ARRIVAL OF NORTHERN FUR SEALS, CALLORHINUS URSINUS, 



ON ST. PAUL ISLAND, ALASKA 



Michael A. Bigg 1 



ABSTRACT 



The age-specific arrival times and relative numbers of northern fur seals, Callorhinus ursinus, on St. 

 Paul Island, Alaska, were determined from an analysis of kill data collected during 1956-82, and a review 

 of the fur seal literature. Arrival times differed by sex, age, and reproductive state. Arrival took place 

 progressively earlier with age in young males and females. Most males age >6 arrived by late June, 

 while most males age 5 arrived by late June to early July, those age 4 by mid-July, those age 3 by late 

 July, those age 2 by mid- to late August, and those age 1 by late September to early October. Females 

 tended to arrive later than males of the same age. Nonpregnant females age >3 arrived by mid-August, 

 while those age 2 arrived by mid- to late September, and females age 1 by October to early November. 

 Pregnant females age >4 arrived mainly by mid-July, about 1 month before nonpregnant females of the 

 same age. For both sexes, the number of seals returning increased between age 1 and age 3. Both sexes 

 appeared to stop arriving earlier and in larger numbers at about the age of sexual maturity. The process 

 of gradual maturation may play a role in inducing a cohort to undertake the return migration at earlier 

 times with age, and to cause a greater proportion to return. 



The northern fur seal, Callorhinus ursinus, inhabits 

 the North Pacific Ocean mainly between lat. 32 °N 

 and 60°N (Fiscus 1978; King 1983). The species is 

 migratory, being pelagic and widely dispersed in 

 winter, and gathering on rookeries to give birth, 

 mate, nurse, and rest in summer. Rookeries occur 

 along the Asian coast on Robben, Kurile, and Com- 

 mander Islands, and along the North American 

 coast mainly on the Pribilof Islands and on San 

 Miguel Island. The presence of large numbers of 

 animals on Robben Island, Commander Islands, and 

 the Pribilof Islands has allowed an annual commer- 

 cial kill for pelts over many years. 



The Pribilof Islands, in particular St. Paul Island 

 and St. George Island has the largest stock of seals, 

 numbering currently about 0.9 million (North Pacific 

 Fur Seal Commission 1984a). The species has been 

 harvested there almost every year since discovery 

 in 1786 (Roppel and Davey 1965; Roppel 1984). Over 

 the years, fishery managers learned to adjust the 

 kill quite specifically for seals of a particular age and 

 sex by making use of the arrival sequence of 

 migrants and their preferences for haul-out sites. 

 For example, Russians in the early 1800's took 

 juvenile males on hauling grounds, and left the 

 breeding adults and pups undisturbed on nearby 

 rookeries. Americans in the late 1800's knew that 

 the largest, and thus oldest, juvenile males arrived 

 before small males (Jordan and Clark 1898). Follow- 



^epartment of Fisheries and Oceans, Pacific Biological Station, 

 Nanaimo, British Columbia, Canada V9R 5K6. 



ing the discovery in 1950 that teeth could be used 

 for aging, the kill was refined further to focus on 

 3- and 4-yr-old males. Although the kill has been 

 directed primarily at young males since the early 

 1900's, females were taken during a herd reduction 

 program from 1956 to 1968. 



Behavioral studies on the Pribilof Islands have 

 documented the arrival times for broad population 

 categories, such as adult and juvenile males, and 

 pregnant females (Jordan and Clark 1898; Barthol- 

 omew and Hoel 1953; Peterson 1965, 1968; Gentry 

 1981). However, these studies could not determine 

 the age-specific arrival times because no method was 

 available to distinguish the age of the live animals 

 being observed. The widely accepted arrival se- 

 quence was for bulls to arrive on land first, followed 

 by progressively younger males, progressively 

 younger pregnant females, and later by mostly 

 young nonpregnant cows (Kenyon and Wilke 1953; 

 Fiscus 1978). This arrival sequence was deduced 

 from preliminary examinations of the age and sex 

 composition of commercial kills and from the arrival 

 times of tagged individuals and to some extent from 

 differences in body size, at least for the 1- and 2-yr- 

 olds. There are no published analyses that describe 

 age-specific arrival times, although some unpub- 

 lished reports give information on arrival times. 



In this study, I determine the arrival times for 

 seals of each age, sex, and reproductive condition 

 on hauling grounds and rookeries of St. Paul Island, 

 the largest of the Pribilof Islands. The study is based 



Manuscript accepted July 1985. 



FISHERY BULLETIN: VOL. 84, No. 2, 1986. 



383 



