BIGG: ARRIVAL OF NORTHERN FUR SEALS 



"nonbreeders" on hauling grounds and the inland 

 edges of rookeries. "Nonbreeders" were thought to 

 consist of idle females and young males. He ob- 

 served a sharp increase in numbers in early August 

 and that most arrived by mid- August. The current 

 study indicated the female component of Peterson's 

 "nonbreeders" were mainly nonpregnant females, 

 plus a few postpartum females. Abegglen et al. 

 (1956) noted an increase in the number of seals on 

 hauling grounds and rookery edges between 15 

 August and 4 September. While this increase may 

 have resulted from a continued influx of nonpreg- 

 nant females at >S yr, it may also have been due, 

 at least in part, to the arrival of some 2-yr-old males 

 and females. 



The increase in number of nonpregnant females 

 during August consisted primarily of 3- and 4-yr- 

 olds. A comparison of the median dates for collec- 

 tion of nonpregnant females at ^3 yr on hauling 

 grounds suggests that arrival times were similar for 

 each age (Table 2). 



Arrival of Females on Rookeries 



Pregnant, >4 Years 



Females gave birth on St. Paul Island during 15 

 June to 10 August, with about 90% of all births com- 

 pleted by 20 July (Bartholomew and Hoel 1953; 

 Peterson 1965, 1968). The general belief that preg- 

 nant females arrived by order of decreasing age ap- 

 parently originated from Wilke (1953). He collected 

 571 females on rookeries from 15 June to 4 Septem- 

 ber and showed the median date of collection for 

 each age became progressively earlier with age. For 

 example, the median collection date for females at 

 MO yr was 7 July, while that for females at 3 yr 

 was 23 August. However, Wilke did not separate 

 pregnant and nonpregnant females in his calcula- 

 tions. The large shift in median dates probably 

 resulted mainly from an influx of young nonpreg- 

 nant females on rookeries during August, as took 

 place on hauling grounds. 



An analysis of arrival times for pregnant females 

 of each age should not include seals that are non- 

 pregnant. Such an analysis can be made using data 

 collected by Wilke between 15 July and 22 July 1953 

 (Table 3). Although Wilke did not record pregnancy 



2000 



1000 



16 21 

 JULY 



26 31 



5 10 15 

 AUGUST 



20 



II 16 21 26 31 

 JULY 



5 10 15 20 

 AUGUST 



Figure 4.— Mean number, and range, of nonpregnant females of 

 northern fur seal killed of ages 2-4 on hauling grounds of St. Paul 

 Island, by date. Data from Lander (1980) and annual reports of 

 the National Marine Mammal Laboratory, Seattle. 



Figure 5.— Mean number, and range, of nonpregnant females of 

 northern fur seal killed at age >5 on hauling grounds of St. Paul 

 Island, by date. Data from Lander (1980) and annual reports of 

 the National Marine Mammal Laboratory, Seattle. 



Table 3. — Median dates of collection of northern fur seal females on rookeries of St. 

 Paul Island during 17 June to 22 July 1953. Data from Wilke (1953) and the current 

 study. 



389 



