Report to show huge rise in beached whale and dolphins on British shores increasing, but many more go unreported

Marine scientists will this week appeal for volunteers to watch for stranded whales and dolphins and report instances as soon as possible to understand why so many are being found on beaches.

It follows the unexplained mass stranding of 33 pilot whales found dead on Rutland island off the coast of northwest Ireland last month and more than 500 dolphins, porpoises and whales stranded on British beaches this year.

New figures to be released this week will show that whale and dolphin strandings are increasing. According to the UK Cetacean Strandings Investigation Programme (CSIP) there have been 9,494 recorded strandings on British coasts in the past 20 years, but the number could be much higher because many are likely to go unreported. Most are washed ashore dead but around 10% are still alive.

Strandings have increased by nearly 25% since central records were first kept by CSIP. "In the first 10 years there were between 300 and 400 a year, but since 2000 there have been more. The highest number was in 2003 when 800 were found stranded. Now there are around 500 a year," said Robert Deaville, project manager for CSIP.

"We under-report the numbers. There are several very good volunteer networks in the south west, Wales and Scotland, but we do need more people in England," said Deaville.

Because cetacean research is hard to conduct at sea, information gleaned from strandings has become essential to understand the health of whale and dolphin populations.

Between 1990 and 2000 there were no recorded strandings of humpback whales on British coasts, but since then there have been 13. "That suggests that humpback whale populations are recovering. It's very sad for the individuals but it reflects the [wider] growth in populations," said Deaville.

Post mortems show that most stranded animals have died from natural causes but high-powered sonar used by navies and the oil and gas industry is increasingly associated with deaths.

However, scientists are wary of attributing too many strandings to the technology. Only around 14 animals found on British coasts in the past decade are likely to have been affected by sonar, said Paul Jepson, a cetacean researcher at the Institute of Zoology.

"It's not very common, but if we are detecting even a few it could mean there are many more at sea. Old records show that well before sonar or the oil and gas industry came there were mass strandings. There is a natural component but the trouble is separating the natural from the man-made.

"Sometimes we cannot find any disease, injury or trauma in stranded animals and they appear in good health. Some of these could be caused by something acoustic," said Jepson.

Scientists think the large pod of pilot whales found dead on Rutland island, Donegal last month was the same seen off the outer Hebrides a few days earlier. It was reported that the Royal Navy had been exercising in the region and could have disoriented the pod with its sonar.

Pilot whales are known to be sensitive to acoustic disorientation and have been found to suffer from a condition known as "gas embolism", which is common to the bends suffered by human divers.

But the navy has insisted its activities had nothing to do with the mass stranding, saying the nearest vessel to the incident was more than 50 nautical miles away, at the Clyde Naval Base, Faslane, and was not using sonar.

Richard Benyon, minister for environment and fisheries, urged people to watch for stranded animals. "Keen-eyed members of the public can also play a vital role in reporting strandings. Dog walkers, ramblers and people all around the country can help us capture information about every whale or dolphin that strands on our beaches."

• For more information about how to report a stranding visit the CSIP website.