Of those 20 areas, nine had particularly high numbers of species living together. The areas are off the coasts of Baja California in Mexico, eastern Canada, Peru, Argentina, northwestern Africa, South Africa, Japan, Australia and New Zealand. The other 11 areas were home to species found nowhere else and therefore highly vulnerable: Hawaiian Islands, Galapagos Islands, San Félix and Juan Fernández Islands, the Amazon River, Mediterranean Sea, Indus River, Yang-tse River, Baikal Lake, Caspian Sea and Kerguelen Islands.
PHOTOS: World's Best Marine Reserve: Cabo Pulmo
The researchers also looked at how pollution, local climate disruption and commercial shipping overlapped with the critical habitats.
"At least 70 percent of the richness areas coincide with regions highly impacted by humans," said Pompa and Ceballos. "This is powerful information that obliges us to enhance marine conservation."
"It's important to protect marine mammals if you want to keep the ocean's ecosystems functional," said study co-author and biologist Paul Ehrlich of Stanford University. "Many of them are top predators and have impacts all the way through the ecosystem. And they're also beautiful and interesting."