Mercury Concentrations in Fish Increase with Harmful Untreated Ship Scrubber Washwater

December 18, 2023
Public Relations

Mercury Concentrations in Fish Increase with Harmful Untreated Ship Scrubber Washwater

It’s the holiday season! And around the world, people are celebrating with traditional fish-focused holiday meals. Take for example, the Sicilians' Feast of the Seven Fishes, the Danish’s Pickled Herring, or the Portuguese’s countless Cod-centered dishes.


Though most consume fish for its versatility, accessibility, religious affiliations and/or believed healthier nutrition, the warning to consume fish low in Mercury always follows. This advice becomes more difficult to heed with an alarming increase in mercury concentrations in fish deriving from their absorption of poisonous ship scrubber washwater.


It’s widely known that the higher the fish is on the food chain, the larger the size or the older the age, the higher the mercury content it will contain as is the case for shark, swordfish and bluefin tuna. But commonly consumed fish such as bottom-feeder bass, halibut and catfish are also known to have higher levels of mercury due to the settlement of this heavy metal on the ocean floor sediment where they feed. Similarly, lower trophic level fish like perch, trout, cod and herring, which account for 90% of the fish catch from the Baltic Sea, known to contain some of the highest levels of dirty scrubber effluents and consequently the highest levels of heavy metals, are now presenting with higher levels of mercury.


With diets ranging from zooplankton like the affected copepods we discussed in July’s article, and other lower trophic level species that have been equally impacted by toxic scrubber wastewater, cod, and herring from the Baltic Sea have been found with an average mercury concentration exceeding 3 times the normal levels.


Mercury does not break down. Once it enters the environment from any source, it remains there. In fact, 70% of the mercury circulating in the environment today consists of mercury emitted from human sources in the past. And an estimated 2,220 tons of mercury are currently emitted into the environment as a direct result of human activity. Overwhelmingly, 20% of this accounts for the ocean pollution contributed by marine shipping such as toxic scrubber discharge, offshore industrial operations, and waste disposal at sea. Once in the ocean, certain microorganisms can convert the inorganic mercury into methylmercury, its most dangerous form.


Methylmercury bioaccumulates as it moves up the food chain, and as it distributes throughout tissue, especially fatty tissue, there is no method of cleaning or cooking fish that will remove the methylmercury. An excess of mercury leads to neurological impairment, kidney failure, lung disease, inflammable skin, and gastrointestinal issues, if not death.


For this and other toxic chemical reasons, it is now recommended that children, adolescents, and women of childbearing age including pregnant and breastfeeding women should limit their consumption of contaminated fish such as Baltic Sea herring, trout, cod or salmon to no more than two to three times a year. Other consumers should limit their consumption to no more than once a week.


While the holidays might feel odd to celebrate without these staple dishes, it’s important to protect the health of our people by reducing our fish intake until we fix this issue. Equally, we need to protect the health of our oceans by eliminating the use of these harmful, polluting ship scrubbers as they progressively destroy marine life.


1.    Excessivemercury concentrations found in Baltic fish – report - LRT

2.    Human Health andOcean Pollution - PMC (

3.    BIO603 Ida Vartia V22.pdf (

4.    Dioxinsand PCBs in Swedish food (


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