Gray whales reach sexual maturity at eight or nine years. A female will give birth only once in two years, usually to a single calf after a 12-13 month gestation period. Most gray whales calve in the lagoons of Baja California, and as with all cetaceans , the young are born underwater and are able to swim immediately. The calves depend on a diet of rich milk for at least six months. They are weaned the following summer at seven to eight months, though young whales will often stay with their mothers for an additional one or two years.
Gray whales obtain most of their food during the summer months in the Arctic.
Their diet consists mainly of amphipods and other crustaceans, along with some mollusks and other invertebrates. Reversal of this oral suction allows the whale to force the sediment back out of the mouth trapping food items in its baleen plates. When in their southern breeding grounds, the whales typically fast, although they are known to feed on sardines . Conservation Status Records from the nineteenth century show that as many as 1,000 gray whales passed San Diego each day during their southern migration. With complete protection in the eastern Pacific beginning in 1970, this species has made a remarkable recovery to its pre-whaling population size.