Decline in bottlenose dolphins is linked to pollutants

 Debs Wallis and Jeff Loveridge examine a stranded bottlenose dolphin, photo by Jan Loveridge

Are PCB's responsible for decline in Bottlenose dolphins?

August 2011. Many people have been thrilled at the sight of playful, energetic bottlenose dolphins around our coasts. But they may be surprised to hear that numbers of the local population around Cornwall are gradually declining. Conservationists are concerned that eventually we may lose them altogether. The reason for the drop in numbers has been a mystery until now, as local scientists think they know why.

Strandings
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. Having researched the decline in the population over a number of years, the Marine Strandings Network was keen to know why they died and wanted to find out more. Could there be a reason why, despite sightings of calves and young dolphins, the pod didn't appear to be thriving?

Post mortem
The dolphins were post mortemed by local veterinary pathologists at AHVLA Polwhele, as part of the UK Cetacean Strandings Investigation Programme, which is funded by Defra. Samples were tested for various diseases by Nick Davison, a scientist at the AHVLA.

Nick explains "Dolphins carry lots of diseases and can live for years with some of them without being adversely affected. However, there are certain bacteria that some of them carry that can severely compromise their health and may lead ultimately to their death. One of these is a bacterium called Brucella ceti, which affects many different species of cetaceans all over the world.  Even healthy looking animals can carry the disease."

Brucella ceti
"We ran a wide range of routine tests on the samples we had stored from all eight of the bottlenose dolphins, including the test for Brucella ceti. What really surprised us was that half of the bottlenose dolphins we tested carried Brucella, which is a rare thing to find in this species and in this region. In fact, no other isolations of Brucella species were found in 15 bottlenose dolphins from the rest of England and Wales between 1989 and 2008, and only one out of 36 dolphins examined in

Scotland during the same period. We also discovered that the levels of Brucella in the Cornwall dolphins were much higher than in the other species we'd examined."

PCBs
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 toxic chemicals that used to be added to paints and cements, fire retardants, adhesives and hydraulic fluids. Because of their toxicity, they were banned about 20 years ago, but they still persist in the marine environment, particularly in coastal waters. They get into the fish the dolphins eat and are readily absorbed into fat, so the dolphins' blubber is an ideal reservoir.

High exposure to PCBs may increase susceptibility of the bottlenose dolphins to infections, including Brucella ceti. PCBs have also been linked to fertility and reproductive issues in dolphins and can affect the immune system, exposing the dolphins to a number of serious diseases and conditions such as meningitis swelling of the brain.

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 says,"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 bottlenoses are such a local icon; they're part of Cornwall's heritage and one of the reasons that people visit the county. It would be a tragedy if we lost those precious moments when we see them leap and flip in the air and I'd hate to think that our children and grandchildren might be denied that pleasure. We need to keep monitoring how they're doing, and do whatever we can to protect them. Research like this by Nick Davison is crucial if we're to understand what's going on in our marine environment."

Although Brucella can be passed to humans, the risk from dolphins is very small indeed. Nevertheless, the AHVLA recommends that people don't swim with dolphins or touch any bodies they find on the beach, just in case. Instead, they urge the public to call the Trust's Marine Strandings Network on 0845 201 2626 if they find a stranded dolphin.

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