Greenpeace boat the Esperenza is due to arrive in South West waters on Thursday to investigate the growing number of dolphin deaths around the region.
Thw winter is the worst time for small cetaceans
Crew members say they want to talk to pair trawler fishermen who are blamed for the deaths.
The charity says it felt compelled to act before the creatures become extinct.
In a joint report, The Net Effect?, the groups estimate around 10,000 animals die annually on UK and French coasts.
Figures for 2002-2003 show 250 cetaceans were found dead on Cornwall's beaches while 80 were found on Devon's shores.
This year conservationists expect the figure to be even higher.
They welcome European Union initiatives aimed at protecting the creatures, but say they do not go nearly far enough.
The Esperanza, is spending about six weeks in the English Channel, the Celtic Sea and the Bay of Biscay.
Its crew will document the amount of bycatch (dolphins and porpoises caught accidentally) in the nets, while a Whale and Dolphin Conservation Society team will study the behaviour of the mammals round the nets.
Some show signs of trauma
The report, which pulls together existing knowledge on cetaceans (whales, dolphins and porpoises) around the UK coast, blames the huge nets towed by two vessels for much of the carnage.
These pair trawls are used in the sea bass fishery, which runs from October to April. The report says fishing for mackerel, hake, albacore tuna and horse mackerel also threatens common and Atlantic white-sided dolphins, bottlenose dolphins and long-finned pilot whales.
But Andrew Pillar of Interfish, a fish processing company in the west of England, told BBC News Online observers from the Sea Mammal Research Unit had confirmed that mackerel pair trawls did not threaten cetaceans.
Hundreds of dead cetaceans wash up on the coasts of south-west England and north-west France each winter.
The two groups say: "Many have sustained injuries - broken beaks, torn flippers, bruising and lacerations - which tell the story of a prolonged death in fishing nets.
"The bodies of thousands of others never wash up and are claimed by the ocean. It is estimated that around 10,000 dolphins and porpoises are killed in nets in this region every year."
They say fishing methods could be pushing some species towards extinction, and that the common dolphin could be declining by an unsustainable 5% annually.
Lack of information
Last year Richard Sabin, coordinator of the national whale stranding recording scheme at the Natural History Museum in London, said the numbers of small cetaceans dying in the area were far above the 1.7% of the population which scientists think is the maximum sustainable annual loss.
But he told BBC News Online the baseline data did not exist to say whether any species faced extinction in the UK.
Greenpeace says there must be observers on as many fishing vessels as possible, not just the 5-10% of the fleet proposed by the European Commission.
It also wants action taken against fisheries found to be responsible for any cetacean bycatch.
Images courtesy of Greenpeace.