Higher Levels of Mercury in Florida Dolphins

A joint research venture between John Hopkins University and The National Aquarium comparing the levels of mercury in captive dolphins versus those in the wild has determined that the dolphins in the wild carry higher levels of mercury. The study comes amid growing concerns that levels of the heavy metal might be harming wildlife.

The study, titled "Mercury in dolphins higher downwind of power plants," observed captive dolphins at the National Aquarium in Washington D.C. while those in the wild were studied off the Gulf and Atlantic coasts of Florida. The dolphins in captivity were fed smaller fish from North Atlantic waters where pollution is less prevalent compared to the waters around Florida downstream from power plants, which emit mercury-laden fumes.

However, Edward Bouwer, chair of the Department of Geography and Environmental Engineering at John Hopkins and co-author of the study, cautioned against reading too much into the findings.

"This is just one snapshot, one puzzle piece. 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. This type of research can give us hints about how the type of diet and where it originated can affect mercury-related health problems in captive dolphins, compared to their cousins in the wild."

The different levels of mercury in the dolphins were attributed to their diets. North Atlantic fish are smaller, and lower down on the food chain. Through a process known as bioaccumulation, the predators at the top of the food chain end up ingesting all of the toxins that have built up on the way to the top. Big fish eats small fish. Bigger fish eats big fish and gets the toxicity in the flesh of both the small and big fish. And dolphins are pretty big fish.

Coal and gas-powered plants release mercury as a byproduct, where bacteria in the ocean can turn it into methylmercury, a toxic substance that can cause damage to the nervous system and inhibit development. Studies imply that the same happens in animals.

The researchers also compared their results against previous tests done in Charleston, South Carolina, saying that, "While mercury levels in the wild dolphins off South Carolina were slightly higher than those in the National Aquarium dolphins, readings from the dolphins off the Florida coasts were significantly higher."