The populations of bottlenose dolphins in the seas off Cornwall are declining in numbers - but scientists now believe they know why.

Over a period of six years, volunteers of Cornwall Wildlife Trust's Marine Strandings Network have collected bodies of stranded bottlenose dolphins washed ashore.

They then take them to the pathologists at the Animal Health and Veterinary Laboratories Agency (AHVLA) at Polwhele in Truro to be examined.

There, half of the dead creatures were discovered to be carrying a bacterium called Brucella ceti, which is known to have serious health implications for the animals.

Although the bacteria is not easily passed to humans, people are warned not to swim with the dolphins.

The second shock for the researchers was the very high levels of polychlorinated biphenyls (known as PCBs) in the blubber of two of the dolphins.

One of them had the highest level of PCBs recorded in this species in the UK in the last 20 years.

PCBS are toxins found in paints and cements, fire retardants, adhesives and hydraulic fluids.

They were banned more than 20 years ago, but still appear to be finding their way into the sea, where they can make the dolphins more susceptible to bacteria such as Brucella.

Scientists now think that chemical pollutants such as PCBs may be one of the possible causes of the decline of the local bottlenose dolphins.

Jan Loveridge, Coordinator of the Strandings Network said: “Strandings and sightings data suggest that UK bottlenose dolphin populations, including ours in Cornwall, have declined markedly from historic levels and the loss of any individuals from such a small population will have a strong impact on its ability to survive.

“We're so fortunate to have this local laboratory to help us find out what's killing our dolphins."

The publication describing the findings of this research can be read on the Marine Strandings Network's website www.cwtstrandings.org/publications.htm.