FISHERY BULLETIN: VOL. 84, NO. 2 



rates for this sampling period, the rates were prob- 

 ably 90-100%, as will be shown later on rookeries 

 for the period 1-21 July. A comparison of median 

 collection dates suggests arrival may have taken 

 place slightly earlier with increasing age, but no 

 clear shift in arrival times was evident, as previously 

 believed. Unfortunately, the true age-specific arrival 

 times of parous females cannot be determined 

 readily from these data. The main difficulty is that 

 the pregnant females used in the analysis included 

 not just parous seals, but postpartum seals as well. 

 Postpartum seals usually remain on land for 2 d, 

 then go to sea to forage for 8 or 9 d, and repeat this 

 pattern about 10 times throughout the nursing 

 period (Peterson 1958; Gentry and Holt in press). 

 The potentially complex effect that returning post- 

 partum females could have on the trend in the num- 

 ber of parous females arriving of a particular age 

 must be considered. Other difficulties were the small 

 sample sizes, and the fact that the sample sizes taken 

 on each date did not reflect the increase in numbers 

 on rookeries. At this time, while a slight shift in ar- 

 rival times of parous females may exist with age, 

 more research is needed for confirmation. 



Nonpregnant 



l-YEAR-OLD.-Jordan and Clark (1898) felt year- 

 ling females did not arrive on rookeries before 

 September. As noted earlier for yearling males, 

 Kenyon and Wilke (1953) felt yearlings returned to 

 the Pribilof Islands mainly during September to 

 November, and only a few individuals were involved. 

 The date of arrival for most yearling females is 

 unclear, although it is probably after yearling males, 

 during October to early November. Only a small 



number of yearling females had arrived by late 

 September to early October compared to males. 

 However, they arrived presumably no later than 

 early November, because few yearlings were pres- 

 ent on the rookeries after that time. 



2-YEAR-OLDS. -The arrival of 2-yr-old females 

 on rookeries began in August, a similar time to that 

 seen on hauling grounds. Branding studies by 

 Osgood et al. (1915) suggested a few individuals 

 began to arrive about one month after males. The 

 first branded 2-yr-old female was seen on 19 July 

 compared to 12 June for males age 2. Thus, arrival 

 was probably completed also a month later than 

 males, by mid- to late September. Jordan and Clark 

 (1898) reported that 2-yr-old females began to in- 

 crease in numbers by about 1 August, while Ken- 

 yon and Wilke (1953) noted they did not begin until 

 late August, and the current study suggested arrival 

 on hauling grounds began in mid- August. Kenyon 

 and Wilke (1953) believed the largest number were 

 present in October, slightly later than suggested by 

 the current study. Based on the comments by Ken- 

 yon and Wilke (1953) and Kenyon et al. (1954), fewer 

 2-yr-olds returned than 3-yr-olds, but more 2-yr-olds 

 returned than yearlings. 



>4-YEAR-OLDS.-A total of 1,533 females were 

 collected on rookeries during 1-6 July 1956 and 1-21 

 July 1957, a period covering the main pupping 

 season. All females were >4 yr of age. Of these, only 

 2% were nonpregnant, a low rate compared to 31% 

 nonpregnancy for the population as a whole, based 

 on the life table derived by Lander (1981). The low 

 rate likely resulted from the small number of non- 

 pregnant females on the rookeries, as was found on 



Table 4.— Summary of the times of arrival and relative numbers for males and 

 females of northern fur seal rookeries and hauling grounds of St. Paul Island, 

 based on the current study and a review of the literature. 



1 R = rookery; HG = hauling grounds. 



2 NP = nonpregnant; P = pregnant. 



3 Date when essentially all seals would have arrived. 



390 



