Highest levels of mercury ever recorded are found in blubber of dolphins in English Channel

Scientists also found high levels of toxic organic pollutants that were banned as long as 40 years ago

Scientists believe the pollutants weaken their immune system and means the dolphins (pictured) face higher rates of illness and disease than captive counterparts (Antoine Soubigou/GECC)

Bottlenose dolphins in the English Channel have among the highest levels of toxic mercury ever recorded in their blubber and skin, scientists have found.

They also have high levels of other toxic organic pollutants known as PCBs (polychlorinated biphenyls) that were banned in the 1970s and 1980s because of the risk they posed to wildlife and humans. The chemicals – often used as refrigerants – are extremely durable, which is why they have remained in the ecosystem four decades after being outlawed.

“These organic compounds are able to dissolve in fats and oils, and consist of the by-products of various industrial processes and pesticides, among others,” said lead researcher Dr Krishna Das, a zoologist at the University of Liege in Belgium.

“Bottlenose dolphins are often used to study levels of environmental pollutants, as the organic compounds accumulate within their thick layer of fatty tissue,” she said.

Researchers took skin biopsies during boat surveys of 82 wild dolphins living in the Normanno-Breton Gulf in the English Channel. They found chloride-containing compounds (PCBs) from industrial fluids made up more than 91 per cent of organic chemicals in their skin, according to the paper published in Scientific Reports.

Mercury levels were similar to levels described in the bottlenose dolphin populations living in notoriously contaminated areas in the Mediterranean Sea and Florida Everglades.

Scientists believe the pollutants could weaken their immune system and mean the dolphins face higher rates of illness and disease than their captive counterparts. Previous research on the effects of PCB exposure has linked it to reduced survival rate of calves.

Dr Das said: “The reported concentrations are among the highest reported for cetaceans.

“We strongly recommend the Normanno-Breton Gulf be a special area of conservation candidate because it contains the last large European population of bottlenose dolphins.”

About 420 coastal bottlenose dolphins live in the the Normanno-Breton Gulf around the Channel islands.

“The bottlenose dolphins in European waters are protected by the Habitats Directive. Their conservation requires the creation of special areas of conservation and the need for strict protection,” Dr Das said.

“Despite this European directive, human activities are increasing in the Normanno-Breton Gulf. The potential threats include pollutants, noise pollution, particularly construction noise, disturbance by tourism activities and by-catch,” she said.

This latest research follows a study from March this year which found levels of mercury, arsenic and lead at shockingly high levels in the blood of great white sharks swimming off the coast of South Africa.

Despite being present at concentrations that would kill most animals, these toxins appear to have no effect on the enormous predatory fish. The scientists undertaking the tests think the sharks may have a special ability to resist the dangerous effects of the heavy metals.

As both great whites and bottlenose dolphins are top predators, they accumulate high volumes of toxins in their bodies from all the other creatures they eat.