This is according to the American Cetacean Society:
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Status: At one time there were three Gray whale populations: a north Atlantic population, now extinct, possibly the victims of over-hunting; a Korean or western north Pacific stock now very depleted, also possibly from over-hunting; and the eastern north Pacific population, the largest surviving population. Hunted to the edge of extinction in the 1850's after the discovery of the calving lagoons, and again in the early 1900's with the introduction of floating factories, the Gray whale was given partial protection in 1937 and full protection in 1947 by the International Whaling Commission (IWC). Since that time the eastern north Pacific Gray whale population has made a remarkable recovery and now numbers between 19,000 and 23,000, probably close to their original population size.
http://www.acsonline.org/factpack/graywhl.htmThe "California" Gray whales never completely disappeared. They were extremely endangered due to over-hunting and have since rebounded.
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Though two populations, a north-west Pacific or Asian and north-east Pacific or American, are recognized, they are not deemed distinct enough to warrant subspecific status.
http://en.wikipedia.org/wiki/Gray_whaleIn other words, there are not two distinct sub-species of Gray whale.
There is no evidence to support the idea that the variety found off the coasts of California ever vanished completely, nor is there evidence of a global migration taking place. Conversation was solely responsible for the preservation of the species.