Get a different brand of Sardines. My local grocer has several different "types" of sardines packed in everything from hot sauce to water to some type of oil (soy, olive, sunflower, etc.).
Generally, the fish oil shouldn't contain mercury as mercury isn't "fat soluable" (sp?) and won't store in the fat. They are supposedly only stored in the muscle (meat) tissues. However, if it's a big deal, look for fish oil that is "Molecularly Distilled" as this is supposed to have all the contaminates removed and be a pure product.