Successful reproduction probably begins at about ten years. B. Mating activity. Only a portion of the female population mates each year, as some are pregnant from the year before. Nonpregnant females may go into estrus some time between December and June, and most ovulate in February. In the Pacific, female herds meet male herds as they move south into the central and south Bering Sea in January.
Estrous females gather at traditional places separate from pregnant females.
Most mating probably occurs from December through March, when most sexually mature males produce viable sperm. Each herd of estrous females is attended to by one or more large adult males. According to one study, the ratio of males to females averaged 1 to 23. The males display visually and vocally from the water while the females rest. A display occurs both at and below the surface and lasts about two to three minutes.