Navy technology responsible for beached dolphins' death

A Navy helicopter was using controversial sonar equipment off the Cornish coast days before 26 dolphins died in a mass stranding.

Officials at the Ministry of Defence admitted last night that the sonar "dipper", designed to hunt submarines, had been used by a Merlin helicopter on training exercise.

The mass dolphin stranding was the biggest in Britain for 30 years.

Four days later, the dolphins beached in the shallow Fal and Percuil rivers 60 miles away near Falmouth. The mass stranding last Monday was the biggest in Britain for 30 years. Conservation groups want a full investigation. Marine wildlife and underwater acoustics experts said loud pulses from the sonar may have scared or confused the common dolphins into Falmouth Bay, where they became disorientated. An investigation by The Sunday Telegraph discovered that the sonar was being used as part of an exercise involving the nuclear-powered submarine Torbay and a number of surface warships.

The Merlin was equipped with a mid-frequency sonar dipper known as S2089, which is winched down into the sea to detect submarines beneath the surface. Mid-frequency sonar, which transmits pulses of sound just beyond the range of human hearing, has been associated with past strandings of marine mammals.

Research by the US Navy has blamed such sonar for whale strandings. Prof Rodney Coates, an expert on underwater acoustics who has investigated the effect of sonar on whales and dolphins, said middle-range sonars were powerful enough to damage an animal's hearing if it was close enough. "They can also have a behavioural impact that can lead them to swim into the shallower water."

Immediately after the dolphins died, military officials insisted that the only vessel using sonar beforehand had been a survey ship operating a low-powered seabed scanner. Later statements have contradicted this position.

Sarah Dolman, science officer at the Whale and Dolphin Conservation Society, said: "The Royal Navy activity in the days leading up to the strandings involved intense noise sources and this makes it a credible suspect."

Experts at the Institute of Zoology, who run the Cetacean Strandings Investigation Project (CSIP) are examining the dead animals. Rescuers saved seven of the stranded dolphins, but 24 died and two more had to be put down. The naval exercises took place south of Portsmouth and the S2089 was used on Thursday, June 5. The survey ship Enterprise was scanning the seabed using high-frequency sonar.

Prof Ian Boyd, director of the Sea Mammal Research Unit at St Andrews University and an expert on the effect of sonar on whales, said he did not believe the high-frequency sonar would have been powerful enough to harm the dolphins. He added, however: "There is a relationship between the type of sonars the military uses for hunting submarines and the strandings of some species of cetaceans."

Rob Deaville, project manager of CSIP, said: "The role of naval sonar is one of the things we will look at, but we are keeping an open mind. We have seen no evidence yet of physical damage in the animals we have examined so far."

A spokesman for the MoD said: "We are very confident that it is highly unlikely that Royal Navy activities resulted in what happened to these mammals."