California Attorney General Bill Lockyer is suing five major grocery store chains to force them to warn customers that tuna, swordfish, and shark may contain dangerously high levels of mercury. In the suit, Lockyer claims that Safeway, Kroger, Albertson’s, Trader Joe’s, and Whole Foods are violating Proposition 65, which requires companies to provide “clear and reasonable” warnings before exposing people to known carcinogens and reproductive toxins. Mercury has been linked to cancer, birth defects, and reproductive, neurological, and behavioral problems. If the court rules in Lockyer’s favor, the stores would be prohibited from selling fish without posting warnings; they could also face millions of dollars in fines for failing to comply with the law.