides: an older, mostly banned class. Chlordane was used on crops and termites until 1988; DDT against mosquitoes until 1973.
• PCBs: used in hydraulic fluids, electrical equipment, paints and dyes.
• Mercury: natural, released by volcanoes and some minerals, and a major pollutant from burning coal and waste.

All three persist, bioaccumulate up the food chain, are toxic to animals including people and killer whales, and travel long distances through the atmosphere to Puget Sound.

Why they matter

Harm comes mostly from long-term, repeated exposure. Fetuses and newborns are most at risk, taking in more per unit of body weight. Mercury can damage the nervous and immune systems; chlordanes suppress immunity; DDT breakdown products disrupt hormones; PCB toxicity mostly affects the immune system. Chlordane is classed as a Class 2 human carcinogen, and DDT as a "probable" one.

PCBs in Puget Sound seafood. Port Susan clams and Chinook sit at the low end. Data: West and others (2017), Washington Department of Ecology (2018), USFWS (2020). Chart: USGS.

What was measured

All values in parts per billion, wet weight; ND means not detected.

Findings

Contaminants were low in every sample, in line with other Puget Sound studies, and below levels that would call for new advisories. DOH compared them with screening levels based on eight (general public) and twenty-three (high consumers) 8-ounce meals a month. Because Chinook is often eaten with the skin, no reduction for cleaning or cooking was assumed.

Softshell clams — low in fat, so low in these compounds:

• PCBs and pesticides were mostly not detected.
• Pesticides allow more than 500 meals a month; PCBs (under 1 part per billion) more than 200.
• Mercury allows 47 meals a month, far above DOH's advisory triggers of 8 (general public) or 23 (high consumers).

Chinook salmon:

• Pesticides: higher than in clams but under screening levels; more than 500 meals a month, and more than 200 for DDT.
• PCBs: an average of 47 ppb, slightly lower than earlier Puget Sound Chinook (54 ppb) but slightly higher than coastal Chinook. That allows 4 meals a month — matching the current advisory for non-resident Chinook — or 8 if skin and belly fat are removed and fat is cooked off.
• Mercury: not measured; earlier Puget Sound salmon results suggest it is not a concern.
• Dioxin-like PCB 126: USFWS measured 180 PCB compounds. PCB 126, especially potent, turned up in 7 of 15 fillets but no clams. DOH, with the state's Fish and Wildlife and Ecology departments, will decide whether further advice is needed.

Current Puget Sound advice is two meals a month of Blackmouth (resident Chinook) and four of mature Chinook returning from the ocean: doh.wa.gov/fishmap.

How PCBs cycle through a watershed and its food web. Diagram: USGS.

Eat fish, wisely

DOH urges everyone to eat at least two fish meals a week, as the American Heart Association recommends. Frequent eaters can cut their exposure:

• eat a variety of low-contaminant fish (DOH guidance) and follow local advisories;
• give young children and small adults smaller portions;
• choose younger, smaller fish (within legal limits), which carry less PCB and mercury;
• remove skin, fat and organs, and eat fillets without skin;
• grill, bake or broil so fat drips away;
• avoid smoking fish, which adds cancer-causing PAHs.

Sources

Based on Contaminants in Fish and Shellfish in the Stillaguamish River and Port Susan Marine Areas, Washington, by Patrick Moran, Franchesca Perez and Dave McBride, USGS Fact Sheet 2020–3043, U.S. Geological Survey; a work of the United States government in the public domain. The map, the PCB chart, the contaminant table and the PCB-cycle diagram are taken from the fact sheet's PDF. Its photographs, courtesy of the Stillaguamish Tribe, the Washington State Department of Health and the Alaska Seafood Marketing Institute and used with permission, are not reproduced.