how it was explained to me by a holistic doctor was that the shorter the life span of the fish before they are cuaght and processed the less mercury it will absorb ,salmon, mackeral and tuna have the highest beacause of there longer life span before being caught and processed , sardines have the shortest life span before being caught and processed so they contain less murcury ,if you decide to eat sardines for protein make sure you get the sardines that are packed in water and not other oils like soybean or canola these oils can affect you. If you take algin before and after eating sardines you should not have any problems with mercury