Lockyer filed a suit against three Californian tuna producers - Tri-Union Seafoods, Del Monte and Bumble Bee Seafoods - in June 2004, for failing to label their albacore and light tuna products with warnings over potential mercury content. He claims that the omission is in violation of Proposition 65, a 1986 law requiring that companies provide "clear and reasonable" warnings before exposing people to known carcinogens or reproductive toxins.
The case is scheduled to commence in a San Francisco court October 19.
The USDA's Dietary Guidelines for Americans contains the advice that people should eat two fish meals a week. A growing body of scientific evidence has linked fish consumption to a number of health benefits, including cognitive function, heart health, joint health and respiratory health, largely because of its high omega-3 fatty acid content.
However all types of fish contain some amounts of mercury. For most people this is not a cause for concern, but a build-up in the blood stream can lead to reproductive problems in women and affect the development of the nervous system in children.
The FDA therefore advises that people eat up to 12 ounces (two average meals) a week of fish and shellfish that are lower in mercury, such as canned light tuna, salmon, pollock, and catfish.
As albacore tuna and fresh tuna steaks contain more mercury than light tuna, the agency says that consumption of this variety should be limited to 6 ounces per week.
This advice is particularly important for children, pregnant women and women who might one day become pregnant. Pregnant women and children are also advised to avoid eating shark, swordfish, king mackerel, or tilefish, as these are particularly high in mercury.
In a letter sent to Lockyer on 12 August, FDA commissioner Lester Crawford said there is no scientific basis for the introduction of proposition 65 warning labels on tuna, and that the lawsuit could have "adverse public health consequences".
He claims that the existing warnings over mercury are sufficient and that the agency's approach has been carefully thought out to avoid scaring the public off eating fish altogether.
The FDA's stance on the matter has met with applause from the tuna industry.
"We are pleased that the FDA continues to recognize that canned tuna is a healthy food and provides many nutritional benefits to the public," said David Burney, executive director of the US Tuna Foundation. "Tuna should be treated as an important source of nutrition and an important food source for the low-income community, not a political football."
At the time the lawsuit was filed, Lockyer said: "This is a crucial public health issue. Prenatal exposure to mercury can cause serious disabilities in infants and children. We're not trying to eliminate tuna from people's diets. We're trying to enforce the law and protect the health and safety of California women and children."
The attorney general's office is reported to have responded to the FDA, claiming that its comments are timed to quash California's case in the impending trial.