Wild dolphins have weaker immune systems than captive dolphins because of polluted sea water, a study has found.

Researcher`s compared the immune health of four groups of bottlenose dolphins living in aquariums and off the coast of America.

Pollutants in the oceans around Florida and South Carolina were found to be putting a strain on the wild dolphins' immune systems, making it more difficult for the animals to fight off bacteria, fungi, viruses, and parasites.

Wild dolphins' immune systems appeared to be "chronically activated" as a result of the unhealthy environments they were living in, said the study's lead author Patricia Fair, a research professor at the Medical University of South Carolina.

“This is likely a result of encountering and fighting off illness caused by pathogens, parasites and anthropogenic pollutants in the ocean that do not exist in closely managed zoological habitats,” she explained.

“The key to a healthy immune system is a balance between being able to recognise harmful organisms and over-stimulation and this study demonstrates the importance of the environment in these responses.”

Dolphins living in the ocean near Charleston, South Carolina were exposed to the highest levels of industrial pollution, and suffered more disease as a result.

Because dolphins are high on the food chain, they accumulate any toxins ingested by their prey, the researchers said. Industrial pollutants released into the water accumulate in micro-organisms, these are eaten by fish, which are in turn eaten by dolphins. At each step up the food chain the toxins become more concentrated.

The researchers suggested the findings of the dolphin study could have implications for human health in the area too, as local fishermen and residents are exposed to and consume the same toxins

Meanwhile, large amounts of toxic mercury were found in dolphins living in the Indian River Lagoon in Florida.

Annual dolphin hunt in Taiji, Japan Show all 15 1 /15 Annual dolphin hunt in Taiji, Japan Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan A bottlenose dolphin was seen floating on back before slaughter Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Fishermen hiding their culture and tradition Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Remaining pod swims just a few feet from the slaughter of their family Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Dolphin drive out to sea Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Lathered in blood, fishermen receive more transfers of dolphin carcasses Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Fishermen enter the cove just after sunrise Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan A juvenile Bottlenose barely surfaces during drive out. The chances of survival are slim after 5 tormenting days in the cove Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Cove Guardians Jac and Ian document the slaughter Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan SSCS Cove Guardian Leader Melissa Sehgal interviews for CNN Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Fishermen in wetsuits hunt dolphins at a cove in Taiji, western Japan; U.S. ambassador to Japan Caroline Kennedy has expressed deep concern over the traditional dolphin hunt. Local fisherman corral dolphins in a secluded bay before killing many for meat Adrian Mylne/Reuters Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan The selection process of dolphins, during the annual dolphin hunt in Taiji. With 250 dolphins, this was the largest round-up in years Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan The agitated dolphins in the cove during the selection process. According to Sea Shepherd, Japanese fisherman rounded up more than 250 dolphins, including babies and juveniles Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan Japanese fisherman are shown in the cove. Taiji town claims the hunt is an important ritual dating back centuries Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan A rare albino calf swims close to his/her mother as the pod was herded into the cove. Dolphins captured in the cove are either sold into captivity, or slaughtered and sold for consumption, despite pleas from animal conservationists around the world against the event Sea Shepherd/EPA Annual dolphin hunt in Taiji, Japan The annual dolphin hunt in Taiji, Japan The process of selecting dolphins during the annual cull, which the mayor of the town defends 'on scientific grounds'

Doctor Gregory Bossart, a co-author of the study and chief veterinary officer at Georgia Aquarium, said the implications of chronic immune system activation in wild dolphins could be severe.

"In humans, this type of prolonged smouldering inflammation is associated with cancer, auto immune disease, cardiovascular disease, and increased vulnerability to infectious disease,” he said.

By comparison, the dolphins kept in the Georgia Aquarium in clean water had healthier immune systems which appeared to be under a lot less stress, Dr Bossart said.

“Dolphins in human care are exposed to fewer pathogens because of environmental controls of water and food quality and preventative medical programmes. Thus, their immune responses tend to be more focused and short acting. Our findings suggest that the wild dolphins of our study have immune systems that are chronically activated and challenged."