About 800 whales, dolphins and porpoises, known collectively as cetaceans, are dying in fishing nets every day, researchers say.
By Alex Kirby
BBC News Online environment correspondent
They believe as many as 300,000 cetaceans die annually as bycatch, probably the biggest danger they face.
Spotted dolphin (Image: WWF-Canon/Sylvia Earle)
Most of the animals drown in the nets, some die of exhaustion, and sharks attack others.
The researchers say there are effective ways of reducing bycatch, and these cut deaths significantly.
The team includes Dr Andrew Read, of Duke University Marine Laboratory, US, and Dr Simon Northridge, of the Sea Mammal Research Unit (SMRU), St Andrew's University, UK.
Both are members of the scientific committee of the International Whaling Commission, which holds its annual meeting in the German capital, Berlin, from 16 to 19 June.
They have compiled the first global estimate of deaths from fishing gear of both cetaceans and pinnipeds (seals, sealions and walruses).
WWF, the global environment campaign, says the thousands of kilometres of nets set in the world's oceans daily pose the greatest danger to cetaceans' survival.
Dolphin: One victim of the nets
It says the nets are often invisible both to sight and sonar, and are too strong for small cetaceans to break through.
It is urging IWC member states to support a resolution at the Berlin meeting aimed at reducing cetacean deaths from this cause.
Dr Read, who co-chairs WWF's cetacean bycatch task force, said: "This level of bycatch is significantly depleting and disrupting many populations of whales, dolphins and porpoises.
"It will lead to the loss of several species in the next few decades if nothing is done.
"Where measures were taken to reduce bycatch, mortality dropped significantly. This demonstrates that it is possible to reduce bycatch while maintaining viable fisheries."
WWF says the most endangered cetaceans are:
the European harbour porpoise, with about 600 animals left
the vaquita, the world's smallest porpoise: found in Mexican waters, only 500 or so survive
Maui's dolphin, with probably fewer than 100 animals living off New Zealand
about 70 Irrawaddy dolphins in the Philippines
the franciscana (the La Plata dolphin), found off the coast from Brazil to Argentina
the few remaining baiji dolphins in China.
Dr Susan Lieberman, the head of the WWF delegation to the IWC meeting, said: "More whales, dolphins and porpoises die every year from entanglement in fishing gear, especially gillnets, than from any other cause.
"WWF calls on the IWC, as the globally recognised forum for cetacean conservation, to address this crisis and adopt a formal action plan for bycatch reduction."
Dr Northridge told BBC News Online: "There are various ways to tackle the problem - acoustic devices called pingers on gillnets, for example.
"Another is to try to persuade the fleets to change their gear or use alternative techniques. The Italians are trying to get their boats to abandon driftnets and use longlines instead.
"We hope approaches like these will help to reduce the number of cetaceans dying in the nets."