Study Compares Mercury Levels In Wild vs. Captive Dolphins
May 23, 2012

Study Compares Mercury Levels In Wild vs. Captive Dolphins

Researchers from Johns Hopkins University and the National Aquarium have completed a study, funded by the National Aquarium and by the Center for Contaminant Transport Fate and Remediation at The Johns Hopkins University, which compared the mercury levels of captive dolphins and wild dolphins. Captive dolphins are fed a diet of small fish from the North Atlantic, while the wild dolphins eat aquatic creatures that have potentially higher mercury levels.

The researchers discovered that the captive dolphins had lower levels of mercury, especially when compared to dolphins off the Atlantic and Gulf coasts of Florida, a state that is in the path of mercury fumes from power plants. The study, published in a recent issue of Science of the Total Environment, concentrated on a small number of dolphins.

Even with the significant results of the study, Edward Bouwer, co-author of the study and chair of the Department of Geography and Environmental Engineering at Johns Hopkins, warned against coming to any wide spread deductions from the study.

"This is just one snapshot, one puzzle piece," Bouwer stated. "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."

Mercury, in the form of a gas produced from coal at oil-fired plants, transfers from the air into the ocean, where bacteria cause it to change into methylmercury. From there, it enters into the food chain, in time ending up in the large fish that dolphins consume and into the dolphins´ bloodstream.

Mercury can cause many health problems when consumed by humans, primarily the type known as methylmercury, which can damage the brain and other parts of the nervous system, predominantly in children. If dolphins consume large amounts of methylmercury, similar health problems can occur.

The researchers, with these factors in mind, studied blood samples from captive dolphins in aquariums and wild dolphins, whose diets vary in location. Blood samples were collected from seven captive dolphins from the ages of two to thirty-eight, and analyzed for levels of mercury, methylmercury, and a different chemical known as selenium, which may help alleviate the damaging effects of mercury exposure.

These results were compared to the blood work obtained from capture and release efforts in Indian River Lagoon on Florida's Atlantic coast, Sarasota on Florida´s Gulf Coast, and off the coast of Charleston, South Carolina.

Lead author of the study and a postdoctoral fellow in the Department of Geography and Environmental Engineering in Johns Hopkins' Whiting School of Engineering, Yongseok Hong stated, “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.”

“The difference in mercury exposure was attributed to differences in the dolphins' diets,” Hong stated. “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. Lower levels of mercury are expected in these waters, compared to the waters off Florida.”

The National Aquarium's director of animal health Leigh Clayton remarked that her staff was eager to participate in the study because it gave them a chance to have a better understanding of the diet that they feed the aquarium´s dolphins.

“It is important that we gain a better understanding of the mercury levels in the North Atlantic food chain in order to ensure we're providing the best diet possible to our dolphins,” Clayton stated. “The research we have done with Johns Hopkins has provided helpful information for our marine mammals team and allows us, at this time, to have confidence that our current fish food sources do not have excessively elevated mercury levels.”

In December of 2011, after the study was completed, the U.S. Environmental Protection Agency (EPA) implemented firm standards with the objective of reducing mercury levels from air pollution, a main cause of mercury pollution within ocean waters.

Co-authors of the study include Clayton, the lead author, Sue Hunter, the National Aquarium's director of animal programs and marine mammals, and Erik Rifkin from the National Aquarium Conservation Center. Three high school students, Debbie Brill, Sara Hamilton, and Amelia Jones, aided the researchers in sample examinations.