Mercury levels of yellowfin tuna are associated with capture location / by Francisco Blaha

I blogged on a 2019 paper on a model of mercury distribution on the pacific here

An update of this blog post was published by the Secretariat of the Pacific Community Fisheries Newsletter in June 2018, you can read the article here

Original from August 2017:
One of the questions I get asked quite often refers to the “dangers” of mercury (Hg) when eating pelagic fish, particularly tuna. A lot has been commented on social media and the news, and it seems to be a topic that never ends. So I was interested to see this recent article that states in the title “Mercury levels of yellowfin tuna (Thunnus albacares) are associated with capture location” so there is not much doubt about it!

Before I dig into the article lets clear the basics; methyl mercury (MeHg - which is the way mostly found in organisms) is naturally occurring and is normally found in higher concentration in organisms originating from areas or volcanic origin, now on top of these baselines that change regionally, we have the one originating from contamination, that can come from various sources.

The key issue with Hg is that bio-accumulates, meaning that the older the individual and the higher in trophic chain (who eats who) the higher the potential levels to be found are, furthermore different groups of “fish” have different capacities to metabolize it (i.e. get rid of it naturally), sharks for example have a very low capacity to do this and being a apex (at the top) predator Hg is found in higher concentrations than in tuna for example, that has better ways to deal with it.

Furthermore, exposure plays a big role, in simpler words if you eat yourself a 100kg shark that has levels way above the maximum recommended by the regulators, not much would happen (beyond indigestion!) yet if you eat 200g every day for 20 years chances are you’ll be in problems.

The confirmation and quantification of the neurological impact of Hg were demonstrated by a severe case industrial pollution in Japan in the 1950’s. Effluent from an industrial plant in the Minamata Bay area contaminated the aquatic environment and local people whose diet was mostly based on seafood from these waters suffered severe health effects.

Yet this was an extreme case, most of the tuna we consume today is at the safe levels (because many of the big -ergo old- ones are been fished already, but that is a fisheries issue) and furthermore, our diet is not just based on tuna.

The demonstrated benefits of including seafood in the diet outweigh the potential risk associated with it. This was researched in huge study and consultation by FAO and WHO (World Health Organization) in 2010 (so is not a conspiracy of the seafood industry!).

One of my mentors in FAO, David James wrote this seminal book on this Risks and Benefits of Seafood Consumption (is not too big so you should read it). One of its conclusions was: 

that the benefits of seafood consumption vastly exceed the risks, except under extreme circumstances involving excessive consumption of a few species” and in particular for pregnant mothers: When comparing the benefits of LC ω-3 PUFAs (omega3 oils) with the risks of methylmercury among women of childbearing age, maternal fish consumption lowers the risk of suboptimal neurodevelopment in their offspring compared with the offspring of women not eating fish in most circumstances evaluated”

Now, back to the paper that I was talking at the beginning of this post, there is another reason to insist (and pay) for provenance and traceability. It looks like the base levels of MeHg from tuna caught in different parts of the world have different levels of methyl mercury, which in principle should be surprise since it levels above natural baselines (i.e. contamination) is associated with population and industrialization, of which we don't have much of both here in the Pacific.

Furthermore, as testing for Hg requires a very complex, regulated and independently certifiable structure of determinations, it is a HUGE expense for the Pacific countries since we have to send the samples either to AUS, NZ, Thailand or Singapore (hence think about the logistics to send frozen samples!). We had the lab capacity at USP in FIJI but the certifications costs were outrageous for the level of samples required.

This paper is good news, as gives us in the Pacific the chance to sustain something we knew, our fish has very low levels of Hg, but most importantly, could argue for the reduction in the frequency of sampling and therefore the costs for the battled seafood safety Competent Authorities in the region.

The paper highlights are:

  • Mercury levels of 117 wild yellowfin tuna, a commercially important species caught worldwide, were measured.

  • Fish were captured from 12 known locations around the globe, representing four major yellowfin stocks.

  • Geographic origin is an important factor that determines mercury levels in yellowfin of similar size.

  • Low mercury fish clusters were found and argue for traceability as a tool to reduce human mercury exposure.

And the abstract says: (unfortunately you have to pay for the paper, I hate that!)

Current fish consumption advisories focus on minimizing the risk posed by the species that are most likely to have high levels of mercury. Less accounted for is the variation within species and the potential role of the geographic origin of a fish in determining its mercury level. Here we surveyed the mercury levels in 117 yellowfin tuna caught from 12 different locations worldwide. Our results indicated significant variation in yellowfin tuna methylmercury load, with levels that ranged from 0.03 to 0.82 μg/g wet weight across individual fish. Mean mercury levels were only weakly associated with fish size (R2 < 0.1461) or lipid content (R2 < 0.00007) but varied significantly, by a factor of 8, between sites. The results indicate that the geographic origin of fish can govern mercury load, and argue for better traceability of fish to improve the accuracy of exposure risk predictions.