A critically endangered species of marine mammal, the vaquita porpoise that lives in the waters off Mexico, could be extinct in 4 years, wildlife conservationists warn.

The rare porpoises are dying in great numbers as they become entangled in fishing nets, and world governments have not done enough to protect them from illegal fishing operations, a study has found.

Possessing a distinctive "smiling" look to their faces, the vaquita -- Phocoena sinus -- live in the Gulf of California, and are the smallest of the world's porpoises at just 4 or 5 five feet long.

The animals are notoriously shy, experts say.

"They hardly ever appear," says Rebecca Lent, executive director of the Marine Mammal Commission. "Most of the ones we have are ones that came up dead in fishing nets."

The commission is an independent federal agency charge with protecting and conserving marine mammals.

The International Union for the Conservation of Nature has listed the vaquita porpoise as critically endangered, with estimates of only around 200 still living in the wild.

The vaquita give birth to one baby porpoise only every other year, with experts reporting the population dropped almost 20 percent in just the past year.

It's chiefly illegal fishing that is to blame for that drop, wildlife experts say.

The porpoises are not the targets of the illegal fishing, they point out; the boats are trawling for an endangered species of fish known as a totoaba, highly prized in Asia for the purported medicinal benefits of its swim bladder.

The have begun utilizing gill nets, vertical nets that capture and hold fish by their gills, conservationists say.

"It's essentially a wall across the environment," says Peter Thomas, the commission's International and Policy Program director. "It's not visible to the vaquita," so they become tangled in the nets and die, he says.

Although fishing with gill nets is outlawed is some areas of the vaquita's habitat, the IUCN says boats equipped with gill nets have been seen in those protected areas.

Enforcement of fishing regulations needs to be stronger, Lent says, urging that the vaquita's entire range be given protected status and gill net fishing banned there.

She also urges measures that would make even having gill nets aboard a ship in the protected area illegal.

"We're really down to the wire now" to save the vaquita, Lent says. "We really only have a couple of years."

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