9. Heavy Metals in Seafood Mussels. Risks for Human Health

Life on earth is threatened both by environmental pollution and overpopulation. Natural phenomena contributing to pollution have always existed but anthropogenic activities are increasing contamination of air, soil and water. Waters are especially polluted by metals. The main threats to life from toxic metals are associated with exposure to Pb, Cd, Hg and As, which accumulate in organisms. Overpopulation means more food is required but the amount of arable land is declining due to human requirements. One way of overcoming these consequences of overpopulation is to exploit seas and oceans. Thus, the consumption of seafood has increased in recent years, especially in coastal regions. However, as many traditional fishing grounds have been over-fished, aquaculture seems a viable solution to these problems. Marine mussels are an excellent candidate for aquaculture. However mussels accumulate a wide range of metals in their soft tissue. Thus, the determination of the concentrations of potentially toxic substances in mussels is essential because of their usage as seafood and the potential adverse effects of their consumption on human health. Moreover, as contamination by metal pollutants continues and is even increasing in some parts of the world, particularly in less developed countries, it is also important to determine the level of pollution in the marine environment, especially in regions where aquaculture is foreseen and where the local population consumes large amounts of mussels.

In this review, these issues are presented and discussed using the Mediterranean mussel Mytilus galloprovincialis as an example. When this mussel is in its best condition, the meat weight can approach 50% of the total wet weight, i.e., M. galloprovincialis is very efficient at converting low value victuals into high quality animal protein. For all these reasons, M. galloprovincialis is an ideal candidate in attempts to alleviate famine through aquaculture. Mussel farming is a relatively “clean” operation because no pollutants are released into the environment. The mussels remove suspended materials from the water column, thereby improving the local water quality. However mussels accumulate metals. Metal concentrations in M. galloprovincialis are valuable indicators of contamination of coastal and estuarine ecosystems. “Mussel Watch”, introduced in 1975, is the longest continuous contaminant monitoring program of coastal waters for assessing spatial and temporal trends in coastal contamination, providing a baseline for assessing the impacts of anthropogenic and natural events and identifying contamination hot spots.

In Europe, the production of cultivated mussels is almost five times higher than wild collected mussels. Recently, the production of M. galloprovincialis in Europe has been increasing rapidly in Greece and Turkey but Spain is still the largest producer of mussels in Europe. The production of the mussel M. galloprovincialis in Spain is about 25% to the total world production, over 200,000 t y⁻¹. Only China has a larger production of these mussels than Spain, about 600,000 t y⁻¹. However, the bioaccumulation of toxic contaminents, mainly Cd, Pb, Hg and As, remains an issue concerning the consumption of mussels. A review of literature data revealed large variations in the Cd, Pb, Hg and As concentrations in M. galloprovincialis from their endemic areas, e.g., Mediterranean, Adriatic and Black Sea and that the concentrations of these toxic metals were generally in the following order: As > Pb > Cd > Hg. The levels of Cd, Pb and Hg were the lowest in mussels from the Aegean Sea, compared to the others investigated Seas, but a very high level of As was found.

The guidelines on the trace elements for seafood safety set by different countries and associations are reviewed. The defined legal limits on permissible concentrations of non-essential elements that are toxic in traces, such as Cd, Pb, Hg and As, in mussels on a wet weight basis in mg/kg are in the ranges: for Cd from 0.1 to 4.0; for Pb from 1.0 to 6.0; for Hg from 0.05 to 10 mg/kg and for As from 0.5 to 86 mg/kg. The reviewed Provisional Tolerable Weekly Intake values appear to support the conclusion that risk to human health from dietary exposure to the investigated elements from Mediterranean mussels is relatively low. Comparison of the published data with European legislation showed that the levels of Cd, Pb and Hg generally did not exceed the existing limits in all the mussels analyzed, excluding mussels from hot spots, such as lagoons and harbors in the Mediteranean, Adriatic and Black Sea.

Overall, the assessment that the investigated toxic elements may pose a health risk to heavy mussel consumers, especially related to the levels of Pb and Cd in M. galloprovincialis from hot spots in the all investigated Seas. Although consumption of these mussels provides proteins, essential minerals and vitamins, and thus, some protection from certain diseases, the risks and benefits of their consumption are still hard to assess because of the metals bioacumulated from the marine environment, with their reviewed toxicity. The final conclusion of this review is that M. galloprovincialis is an excellent candidate for aquaculture but that extreme care must be taken when choosing the location of farms. Care should also be taken by coastal populations when consuming wild mussels from contamination hot spots.