Mercury levels

Due to their high position in the food chain and the subsequent accumulation of heavy metals from their diet, mercury levels can be high in larger species such as bluefin and albacore.

In 2009 a California appeals court upheld a ruling that canned tuna does not need warning labels as the methylmercury found in tuna is naturally occurring.
In March 2004 the United States FDA issued guidelines recommending that pregnant women, nursing mothers, and children limit their intake of tuna and other types of predatory fish. However, in January 2009 the FDA released a draft report that found a greater risk to children whose mothers do not eat fish during pregnancy, concluding, "Benefits tend to increase as both fish consumption and exposure to methylmercury increase. The benefits are the size of a fraction of an IQ point through the 95th percentile of exposure to methylmercury (involving the consumption of 44.2 grams of fish per day), but then increases to the size of about 1.5 IQ points at the 99th percentile of exposure (involving the consumption of about 98 grams of fish per day), and to about the size of three IQ points at the 99.9th percentile of exposure (involving 205.7 grams of fish per day).

The Chicago Tribune, cited for its inaccurate reporting on tuna related topics, reported that some canned light tuna such as yellowfin tuna is significantly higher in mercury than skipjack tuna, and caused Consumers Union and other activist groups to advise pregnant women to refrain from consuming canned tuna. This was considered extreme and thus not adopted by leading scientific and governing bodies.

The Eastern little tuna (Euthynnus affinis) has been available for decades as a low-mercury, less expensive canned tuna. However, of the five major species of canned tuna imported by the United States it is the least commercially attractive, primarily due to its dark color and more pronounced 'fishy' flavor. Its use has traditionally been restricted exclusively to institutional (non-retail) commerce.

A January 2008 report conducted by the New York Times has found potentially dangerous levels of mercury in certain varieties of sushi tuna, reporting levels "so high that the Food and Drug Administration could take legal action to remove the fish from the market."