Netted vaquita (NOAA)
A succession of measures (described here) by the Mexican government and nonprofit agencies has failed to stem the decline. Now the government is going for the last resort: capture of vaquitas to be maintained in open-water pens, where they will hopefully survive until the Gulf can be cleared of poachers (if that's possible) or create a viable captive population.
This is very chancy stuff: the vaquita has never been maintained in captivity. Cetacean-keeping is still something of an art, and a controversial one. It is possible to maintain captive populations, the outstanding example being the bottlenose dolphin. Bottlenoses, though, mate easily in captivity (and not just with their own species, but with anything they can entice or coerce, producing a dizzying array of hybrids) and we have decades of experience maintaining them. We don't know if vaquitas will take well to a restricted pen and unusual conditions. We don't know how many breeding females there are to begin with.
Vaquita (Phocoena sinus) (NOAA)
But I don't think there's an alternative, given the way so many anti-poaching measures have fallen short - at least, no alternative less drastic than the extreme of flooding the Gulf with military forces with shoot-to-kill orders, the military is lending a hand, as U.S. Navy dolphins are among the forces being deployed now to find and bring in the vaquitas. The rest of us can only cross our fingers and wish the vaquitas good luck.