Despite the many health benefits of eating fish, most commercially harvested fish are contaminated with mercury. The most common form of mercury in fish is methylmercury, a neurotoxin that is especially dangerous to the developing nervous system. Although present in only small quantities in the environment, mercury accumulates in living organisms. Among fish, accumulation of mercury is prevalent but variable, primarily due to differences in trophic level and body size, such that mercury concentrations tend to be high in larger, longer-lived predatory fish. Therefore, 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.
New measurements from fish purchased at retail seafood counters in 10 different US states show the extent to which mislabeling can expose consumers to unexpectedly high levels of mercury. The issue at hand are fishery stock substitutions which falsely present a fish of the same species, but from a different geographic origin.
A new study conducted at the University of Hawaii compared two kinds of fish sold at retailers: those labeled as Marine Stewardship Council-certified Chilean sea bass (Dissostichus eleginoides), and those labeled simply as Chilean sea bass (uncertified). The certified version is supposed to be sourced from the Southern Ocean waters of South Georgia, near Antarctica, far away from human-made sources of pollution. Certified fish is often favored by consumers seeking seafood harvested in a sustainable fashion but is also potentially attractive given its consistently low levels of mercury.
However, in a previous study, the scientists had determined that 20% of fish purchased as Chilean sea bass were not genetically identifiable as such. They used a mtDNA marker (control region flanked by tRNA proline and 12S rRNA) to determine both species and stock population. About 15% of the Chilean sea bass positively identified, were not sourced from the South Georgia fishery.
In the new study, the scientists used the same fish samples to determine their mercury content. When they compared the mercury in MSC-certified sea bass with the mercury levels of, non-certified sea bass, they found no significant difference in the levels.
It turns out that the fish with unexpectedly high mercury originated from some fishery other than the certified fishery in South Georgia. Actually, the DNA analysis indicated they were from Chile. Thus, fishery stock substitutions are also contributing to the pattern by making certified fish appear to have more mercury than they really should have given their origin. Certain fish had very high mercury levels - up to 2 or 3 times higher than expected, and sometimes even greater than import limits to some countries.
Although on average, MSC-certified fish is a healthier option with respect to mercury contamination than compared to uncertified fish, our study showed that fishery-stock substitutions, can result in a larger proportional increase in mercury consumption than species substitutions for consumers, and that variation in mercury contamination among fishery stocks may be considered in future seafood consumption guidelines.