Marko PB, Nance HA, van den Hurk P (2014) Seafood Substitutions Obscure Patterns of Mercury Contamination in Patagonian Toothfish (Dissostichus eleginoides) or “Chilean Sea Bass”. PLoS ONE 9(8): e104140. doi:10.1371/journal.pone.0104140
What is mercury? Why is it in fish?
Mercury pollution is a byproduct of human activities including fossil fuel combustion and processing metal ores. Methylmercury, the most common form of mercury, is a powerful neurotoxin. (For more information on mercury, check out this OceanBites article!) The toxin is incorporated in lower trophic level organisms (e.g., algae, plankton) and travels up food chains, all the way to top predators, in a process known as bioaccumulation. Although mercury is present at low concentrations in the environment, the neurotoxin accumulates in higher concentrations as it travels up the food chain. Concentrations found in different fish species vary widely (Figure 1). These differences are generally attributed to body size and trophic level. For example, fish that have longer lives and reach larger body sizes eat more and thus, accumulate more toxins in their tissue.
Size and lifespan differences also contribute to a considerable amount of variability in mercury concentrations within each species (note the large error bars in Figure 1). Variability also comes from within species with different geographical locations. Two recent studies (Dawson et al. 2007; Hanchet et al. 2011) show that subpopulations of Chilean sea bass (a.k.a. Patagonian toothfish), a southern hemisphere fish species, have vastly different concentrations of mercury: ranging between ~0.15ppm and greater than 1ppm. For reference, concentrations greater than 0.50ppm are banned from importing to New Zealand, Canada and Australia; the legal limit for the US and European Union is 1ppm.
Chilean sea bass stocks located at higher latitudes in the southern hemisphere (that is, close to the pole) have lower concentrations of mercury relative to body size than stocks located closer to the equator. In fact, these high latitude stocks, such as the South Georgia/Shag Rocks (SGSR) stock, have mercury levels far below the legal import limits (~0.23ppm) while fish from the stocks near Patagonia have levels nearly three times greater (~0.73ppm). Additionally, the SGSR stock is considered a sustainable seafood choice by Seafood Watch and officially certified by the Marine Stewardship Council (MSC) as a well-managed stock with low by-catch (or, species caught that are not targeted by the fishery). This combination of sustainability and low mercury content makes Chilean sea bass from the SGSR stock quite appealing to wise consumers.
However, seafood mislabeling is a widespread problem. We often think of mislabeled seafood as a hindrance to making sustainable seafood choices, but it can also put the consumer unknowingly at risk of ingesting greater levels of mercury.
The authors set out to collect samples of fish labeled “Chilean sea bass” or “Patagonian toothfish” obtained from U.S. merchants. They used “fresh” fish fillets (in other words, thawed) from retailers in 10 U.S. states. Samples were immediately preserved for analyses.
Researchers collected genetic data from the samples to determine what proportion of the fish were in fact Chilean sea bass (CSB) and not some other species. They found that 20% of fish were mislabeled. Of the fish that were certified by the MSC, only 5% were mislabeled; 40% of uncertified fish were mislabeled. Mercury levels were also measured in 25 of the MSC certified fish and 13 of the non-certified. By comparing mercury levels, we can infer what stock (SGSR or low latitude stocks) the CSB actually came from.
Mercury levels in the fish samples ranged from 0.07ppm up to 1.9ppm. Fish labeled as MSC certified had less than half of the mercury concentration of uncertified fish (Table 1, Figure 2). However, when fish other than CSB were removed from the analyses, the difference between certified and non-certified samples was only marginally significant. Comparing mercury content of the MSC certified samples across haplotypes – or, genetically distinct fish— revealed that haplotypes that were known to be from the South Georgia (SG) stock were significantly lower in mercury than the not known haplotypes (Figure 3).
Although seafood mislabeling has been a known problem for some time, this study is unique in that it shows the prevalence of not only species substitutions, but also stock substitutions. The combination of both frauds makes it difficult for consumers to know what the mercury content of their fish truly is. The study also makes clear that the differences in mercury content are minimized when only MSC certified fish are considered. Although species substitutions occur in both certified and non-certified, the certified fish substitutions tended to have lower mercury content than the non-certified substitutions. The combined effect obscured the true mercury concentrations of certified and non-certified CSB.
Within the true CSB from MSC certified catch, the mercury content differences (and DNA haplotypes) showed that the fish origins were mislabeled (Figure 3). Although a Chilean sea bass may be certified, the stock it came from might not be from the certified South Georgia stock. This fishery-stock substitution is much more difficult to detect than species substitutions and puts the consumer unknowingly at risk of consuming higher levels of mercury. Furthermore, the mercury concentration differences for species substitutions was just ~7%, while stock substitutions could account for a 100% increase in mean mercury concentration! The authors suggest geographic origins of fish should become a stronger focus in seafood consumption. Finally, although there were substitutions apparent in both certified and non-certified, MSC-certified fish still had lower mercury concentrations, identifying these fish as safer for consumers.