A study has found higher levels of toxic mercury in dolphins downwind of power plants than in captive dolphins, U.S. researchers say. Scientists from The Johns Hopkins University and The National Aquarium said they compared levels of the chemical in captive dolphins fed a controlled diet with dolphins found in the wild that dine on marine life that may carry more of the toxic metal. Levels of mercury were lower in the captive animals compared to wild dolphins tested off the Atlantic and Gulf coasts of Florida, a state that is in the path of mercury-laden fumes from power plants, they reported Monday. Since the research was a small, pilot study, researchers cautioned against drawing wide-ranging conclusions. \"This is just one snapshot, one puzzle piece,\" said Edward Bouwer, chair of the Department of Geography and Environmental Engineering at Johns Hopkins. \"What we\'d like to do now is repeat this project with aquariums in other parts of the world. The goal is to get a clearer comparison of mercury-related health risks facing dolphins both in captivity and in the wild.\" The captive dolphins in the study were fed smaller fish from North Atlantic waters where mercury pollution is less prevalent, researchers said. \"The aquarium dolphins were fed a consistent level of small fish --capelin and herring -- that were caught in North Atlantic waters off Newfoundland and New England,\" Johns Hopkins environmental scientist Yongseok Hong said. \"Lower levels of mercury are expected in these waters, compared to the waters off Florida.\" In December, after the study was conducted, the U.S. Environmental Protection Agency adopted strict standards aimed at reducing the release of toxic air pollution from coal- and oil-fired power plants, a key source of mercury contamination in sea water.