17 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 and conservationists are concerned that eventually, we may lose them altogether. The reason for the drop in numbers remained a mystery, but now, local scientists think they know why.

Over a period of six years, volunteers of the Cornwall Wildlife Trust Marine Strandings Network collected the bodies of stranded bottlenose dolphins that had washed ashore and took them to the pathologists at the Animal Health Veterinary Laboratories Agency (AHVLA) at Polwhele, Truro to be examined. Having researched the decline in the population over a number of years, the Strandings Network were keen to know why they died and wanted to find out more about their health and breeding fitness. Could there be a reason why, despite sightings of calves and young dolphins, they didn't appear to be thriving?

The dolphins were necropsied by local veterinary pathologists at the AHVLA Polwhele, under the aegis of the UK Cetacean Strandings Investigation Programme, which is funded by Defra. Samples from them were tested for various diseases by Nick Davison, a bacteriologist at the AHVLA. As Nick explained, "Dolphins carry lots of diseases and can live for years with some of them, without being adversely affected. But 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, and has been found in quite a few of the harbour porpoises, striped dolphins and common dolphins we've examined before in Cornwall. Even healthy looking animals can carry the disease."

Nick Davison continued: "We ran a wide range of routine tests on the samples we'd stored from all eight of the bottlenose dolphins, including the test for Brucella ceti, and 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 was much higher than in the other species we'd examined."

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, and the fact that 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 for around two decades, 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 with a range of pathogens, including Brucella ceti. They have been linked to impairment of the reproductive organs and can lead to sterility in the males, abortion in females and weakness or mortality in offspring. In addition, they can affect the immune system, exposing the dolphins to a number of serious diseases and conditions such as meningitis and encephalitis, which affect the brain. In a recent review of chemical pollutants in marine mammals in the North Atlantic and Arctic oceans, high levels of PCBs were highlighted as the greatest toxicological threat to top marine predators, such as bottlenose dolphins and killer whales and this, the scientists think, 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 survivability. 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 main reasons 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 the think that our children and grandchildren might be denied that pleasure. Local people care very much about what happens to them", she continued, "and we need to keep monitoring how they're doing. We also need to do whatever we can to protect them and research like this by Nick Davison is crucial if we're to understand what's going on in our marine environment."

The public are urged to call the Strandings Network on 0845 201 2626 if they find a stranded dolphin, as their team is trained to collect the body for examination by the AHVLA.

The publication describing the findings of this research can be read on the publications page.