A team of scientists in the Gulf of California has had 25 sightings of the endangered vaquita marina, the world's smallest porpoise, in 20 days.

The monitoring team aboard the Ocean Starr is part of an effort to save the vaquita, which is only found in the gulf, also known as the Sea of Cortez.

The vaquita is threatened by gillnet fishing for totoaba, a huge fish whose swim bladder is prized by chefs in Asia.

Only about 100 are left, according to the International Committee for the Recovery of the Vaquita, and Mexico has tested unmanned drones to help save the critically endangered species.

Scientists did not say what the sightings indicate, and emphasised that they are not taking a census and could have seen multiple sightings of the same animals.

But their plan is to estimate the density of the population in vaquita waters as a base measure for the effectiveness of government programmes to save the porpoise.

"These 25 sightings are, of course, good news and give us hope," said Omar Vidal of the World Wildlife Fund.

"We need, however, to be very, very clear that it is quite early to come with any conclusions on vaquita abundance."

Mr Vidal has called the elusive porpoise "an emblematic species for Mexico, the equivalent of the panda for China".

President Enrique Pena Nieto agreed in a blog posting that the sightings give hope to efforts to rescue the population.

He listed other actions by the government to save the porpoise, including increasing by 11 times the protected area for the animal and enforcing fishing bans, especially the illegal fishing of totoaba.

He said the government is promoting new fishing methods that allow fishermen to continue their livelihoods without harming the porpoise.

Greenpeace said the efforts of Mexican government and fishermen show the "will and commitment to prevent extinction".

But it also encouraged the government to continue working on sustainable fishing methods and to work with China and US to eliminate the illegal market for totoaba.