The Humpback whales, George and Gracie, were brought back to the future in order to save earth and repopulate the species. How was this accomplished?
While Humpback whales have a life expectancy of 45-50 years, George and Gracie have already used up about half of that life span.
Even considering that Gracie is pregnant (she told Spock), they have a gestation period of 12 months. Humpback calves nurse for approximately six months then mix nursing and independent feeding for possibly six months more. Offspring reach sexual maturity at 6-10 years of age so it would be another decade before another 'mature' whale would even be available. It's difficult to imagine creating a viable population from only two mature adults.