It is still not known if some species will ever recover, even after decades of protection.
The statistics say it all. The blue whales of the Antarctic are at less than 1 percent of their original abundance, despite 40 years of complete protection. Some populations of whales are recovering but some are not. Only one population, the East Pacific grey whale, is thought to have recovered to its original abundance, but the closely related West Pacific grey whale population is the most endangered in the world. It hovers on the edge of extinction with just over 100 remaining. [Via]