Possible effects of bull harvest on the harem size in breeding colonies of Cape fur seal (Arctocephalus pusillus pusillus)

Estimates of harem sizes in breeding colonies of the Cape fur seal in Namibia have been made in order to see if the demographic structure of the colonies is affected by the bull harvest. The population size and trends are monitored by aerial photographic surveys during the breeding period. Surveys take place soon after the pupping season in mid-December when the maximum number of animals is expected to be ashore. Photographs of the colonies are taken from the aircraft at an altitude of 350-400 feet. Two colonies were selected for this study, Cape Cross and Van Reenen Bay. Cape Cross is harvested, while Van Reenen Bay is not. At Cape Cross, aerial photographs from surveys in 1982 and 2003 were used, and at Van Reenen Bay, aerial photographs from surveys in 1984 and 2001 were used. Two transects were selected from each colony, and the number of bulls and females inside those transects were counted. One reader counted all areas. The harem size at Van Reenen Bay was in the range of 7.9 and 9.1 females per bull in 1984, and 7.4 to 8.5 in 2001. At Cape Cross, the harem size was in the range of 9.5 to 12.4 females per bull in 1982, and 14.7 to 19.7 in 2003. The significant increase in harem size at Cape Cross indicate that the bull harvest effects the structure of the colony, and that the number of socially mature bulls participating in formation of harems have decreased relative to the number of females in the colony. Decreases in the proportion of socially mature males may have long-term effects on populations because the productivity of colonies is related to the number and age of the males present. This must be considered when choosing harvesting strategies in the future.