The Abundance and Distribution of the Northern Sea Lion (Eumetopias jubata) on the Coast of British Columbia

The abundance and distribution of the northern sea lion, Eumetopias jubata, in British Columbia are described chiefly on the basis of a survey conducted during the summer of 1956. Results of surveys and censuses made in the years 1913, 1916, 1938 and 1955 are included for comparison. Most major rookeries and hauling-out sites were visited in 1956. Some which were missed were surveyed by aircraft in 1957.The numbers of sea lions in British Columbia are estimated to be 11,000–12,000 in 1956–57. They have apparently changed little since 1913 and 1916 when the population is estimated less reliably to have been 12,000–13,000. Some changes have occurred in their distribution, mainly as a result of organized destruction of concentrations centred near to fishing areas. Numbers in the Sea Otter Group have been reduced to about one-quarter of their former abundance. Numbers on the Cape St. James rookery, where the population has seldom been molested, have doubled.Present rookeries include: Triangle, Sartine, and Beresford Islands, in the Scott Island group; Kerouard Islands off Cape St. James; and North Danger Rocks. The Virgin and Pearl Rocks in the Sea Otter Group are no longer rookeries. Sartine Island and North Danger Rocks are recorded for the first time as rookeries. The Scott Islands and Kerouard Islands rookeries accommodate approximately 70% of the entire population, and 90% of the pups during the summer breeding season.Destruction of approximately 1,000 sea lions annually, when many of these are pups, is shown to be ineffective in substantially reducing the total population. Where efforts are concentrated in one area such as the Sea Otter Group, however, a local population can be greatly reduced and pupping curtailed or stopped.Pups are born from late May until late June. Soon after giving birth, the cows are serviced by the harem bulls. The harem structure, which averages about 10 cows per harem bull on the rookeries, begins to disintegrate near the end of July when pups take to the water and dominant bulls are replaced by reserve bulls. Some cows, probably not more than 25% at any one time, may forgo the normal annual pregnancy and continue to nurse a pup for more than a year.It is calculated that more than 70% of the sexually mature females in this population are pregnant in any one year. Natural mortality among the pups appears to be slight, but severe storms in some years may cause heavy pup mortality and constitute an important check on population growth.