Combining mercury content and species identification in this study shows that all species exceed or approach levels permissible by Canada, the E.U., Japan, the U.S., and the World Health Organization. Mercury levels are significantly higher in bluefin akami (sushi from lean, dark red tuna) and all bigeye tuna samples than in bluefin toro (sushi from fatty tuna) and yellowfin tuna akami samples. This is probably because mercury accumulates differently in different tissue types: mercury has an affinity for muscle and not fatty tissue, so the leanest fish tend to have the highest concentration.
But there also seem to be other factors in play. Although yellowfin tuna is very lean, this species tends to have lower accumulation of mercury, likely because yellowfin are typically smaller than other tuna and are harvested at a younger age. Furthermore, yellowfin are tropical and do not thermoregulate like the warm-blooded bigeye tuna and bluefin tuna. Because bigeye and bluefin species eat three times more than yellowfin to maintain their energy level, they might bioaccumulate, or slowly increase the level of toxins over time.
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