Twenty-six common dolphins became stranded in estuarine waters they are not normally known to frequent (Image: Adam Gerrard/SWNS.COM)

An investigation into the UK’s largest ever mass stranding of common dolphins has identified military activity as the most probable cause – although no single activity can be definitively linked to the stranding.

Twenty-six common dolphins died after becoming stranded in the Fal Estuary in Cornwall, southwest England on 9 June, 2008, while a similar number were refloated by volunteers. An investigation into the cause of their deaths (pdf format) by Paul Jepson at the Zoological Society of London and his colleagues has ruled out a lengthy list of possible causes:

• Infectious disease


• Pollution

• Decompression sickness

• Attack by killer whales or bottlenose dolphins

• High-intensity acoustic inputs from seismic airgun arrays or natural sources

However, documents obtained under the UK Freedom of Information act have provided researchers with unprecedented access to military records of navy activity in the area. While there is no evidence of physical injury to the dolphins caused by sonar, “what we are left with is a mass stranding and a naval exercise – we have ruled out pretty much everything else,” Jepson says.

The UK navy had been conducting exercises in the area several days before the stranding, and on the morning of the stranding itself. Jepson suspects that these may have driven the dolphins closer to shore than normal, and that something then caused them to panic and beach.

Sound damage

Military sonar has previously been linked to strandings of cetaceans elsewhere and is known to temporarily deafen dolphins.

However, the Ministry of Defence says that routine exercises including sonar activity had ceased 60 hours before the onset of the stranding and cannot therefore have directly triggered the event. It says it is prepared to work with researchers to further explore how its activities might affect cetacean behaviour.

Sarah Dolman of the UK’s Whale and Dolphin Conservation Society believes they should go even further.

“We are calling for the Ministry of Defence to conduct a full and transparent environmental assessment of all of its activities in this exercise area and in all exercise areas that it operates in,” Dolman says. “The post mortem results have shown us that those dolphins that died were healthy animals prior to stranding. Something frightened them ashore, way up inside the river system, where this species is not generally known to go.”