By Rex Dalton

At the northern end of the Gulf of California, where the Baja peninsula joins the rest of Mexico, the world's most endangered marine mammal is inching closer to extinction.

With adults only 1.5 meters long, the vaquita (Phocoena sinus), a rare porpoise found only in these waters, epitomizes the plight of small cetaceans, which bear the brunt of pollution, ship traffic and fishing because they live in rivers and coastal areas. In China, the Yangtze river dolphin (Lipotes vexillifer) was last seen in 2007 and is now considered extinct. The vaquita--vulnerable to gill nets used by local fisherman--could be the next to go.

On the basis of data gathered in 2008 during an acoustic survey researchers now estimate that only 250 individuals of the species remain, a drop of 56 percent in just over a decade. The finding was presented this week at a scientific meeting of the International Whaling Commission in Agadir, Morocco.

"This information shows we don't have a lot of time to save the vaquita," says Timothy Ragen, executive director of the Marine Mammal Commission in Bethesda, Md., which part-funded the survey.

First documented in 1958, the vaquita is an elusive and poorly understood species. Genetic analyses suggest that its ancestors were Southern Hemisphere porpoises that migrated north during the last ice age. Individuals travel in small groups and rarely attract attention by leaping or splashing.

In 1997, Tim Gerrodette, a marine biologist at the Southwest Fisheries Science Center in La Jolla, Calif., led the first comprehensive survey of the vaquita, estimating the population to be 567 individuals. A decade later, another analysis, based on porpoise population rates and numbers of vaquita caught by fishermen, suggested that the number had dropped to 150.

Fearing that the porpoise's population might become too small to survive, Lorenzo Rojas-Bracho, a marine biologist at the National Institute of Ecology office in Ensenada, Mexico, teamed up with Gerrodette and others in 2008 to undertake a new abundance analysis. The team used the research ship David Starr Jordan, operated by the U.S. National Oceanic and Atmospheric Administration in Washington, D.C., and a small sailing boat, the Vaquita Express, sponsored by the Intercultural Center for the Study of Deserts and Oceans, in Tucson, Ariz., to count vaquita.

The vessels ran coordinated transects in different water depths, tallying sightings and, on some transects, towing hydrophones to catch the porpoise's distinctive communicative clicks. "Coordinating the paths of a sailboat dependent on the wind and a motorized ship was a bit tricky," says Rojas-Bracho.

The team combined the occurrence of vaquita clicks with the total area covered to estimate population size. Although the results show a precipitous decline in numbers since 1997, the findings are better than the earlier prediction. "We are encouraged, as it is not as bad as we feared," says Gerrodette. He and his colleagues were also encouraged by the sight of several newborn vaquitas. "But clearly, the number is not good news."

Now the challenge is to protect the surviving group. In 2005, Mexico created a reserve and later followed this with a ban on gill nets in the area, which covers nearly 2,000 square kilometers in waters near San Felipe off the northern Baja peninsula. Vaquita easily become entangled in the nets and drown.

Rojas-Bracho hopes to introduce alternative methods of fishing that do not rely on the nets. He and his colleagues also plan to deploy an array of 60 acoustic devices on the sea floor to detect population changes on the basis of the frequency and pattern of clicks.

But because of the potential for vandalism by fishing supporters, the locations of these hydrophones cannot be marked with floating buoys. Instead, the team needs to devise an underwater system for locating and releasing the hydrophones. If successful, the system could serve as a model for monitoring other cetaceans.

A more immediate challenge is to expand the protected area. "We need to get all the gill nets out of the water," says Ragen. But a broader ban would be a difficult economic and political challenge, pitting the vaquita against the livelihoods of local fishermen.