Wednesday, October 13th, 2010 | Author:  | 18,937 views - starting Aug 9/09

Tuna are magnificent fish. They are large, stealth predators with powerful, stream-lined bodies that can out-perform even the most sophisticated machine (of comparable size) in strength, resilience, and vigor.

Unfortunately for them, they are also incredibly tasty and one of the most desirable ocean catches worldwide. Unfortunately for humans, they are not nearly as abundant as they once were because over-fishing threatens their survival and neither are they as nutritious because they are becoming increasingly contaminated with pollutants such as heavy metals.

Heavy metals are so called because they literally are heavier than other elements (actually, the term “heavy metal” is rather ambiguous … there are other definitions of what constitutes a heavy metal, but this is the most common). Almost all heavy metals are toxic to humans. Some, such as iron, zinc, and copper, are essential for human health—but only in small quantities, otherwise they too can be toxic.

Others, such as mercury, cadmium, and lead, are toxic at any quantity. These heavy metals accumulate in tissues—especially fatty tissues including the brain—and poison the body. Our bodies lack the enzymes and cellular “machinery” needed to quickly remove these toxic metals from our system.

The result is a gradual buildup of heavy metals in our cells as we age. The health impacts can be profound, especially because heavy metal toxicity causes inflammation. Chronic inflammation is at the root of many common diseases.

Heavy metals also tend to accumulate in the tissues of most animals and plants (this is referred to as bioaccumulation). Higher concentrations of heavy metals are found in animals that:

Atlantic blue marlin (Makaira nigricans) is also a large predatory and long-lived fish that bioaccumulates high concentrations of pollutants including heavy metals. Populations are in decline due to decades of intense fishing pressure. This year, Greenpeace International added this species to its seafood red list but it is not yet listed by the IUCN.

Atlantic blue marlin (Makaira nigricans) is also a large and long-lived predatory fish that bioaccumulates high concentrations of pollutants including heavy metals. Populations are in decline due to decades of intense fishing pressure. This year, Greenpeace International added this species to its seafood red list but it is not yet listed by the IUCN.

   - are carnivorous

   - are top predators (and eat smaller, fatty animals)

   - are large and fatty

   - are long-lived

   - live in environments contaminated with heavy metals.

When we eat animals that are full of heavy metals, we ingest poison along with nutrients. The key to minimizing toxicity in your body is to be very selective of which animals you eat. It’s now common knowledge that food harvested from marine and freshwater sources is contaminated with pollutants such as heavy metals.

Enter tuna: a large, fatty, long-lived carnivorous fish that is the top or close-to-the-top predator in most marine food chains. With a ravenous appetite and an incredibly high metabolic rate that forces them to devour the largest and fattiest fish they find, plus an ocean environment that becomes more polluted each day, tuna accumulate more heavy metals than the majority of other fish species humans eat.

Numerous studies have measured pollutant levels in fish to determine the safest species to eat. Tuna has notoriously topped every list as one of the most toxic species, and heavy metal toxicity varies among tuna species.

Despite this, and despite that many (arguably all) species of tuna are endangered, global tuna consumption is still on the rise.

A study published this month measured the concentrations of mercury, cadmium, and lead in 45 popular brands of canned tuna sold in Italy. The researchers also measured heavy metal concentrations in freshly caught tuna from the Tyrrhenian Sea (off the west coast of Italy). What they found wasn’t surprising.

Canned and freshly caught tuna were contaminated with all 3 heavy metals. The good news is that compared to the safe weekly intake set by the World Health Organization (WHO) for cadmium and lead, tuna contained an average of 0.40% of the safety levels of cadmium and 0.56% of the safety levels of lead.

The bad news is that 9% of the canned tuna samples and 20% of the freshly caught tuna samples had mercury levels exceeding the safety guidelines set by the European legislation.

This may not seem like a lot, but it’s difficult to know how much mercury is contained in the tuna you eat because several factors affect this. Tuna that are larger, older, living in polluted waters, and eating contaminated fish will contain higher levels of mercury. These are details that you rarely know about the fish you purchase.

August 2006 saw some pioneering blue water spearfishing off New Zealand’s West Coast. Five Pacific Blue Fin Tuna up to 292 kg were landed by New Zealand blue water spearfisherman.

“August 2006 saw some pioneering blue water spearfishing off New Zealand’s West Coast. Five Pacific Blue Fin Tuna up to 292 kg were landed by New Zealand blue water spearfisherman.”

Previous studies have suggested that canned tuna may contain lower levels of pollutants than fresh fillets because most small catches and smaller tuna species such as skipjack and albacore are canned whereas the larger fish such as bluefin tuna are sold fresh or frozen.

However, keep in mind that a small tuna is still a big fish—tuna average body lengths greater than 1 m and life spans of more than 10 years. What’s more, the names “albacore” and “skipjack” can refer to a number of different tuna species around the world, depending on whom you’re talking to.

There’s something else that’s very important and alarming. The WHO, setting a standard for the vast majority of countries, has set safety levels of mercury in fish at 0.50 mcg/g wet weight of fish. This means that only fish containing less than 0.5 micrograms of mercury per gram of uncooked meat are safe to eat. One exception is Japan, where any fish with more than 0.40 mcg of mercury/g wet weight of meat is not fit for human consumption.

However, the safety levels for predatory fish, such as tuna and sharks, are set at 1.0 mcg/g wet weight of fish. This is twice the amount of mercury that is allowable in non-predatory fish! This double standard has been set because few predatory fish contain less than 1.0 mcg/g wet weight, let alone less than 0.5 mcg/g wet weight.

BUT, mercury is toxic no matter where it comes from. If the WHO regards most fish to be unsafe when tissue levels exceed 0.50 mcg/g wet weight, then that same standard should be applied to predatory fish. Mercury is not less toxic to human health if it comes from predatory fish. That means if predatory fish rarely contain less than 0.5 mcg/g wet weight, they should not be eaten.

So what to do? If you eat fish, be very mindful of the species and sources you choose. Fortunately, the internet has become a sea of information to help you choose safe and sustainable fish.


Seafood Watch

Fish Online

Australian Marine Conservation Society

New Zealand Best Fish Guide

WWF Seafood Guides, listed by country

Also visit:

Marine Conservation Society

Seafood Choices Alliance

Blue Ocean Institute

CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora)


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7 Responses

  1. 1
    got mercury 

    Great article! Another excellent resource for learning more about mercury in seafood is the public health project The fish calculator helps you gauge how much mercury may be in the seafood you are eating

  2. I am happy for this important amazing web page. Being a passionate blogger, I am happy to find out other people today are writing to contribute to the community. I just wanted to comment to let you know I am familiar with your cause many writers don’t get the credit they deserve.

  3. 3

    Thanks for the great info. I like this post. I learned alot.

  4. 4
    Elisha Sanzone 

    I always thought eating tuna was fishy! Haha! Its too bad were losing so many ocean fish and the pollution too.

  5. I am an MD from southern Northern Ontario. Two of my patients are fishing guides. One on the English River and a year ago he ate 49 shore lunches of pickerel as they call it here or walleye. He had three times the toxic level of Hg in his blood. After a few months it reverted to normal values.
    The English Wabigoon river systems were notorious in the seventies for causing Minamata disease in natives mainly as fish is their staple food.
    Then I tested another fisihing guide from the Wabikanagami river system and he also had toxic levels of Hg. A bit early to jump to conclusions but the fact that both western and eastern Northern Ontario is significantly contaminated with Hg.
    I tested several other avid fishermen but none of them had any levels to speak of. Some I also tested for Pb. Nil or almost nil.
    So it seems that fishermen who come to fish on N Ont lakes are safe, only fishing guides and Ojibway or Cree are in any danger.
    Gotta go play ping pong more later.

  6. I resported this to the Govt of Can website and to my surprise they responded politely but said there is no longer any toxic level of Hg in the English River.
    Then I started to research this but didnt get very far.
    I only have guesses as to where the Hg came from and have no idea what is the level of Hg in natives of the Kenora or Wawa districts. Occasional sport fishermen are obviously not affected.
    So where does the Hg come from?
    Leached from bedrock by acid rain? Mining tailings? Pulp and paper mills? Apparently they are closed in that area.
    Or a fourth theory I heard from Hg from coal fired plants in China sending a cloud of mercury filled smoke around the northern hemisphere?
    Which might explain the mercury levels in ocean tuna.
    Where is the mercury from? Local or global source?
    Anyone with supporting or conflicting info?

  7. 7

    hi alexander!
    thanks so much for your comment and great info … here’s a study you may find interesting & helpful … let me know if you want me to email you the pdf … :)