Having a sustainable seafood strategy is becoming even more important: Mercury levels are increasing in some of the most popular fish in the American diet, according to a new study out of Harvard and published in the journal Nature. The research found that from the 1970s through the 2000s, methylmercury levels in Atlantic cod climbed 23 percent as a result of overfishing. The model used in the research also predicted mercury levels in Atlantic bluefin tuna would increase 56 percent between 1969 and the present as a result of higher seawater temperatures. Because overfishing and changing seawater temperatures are causing fish to alter their diets – often to include fish that are higher or lower in mercury content -- people who distribute and serve fish need to understand how environmental factors are impacting the food chain. (E.g. As Healthline notes, Atlantic cod had high levels of mercury until their main food source, herring, were overfished. Then as herring returned, mercury levels in cod increased again.) If you or your guests feel strongly about having tuna and cod on the menu, use suppliers that lobby for tighter regulations on fishing and make efforts to stop climate change and reduce pollution.