Annual Mortality of Territorial Male Fur Seals and Its Management Significance

Abstract

The ages of 249 territorial male fur seals taken on St. Paul Island, Alaska, ranged from 7-17 years; the majority were aged 9-11 years. The ages of 156 males that died from natural causes were similar to the 249 killed. An annual mortality of 0.38 was estimated from the distribution of ages among these 405 adult males. An important cause of death is believed to be fighting. Few males under 10 years of age are successful in holding a breeding territory. Fifty percent of the territories on which males were killed were reoccupied within 24 hours, most replacements probably coming from the water rather than the land. Commercial use of young males could probably be increased without adversely affecting the productivity of the herd. The primary objective of current research on fur seals (Callorhinus ursinus) of the Pribilof Islands, Alaska, is to determine the level at which the breeding population will yield the maximum number of salable skins. Fur seals of the eastern North Pacific Ocean spend the greater part of their lives on the high seas, returning to the Pribilof Islands of St. Paul, St. George, and Sea Lion Rock in summer to bear their young and to breed. Here, the Bureau of Commercial Fisheries of the U. S. Department of the Interior supervises the annual harvest of 2to 5year-old males. Females are taken only in years when recruitment of this sex exceeds the number needed to maintain the population at the desired level. The number of males killed depends on the number of seals available on traditional hauling grounds during the period of harvesting. The degree to which a year class of males is utilized can be regulated by varying the length of the killing season, by imposing minimum and maximum limits on the size of seals taken, or by other means of varying harvest effort. The large number of mature males on the hauling grounds probably indicates that optimal use has not been achieved. Accurate estimates of the recruitment needed to maintain the population of adult males at a selected level have not been possible because the mortality was not known. Considerably more information is necessary before the optimum level for maximum production will be known. This paper presents information on the annual mortality rate of adult males as derived from the age composition of 249 territorial males killed on two areas of Northeast Point Rookery (St. Paul Island), two areas of Sivutch Rookery (Sea Lion Rock), and 156 adult males that died from natural causes on St. Paul Island beaches, in the summer of 1965. Also included is information on the rate at which animals killed were replaced. Territorial males are physically mature animals that haul out on the fur seal rookeries of the Pribilof Islands where they vigorously defend territories from late May to early August. Pregnant females returning to the rookeries accumulate in the territories, forming harems of various sizes. Not all of the territorial males acquire harems. As a group, however, they are essentially the only males active in breeding. The following biologists of the Marine Mammal Biological Laboratory assisted in collecting the data and determining ages: C. Fiscus, H. Kajimura, A. Roppel, and V. Scheffer.