Olesiuk: Prey consumption of Phoca vitulina 



499 



tuaries along the coast of Washington and Oregon 

 ( Jefferies 8 ). Alternatively, these influxes also coincided 

 with the earliest low tides of the year in which the 

 sandbars utilized as haulouts were exposed during day- 

 light hours and may therefore merely indicate that 

 seals preferred to occupy areas where they could 

 haulout during daylight. 



Although numbers in Boundary Bay declined after 

 August (i.e., the end of the pupping season), the over- 

 all proportion of the population in estuaries continued 

 to increase and peaked at 18% in September. This was 

 due largely to a migration of seals from Boundary Bay 

 to the Fraser River and an influx of seals into many of 

 the smaller estuaries, where peak abundance gener- 

 ally occurred in September-November coinciding with 

 the return of spawning salmon to their natal rivers 

 (Olesiuk et al., 1990b). Weighted seasonally, 10.3% of 

 the total population inhabited estuaries. 



Population parameters 



As expected, the relative number of animals in age- 

 classes decreased with age (Fig. 4). However, piece- 

 wise log-linear regressions indicated that the rate of 

 decline for both sexes changed abruptly at 4 years of 

 age, which roughly coincided with the onset of sexual 

 maturity (see Tables 1 and 2). The regressions for fe- 

 males and males aged 1-4 years were not significantly 



"Jefferies, S. J. 1986. Seasonal movements and population trends of 

 harbor seals (Phoca vitulina richardsi) in the Columbia River and 

 adjacent waters of Washington and Oregon. Final Rep. Mar. Mam- 

 mal Comm., Contr. No. MM2079357-5, 41 p. 



A PUPS 



• JUVENILES 



ADULT FEMALES 



£ ADULT MALES 



AGE (yearsl 



Figure 4 



Exponential rates of decline in the size of age-classes of har- 

 bor seals as a function of age (Olesiuk 1 ). The trend lines 

 represent piecewise log-linear functional regressions fitted by 

 least squares criteria and scaled to an initial cohort size of 

 1000. 



different (2=0.18; P>0.50), so juveniles of both sexes 

 were pooled. For females, the rate of decline decreased 

 beyond age 4 years, whereas for males the rate in- 

 creased beyond age 4 years: 



XT _/V p -0.217S.t 

 AT_ _IU p-0.1653.t 



*'/r«« iy ftx) c 



AT -XT p-0.2878.1 



for both sexes aged 1-4 year [17] 



for females aged >4 years [18] 



for males aged >4 years [ 19] 



The N s , xl series were truncated at 29 years for females 

 and 20 years for males. The truncation points, MA : 

 and MA m , represent the oldest specimens collected by 

 Bigg (1969) and also the ages by which the size of age- 

 classes diminished to less than 0.5% the number of 

 new recruits (see Tables 1 and 2). It should be noted 

 that the rates of decline in the size of age-classes do 

 not entirely reflect mortality because the population 

 was non-stationary, such that the number of seals be- 

 ing recruited (i.e., the initial size of cohorts) had in- 

 creased over time. When corrected for an intrinsic rate 

 of increase of 12.5% per annum (Olesiuk et al., 1990a), 

 the exponential decays represent finite annual mortal- 

 ity rates of 9.5% for juveniles aged 1-4 years, 4.6%> for 

 adult females aged >4 years and 15.6% for adult males 

 aged >4 years (Olesiuk 3 ). The finite per capita birth 

 rate, (3, was calculated to be 29.8%, which implied that 

 mortality during the first year was 27.0%. The finite 

 per capita mortality rate, d, was subsequently esti- 

 mated to be 13.3%. 



The estimated number of seals in each sex- and age- 

 class indicates that the population was markedly 

 skewed toward younger age-classes (see Tables 1 and 

 2). The mean age was only 4.0 years (4.7 years for 

 females and 3.2 years for males). A total of 74% of all 

 individuals were aged <5 years and 91% were aged 

 <10 years. As noted previously, this skewed age- 

 structure was not due entirely to high mortality ( mean 

 life expectancy was 8.2 years — 10.4 years for females 

 and 6.0 years for males; Olesiuk 4 ), but also because 

 the population had been exponentially increasing since 

 1970 (Olesiuk et al, 1990a). Given a growth rate of 

 12.5%, the total population numbered only 8,770 in 

 1983 and 4,870 in 1978, such that the initial size of 

 the 1983 cohort, represented by 5-year-olds in the popu- 

 lation in 1988, was only 55% the intial size of the 1988 

 cohort, and the initial size of the 1978 cohort, repre- 

 sented by 10-year-olds in the population in 1988, only 

 31% the initial size of the 1988 cohort. In other words, 

 the population in 1988 was skewed toward younger 

 animals because most of its constituents had been re- 

 cruited in recent years. 



Bigg (1969) reported that mean body mass of pups 

 increased from 10.2 kg at birth to 24.0 kg by the end 

 of the 5-6 week nursing period, but there appeared to 

 be little further increase in body mass during the 



