Campaigner Lindy Hingley with a dead dolphin
A total of 106 dolphins and porpoises have washed up on Cornwall's beaches and in the nets of fishing boats in just eight weeks, according to Cornwall Wildlife Trust.
The toll for the whole of last year was 205 while in the two previous years the numbers had been under 100.
Large trawlers are being blamed for the alarming increase - with French boats said to be the worst offenders as they work in pairs.
It is understood they are competing with dolphins for fish such as mackerel, herring, bass and sprats and experts say they are wiping out entire family groups.
The mammals get caught up in the nets used by trawlermen and are suffocated when held under the water.
Lindy Hingley, founder of Brixham Sea Watch, said: "It's murder. It's a massacre.
"It takes 20 minutes for them to die, and it's an appalling death."
Since Christmas, small trawlers operating in an area from Mevagissey in Cornwall to Plymouth in South Devon have hauled dozens of dead and rotting dolphins.
Martin 'Beaver' Thomas, a trawlerman based in Polperro, Cornwall, said it had become so common they don't even talk about it anymore.
He said: "We have been trawling up dead and stinking dolphins. The smell is horrific. All the boats around here have been bringing up dead dolphins.
"We have all towed up the odd dead dolphin in the past and talked about it in the pub. But now it's not a rare occurrence."
Lindy Hingley added that more than 30 dead dolphins and porpoises had been found in Devon so far this year.
She said: "More have been found in Cornwall because the boats catching them have been working off Cornwall.
"It's turning out to be a pretty grim year."
A spokesman for Devon Wildlife Trust, which has launched a Devon Dolphins campaign, said: "News of dead dolphins and porpoises is always distressing but this winter the numbers of these beautiful animals being stranded does seem to have exceeded those of recent years. It is now a cause of considerable concern.
"The precise cause of these deaths is often hard to ascertain but the numbers occurring do give greater urgency to the calls for better protection for all our marine wildlife. Devon Wildlife Trust, along with the wider Wildlife Trust movement has led the recent campaign for a Marine Protected Area off the coast of Devon.
"If designated by the government, the Lyme Bay Deeps Marine Conservation Zone would specifically provide protection to an area of our seas off Torbay which we know is important as a feeding and breeding area for dolphins and porpoises."
The Marine Management Organisation said the creatures are legally protected.
A spokesman said: "We're aware of local fishermen's concerns and have been working with other organisations to look into these."
Rob Deaville, UK project manager of the Defra-funded Cetacean Strandings Investigation Programme, said they were told of 76 dead dolphins in January alone.
Of those, 13 were taken away for post-mortem examinations - and five showed clear signs they had been caught in nets.
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