They say more than half of all species are of conservation concern, with two on the “knife-edge” of extinction.
Lack of action over polluted and over-exploited seas means that many will be declared extinct within our lifetimes, the letter says.
Even large iconic whales are not safe.
“Let this be a historic moment when realising that whales are in danger sparks a powerful wave of action from everyone: regulators, scientists, politicians and the public to save our oceans,” said Mark Simmonds.
The visiting research fellow at the University of Bristol, UK, and senior marine scientist with Humane Society International, has coordinated the letter, which has been signed by experts across the world.
“Save the whales” was a familiar green slogan in the 1970s and 1980s, part of a movement that helped bring an end to commercial whaling.
While stricken populations in most parts of the world have had a chance to recover from organised hunting, they are now facing myriad threats from human actions, including plastic pollution, loss of habitat and prey, climate change and collisions with ships.
By far the biggest threat is becoming accidently captured in fishing equipment and nets, which kills an estimated 300,000 whales, dolphins and porpoises a year.
Hundreds of scientists have expressed the same concern – that we are moving closer to a number of preventable extinctions. And unless we act now, future generations will be denied the chance to experience these intelligent social and inspiring creatures.
They point to the decline of the North Atlantic right whale, of which only a few hundred individuals remain, and the vaquita, a porpoise found in the Gulf of California, which may be down to the last 10 of its kind.
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And they say it is almost inevitable that these two species will follow the Chinese river dolphin down the path to extinction. The dolphin, also known as the baiji, was once a common sight in the Yangtze River but is now thought to have died out.
The letter, which has been signed by experts in the UK, US, Mexico, South Africa and Brazil, among others, points out that these “dramatic” declines could have been avoided, but that the political will has been lacking.
Dr Susan Lieberman of the Wildlife Conservation Society said she signed the letter to help scientists raise these issues more widely.
“It is critical that governments develop, fund, and implement additional needed actions to better protect and save these iconic species – so they don’t end going the way of the baiji,” she told BBC News.
The scientists say that more than half of the 90 living species of whales, dolphins and porpoises, are of conservation concern, and the trend of acting “too little, too late” must end.
They are calling on countries with whales, dolphins and porpoises (cetaceans) in their waters to act to monitor threats and do more to protect them.
Sarah Dolman of Whale and Dolphin Conservation, UK, said accidental capture in fishing gear, known as bycatch, is an issue around UK waters, causing the deaths of thousands of cetaceans and other animals, including seals and birds, a year.
These include harbour porpoises and common dolphins, and increasing numbers of minke and humpback whales off the coast of Scotland.
She said entanglement in fishing nets was a “horrible way to die” with some animals surviving with broken teeth or beaks, or losing their young.
She told BBC News: “We have a long way to go before we can be confident the fish we are eating is not causing bycatch of protected species like whales and dolphins.”
The letter is part of a growing movement by scientists and conservationists to raise awareness of the threats faced by whales and their smaller relatives, the dolphins and porpoises.
The matter was discussed in September at a meeting of the scientific conservation committee of the International Whaling Commission, which has a core mission to prevent extinctions.
Members have set up an “extinction initiative” to work out how many extinctions we may be facing and what more we can do to prevent them.
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