Mercury in Dolphins: Study Compares Toxin Levels in Captive and Wild Sea Mammals

Amid growing concerns about the spread of harmful mercury in plants and animals, a new study by researchers from The Johns Hopkins University and The National Aquarium has compared levels of the chemical in captive dolphins with dolphins found in the wild. The captive animals were fed a controlled diet, while the wild mammals dined on marine life that may carry more of the toxic metal.

The study found lower levels of mercury in the captive animals, particularly 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. The aquarium dolphins are fed smaller fish from North Atlantic waters, where mercury pollution is less prevalent.

The findings were published in a recent issue of Science of the Total Environment.

Although these results represent a significant case study, the research focused on a small number of animals, and Edward Bouwer, chair of the Department of Geography and Environmental Engineering at Johns Hopkins and supervisor/co-author of the study, cautioned against drawing wide-ranging conclusions from the research.

“This is just one snapshot, one puzzle piece,” said Bouwer. “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.”

Public health officials are concerned about human consumption of mercury, particularly in a form called methylmercury, because it can damage the brain and other parts of the nervous system, especially in young children. Dolphins that ingest too much methylmercury can suffer similar harm.

Mercury is emitted as a gas from coal- and oil-fired power plants. Some makes its way into the ocean, where bacteria turn it into methylmercury, which moves up the food chain. Eventually, it turns up in the large fish that serve as dinner for wild dolphins. Once ingested, the heavy metal makes its way into the animals’ bloodstream, where it can begin to damage the nervous system.

With this in mind, the Johns Hopkins-National Aquarium researchers wanted to investigate mercury exposure differences between captive and wild dolphins whose diets came from different regions. The team collected blood samples from seven aquarium dolphins, ages 2 to 38, and tested them for mercury, methylmercury and a third chemical, selenium, which appears to help ward off the toxic effects.