BIGG: ARRIVAL OF NORTHERN FUR SEALS 



the hauling grounds at this time (Figs. 4, 5). The rate 

 was probably biased downward by the fact that non- 

 pregnant females stayed on land for a slightly 

 shorter period of time than nursing females. Using 

 data given by Gentry and Holt (in press), nonnursing 

 females appeared to stay on shore for only about 

 64% as long as nursing females. Nonnursing females 

 make about half as many visits to land as nursing 

 females, but stay about one-third longer for each 

 visit. 



A gradual increase in the nonpregnancy rate took 

 place on Polivina rookery during early to mid-July: 

 1 July - 0% (n = 280), 6 July = 2% (734), 11 July 

 = 1% (198), 16 July = 3% (148), and 21 July = 6% 

 (173). When weighted for the shorter period of stay 

 on land by nonpregnant females, the rates increased 

 from 0% by 1 July to 10% by 21 July. Presumably, 

 the increasing rate during July resulted from the 

 arrival of more nonpregnant females age >A. 

 Numbers of nonpregnant females began to increase 

 particularly by mid-July. 



DISCUSSION 



Northern fur seals arriving on St. Paul Island can 

 go first to rookeries located on beaches just above 

 high tide, or to hauling grounds more inland. The 

 typical arrival sequence (Jordan and Clark 1898; 

 Kenyon and Wilke 1953; Peterson 1965, 1968) is for 

 the bulls to establish territories for breeding on 

 rookeries in May-June. Pregnant females arrive 

 next on rookeries to pup, mate, and nurse in harems 

 within the territories. Subadult males arrive main- 

 ly during the pupping season and go to hauling 

 grounds rather than rookeries. Although young 

 males of different sizes (i.e., ages) tend to arrive in 

 successive waves with time, studies of marked seals 

 (Gentry et al. 1979) indicate that arrival times of in- 

 dividual subadult males can be quite variable be- 

 tween years. In early August, harem bulls abandon 

 their territories, and the social structure of the 

 rookery disintegrates. Nursing cows then tend to 

 disperse more widely on land, and nonterritorial 

 bulls and some subadult males move on rookeries 

 from hauling grounds. The mixing of seals between 

 rookeries and hauling grounds after July results in 

 less site distinction. The literature is unclear as to 

 the arrival times of subadult and nonpregnant adult 

 females after July, and whether these seals go first 

 to rookeries or to hauling grounds, or go to both 

 simultaneously. Age 2 females arrive later in the 

 season, and go to rookeries and hauling grounds, 

 while yearlings of both sexes arrive last, and go 

 mainly to rookeries. Seals begin leaving St. Paul 



Island for the southern migration in October to 

 November (Roppel et al. 1965a; Kenyon and Wilke 

 1953). Few remain on the hauling grounds after mid- 

 October, and few on rookeries after early 

 November. 



Table 4 summarizes the age-specific arrival times 

 and relative numbers of seals seen on rookeries and 

 hauling grounds, based on information given in the 

 Results. Two arrival times existed for pregnant 

 females, one by mid-July on rookeries and the other 

 by mid- August on hauling grounds. The second date 

 no doubt resulted from the movement of some post- 

 partum females from the rookeries to the hauling 

 grounds after the harems disintegrated. Thus, the 

 arrival time on St. Paul Island was by mid-July, 

 rather than mid-August. 



The arrival times for nonpregnant females at >2> 

 yr on to St. Paul Island was less certain than for 

 pregnant females because age-specific data on ar- 

 rival times existed from hauling grounds up to mid- 

 August, but not from rookeries after mid-July. Also 

 it was not known whether nonpregnant females 

 went first to rookeries or to hauling grounds. The 

 main arrival time was probably by mid- August, a's 

 was found on hauling grounds. This was likely 

 because nonpregnant females began to increase in 

 numbers on rookeries in early to mid-July, and an 

 interval of 1-1 V2 mo was probably needed for essen- 

 tially all arrivals to be completed. Also, Abegglen 

 et al. (1956) felt that most females on the hauling 

 grounds during August came directly from the sea, 

 although some came from rookeries. From the cur- 

 rent study, some postpartum females go from 

 rookeries to hauling grounds. Perhaps most non- 

 pregnant females go first to the hauling grounds. 



Nonpregnant females >3 yr arrived about 1 mo 

 later than pregnant females. According to R. Gen- 

 try (fn. 4), marked adult females on St. George 

 Island also arrived later when nonpregnant, al- 

 though only about 10 d later. The reason for the dif- 

 ferences in length of delay caused by nonpregnan- 

 cy found in the two studies is unclear at this time. 

 The answer may come when details of the study by 

 Gentry are reported, or perhaps when more is 

 known about movement patterns of adult females 

 between rookeries and hauling grounds. 



The finding that nonpregnant females arrived 

 after pupping suggests nonpregnancy delayed the 

 date of mating. A delay in mating has been reported 

 previously for maturing females, but not for non- 

 pregnant cows. Because parous females pup about 

 1 d after arrival, and mate 5-6 d after pupping 

 (Peterson 1968; Gentry and Holt in press), essen- 

 tially all females that pup will have mated by mid- 



391 



