20 Aug Mercury Contamination In Chilean Sea Bass Complicated By Seafood Substitution
Medical Research: What are the main findings of the study
Prof. Marko: The main finding of the study was that species substitutions and fishery stock substitutions together obscure a complex pattern of mercury contamination in Chilean sea bass (or Patagonian toothfish) that can put consumers unknowingly at risk of ingesting greater levels of mercury than the labeling would suggest. Although it is well appreciated that mercury levels vary dramatically among different species of fish, and that species substitutions have the potential to expose consumers to unwanted mercury, our study shows that for Chilean sea bass, fish mislabeled as to their country or region of origin (but labeled as the correct species) have a high potential to expose consumers to unexpectedly high levels of mercury.
Medical Research: Were any of the findings unexpected?
Prof. Marko: Yes and no. Going into our study we knew two things.
- First, many types of seafood including Chilean sea bass, are frequently mislabeled as other species (species substitutions), but even if correctly labeled as to species, fish can also be incorrectly labeled as to their geographic origin (fishery stock substitutions).
- Second, we knew that levels of mercury, a toxin dangerous to developing human nervous systems, consistently varies not only among species of fish, but also varies geographically within individual species. Chilean sea bass is just such a species that has widely varying levels of mercury accumulation across it’s geographic range: fish from high latitudes in the Southern Ocean (i.e. closer to the south pole) tend to accumulate mercury more slowly than populations of the same species closer to the equator. Why this happens is not well-undertood, but for Chilean sea bass, the differences in mercury accumulation (established in by other research) are large: for example, fish from Chile have approximately 2-3 times more mercury than those from sub-Antarctic islands.
So, our study was focused on characterizing how mislabeling actually interacts with mercury contamination in actual retail situations for a fish that is harvested over a large area of the southern hemisphere’s oceans. What surprised us was how well inferred instances of mislabeling (identified by genetics) corresponded to mercury levels in the same samples. For example, Chilean sea bass labeled as having originated from the sub-Antarctic island of South Georgia – but which had mitochondrial DNA typical of Chile – turned out to have massively higher levels of mercury than typical South Georgia fish.
Medical Research: What should clinicians and patients take away from your report?
Prof. Marko: I am not a clinician so I cannot make a professional recommendation. I can tell you that the four species of fish (shark, tilefish, swordfish, and king mackerel) on the FDA “do not eat list” (for pregnant women, women planning to have children, and children) range from a mean of 0.73 to 1.45 parts per million (ppm) mercury and that, although mean levels of mercury for Chilean sea bass from the FDA are listed at 0.35 ppm, the Chilean sea bass in our study that were labeled as either originating from Chile or inferred to be from Chile (with genetics) averaged 0.86 ppm.
Medical Research: What recommendations do you have for future research as a result of this study?
Prof. Marko: Our study suggests that more mercury data need to be collected from across the geographic ranges of species of fish, particularly those known to show high geographic variation in mercury accumulation rates. Fish from one part of the world might have low levels of mercury, but individuals of the same species – but from different oceans or regions may have mercury levels considered unacceptable to many consumers.