Scientists have identified the 20 most important regions of the world's oceans and lakes that are key to ensuring the survival of the planet's marine mammals such as seals and porpoises. Their analysis also shows, however, that most of these areas are already under pressure from human impacts such as pollution and shipping.

Marine ecosystems around the world are deteriorating rapidly, according to Sandra Pompa, an ecologist at the National Autonomous University of Mexico who led the study, in particular due to habitat degradation, introduction of exotic species and over-exploitation of natural resources. Many species of marine mammals have experienced severe population depletion and several – including the Caribbean monk seal, Atlantic gray whale and the Steller's sea cow – became extinct in the 20th century because of the trade in their fur, blubber and meat.

Pompa led a team of scientists to try and identify which parts of the world's oceans were most crucial for the world's 129 marine mammal populations. Their results, published on Monday in the Proceedings of the National Academy of Sciences, revealed nine key global conservation sites that hold 84% of marine mammal species and 11 "irreplaceable" conservation sites, which contain species that are found nowhere else.

For the study, she split the oceans up into a grid of roughly 10,000 square kilometre boxes and examined which species lived in which boxes. The boxes were also assigned values based on whether they contained important feeding grounds or if they were in migration routes.

The main conservation areas, which contain 108 species, are the coasts of Baja California, north-eastern America, Peru, Argentina, north-western Africa, South Africa, Japan, Australia, and New Zealand. The 11 smaller conservation zones, which each contained unique species specific to them, included areas around Hawaiian and Galapagos Islands, Lake Baikal in Siberia and major rivers such as the Amazon, Ganges and Yangtze.

Pompa also correlated her map with information on human impacts such as climate disruption, ocean-based pollution and commercial shipping. Areas where there was no impact were scored zero, while areas of high impact were scored three. "Seventy per cent of the most impacted areas were near a key conservation site," said Pompa. "We are competing with [the sea mammals] in terms of shipping or ocean pollution. We want to build industry or touristic attractions and it's their home."

The next species of marine mammal that is likely to become extinct, said the researchers, is the Mexican vaquita, a small porpoise that is endemic to a small geographic area in the most northern part of Baja California. It is thought to number around 250 individuals in the wild.

"The Baikal seal, it's also a very small-numbered population," said Pompa. "Maybe you can think about the vaquita escaping the Gulf into somewhere else but the Baikal seal can't. It's a freshwater endemic mammal species. If any disruption in the lake should happen or a new sickness, they're all packed in one lake."

The researchers said that their maps should be the start of a conversation about where to site new conservation areas to safeguard the world's marine mammals. "Perhaps you are a government or NGO, you can use this information as a tool depending on the aim you have," said Pompa.

"Marine conservation is barely beginning. Marine mammals are great species because they represent healthy ecosystems so, if you begin to lose the species that give you a clue to a healthy ecosystem then you start with the degradation of all of the oceans. A visual projection of where is the richness, where are the endangered species, which corridors we need to protect in order to have all the species present in the world, it's a nice start to know where to focus the effort."