Abstract
Seafood mislabeling distorts the true abundance of fish in the sea, defrauds consumers, and can also cause unwanted exposure to harmful pollutants. By combining genetic data with analyses of total mercury content, we have investigated how species substitutions and fishery-stock substitutions obscure mercury contamination in Patagonian toothfish (Dissostichus eleginoides), also known as “Chilean sea bass”. Patagonian toothfish show wide variation in mercury concentrations such that consumers may be exposed to either acceptable or unacceptable levels of mercury depending on the geographic origins of the fish and the allowable limits of different countries. Most notably, stocks of Patagonian toothfish in Chile accumulate significantly more mercury than stocks closer to the South Pole, including the South Georgia/Shag Rocks stock, a fishery certified by the Marine Stewardship Council (MSC) as sustainably fished. Consistent with the documented geography of mercury contamination, our analysis showed that, on average, retail fish labeled as MSC-certified Patagonian toothfish had only half the mercury of uncertified fish. However, consideration of genetic data that were informative about seafood substitutions revealed a complex pattern of contamination hidden from consumers: species substitutions artificially inflated the expected difference in mercury levels between MSC-certified and uncertified fish whereas fishery stock substitutions artificially reduced the expected difference in mercury content between MSC-certified and uncertified fish that were actually D. eleginoides. Among MSC-certified fish that were actually D. eleginoides, several with exogenous mtDNA haplotypes (i.e., not known from the certified fishery) had mercury concentrations on par with uncertified fish from Chile. Overall, our analysis of mercury was consistent with inferences from the genetic data about the geographic origins of the fish, demonstrated the potential negative impact of seafood mislabeling on unwanted mercury exposure for consumers, and showed that fishery-stock substitutions may expose consumers to significantly greater mercury concentrations in retail-acquired fish than species substitutions.
Highlights
Despite the many health benefits of eating fish, most commercially harvested fish are contaminated with mercury [1]
Fish labeled as Marine Stewardship Council (MSC)-certified Chilean sea bass had less than half the THg of uncertified fish (Table 1, Fig. 2), a highly significant result (t-test for unequal variances: t = 22.97, one-tailed p = 0.0050)
Among only those fish that were genetically verified as D. eleginoides, the difference in THg between MSC-certified and uncertified fish was only marginally significant at the 0.05 level and not significantly different at the 0.0167 level (t-test for unequal variances: t = 2 2.15, one-tailed p = 0.0320)
Summary
Despite the many health benefits of eating fish, most commercially harvested fish are contaminated with mercury [1]. Present in only small quantities in the environment, mercury accumulates in living organisms. Accumulation of mercury is prevalent but variable (Fig. 1), primarily due to differences in trophic level and body size, such that mercury concentrations tend to be high in larger, longer-lived predatory fish [3]. The amount of fish and the particular species of fish consumed are considered the most important factors determining the health risk associated with eating seafood contaminated with mercury. The US Food and Drug Administration (FDA) advises pregnant and nursing women, women who may become pregnant, and young children not to eat species that have mean mercury concentrations near 1.0 ppm (shark, swordfish, king mackerel, and tilefish), and not to consume more than 12 ounces per week of other species that have lower mercury levels
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