'Real and imminent' extinction risk to whales
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.