SAN FELIPE, Mexico — In the shallow sea waters of the Gulf of California swims a porpoise that few have seen, its numbers dwindling so fast that its very existence is now in peril.

Known mostly by its Spanish name, the snub-nosed vaquita is the world’s smallest cetacean, a miniature porpoise with a cartoonlike features and dark smudges around its eyes. The species lives only in the fertile waters of the gulf’s northern corner.

The size of its population has always been precarious, but now voracious demand in China for a fish that shares the vaquita’s only habitat has pushed the tiny porpoise to the brink of extinction.

No more than 30 vaquitas are left, according to a November estimate based on monitoring of their echolocation clicks. Half of the vaquitas counted a year earlier have disappeared.