494 



Fishery Bulletin 91(3). 1993 



where p denotes the finite annual per capita birth rate 

 (see Population Parameters). 



The actual size of the population at the end of the 

 pupping season, N p , was thus estimated by correcting 

 the original census counts, C„ for both the estimated 

 number of pups born subsequent to censuses and the 

 mean proportion of seals missed during censuses: 



N p = C,-P,-0.884- 



[3] 



Seasonal fluctuations in the size of the population 

 due to recruitment during the pupping season and mor- 

 tality throughout the year could not be directly moni- 

 tored because low tides suitable for censuses occurred 

 only at night prior to May and after October. Seasonal 

 fluctuations were thus modeled by adjusting the post- 

 pupping population estimate, N p , for births and deaths. 

 The Strait of Georgia population was assumed to be 

 closed to immigration and emigration, which was real- 

 istic considering that harbor seals are non-migratory 

 (Bigg, 19811 and that the density of seals in neigh- 

 bouring areas are much lower than in the study area 

 (Olesiuk et al., 1990a). The size of the population, N„ 

 on the tth date was thus estimated by 



N, = iNJP,)e- 



[4] 



where Z represents the instantaneous daily mortality 

 rate. Assuming that mortality was uniformly distrib- 

 uted throughout the year, Z was obtained from the 

 mean finite annual per capita mortality rate, 3 (see 

 Population Parameters): 



Z = |-ln( 1-3)1 -365 



[5] 



Although it was not possible to monitor seasonal 

 trends in abundance in the entire study area, seasonal 

 trends in estuaries were monitored. Estuaries could 

 be censused throughout the year because seals typi- 

 cally hauled out on logbooms that were exposed at all 

 tides, and also because swimming seals could be 

 counted in these shallow, confined areas. Between May 

 1988, and February 1989, a series of 5 aerial censuses 

 of all estuaries, except Port Moody and the Squamish 

 River estuary, were conducted at approximately 2- 

 month intervals. The aerial counts were supplemented 

 with periodic boat counts made during scat collections, 

 and for Port Moody and the Squamish River estuary 

 (which combined represented only 2-8% of the total 

 number of seals in estuaries) with counts provided by 

 knowledgeable volunteers. All counts for each estuary 

 made in a particular month during 1988-89 were av- 

 eraged and plotted at the mean date of the counts, and 

 abundance linearly interpolated between consecutive 



estimates. The total number of seals outside estuaries 

 on the tth date, N 0I , was subsequently estimated by 

 subtracting the total count for all estuaries, N E „ from 

 the estimated total size of the population, N,. 



Population parameters 



Population parameters were derived from a sample of 

 324 specimens collected by Bigg ( 1969) from the Strait 

 of Georgia and neighbouring waters during 1964-69. 

 The sample was collected following a long period ( 1913— 

 64) over which the size of the harbor seal population 

 had been reduced and maintained below natural lev- 

 els by control kills and commercial harvests (Olesiuk 4 ). 

 Since the control and commercial kills were likely 

 non-selective by sex or age, except for pups which were 

 probably more vulnerable than older animals (Ole- 

 siuk 4 ), Bigg's (1969) post-pup sample was considered 

 to be representative of a population below carrying 

 capacity with a stable sex- and age-structure. Since 

 the study population has been increasing exponentially 

 since 1970 (Olesiuk et al., 1990a), it has also likely 

 attained a stable sex- and age-structure indicative of a 

 population below carrying capacity. 



Age-specific maturation rates were calculated from 

 the data given in Bigg's (1969) Tables 1 and 2. Owing 

 to the small sample size available for males, matura- 

 tion rates of males were smoothed by calculating 

 3-point running means. Age-specific fecundity rates (i.e., 

 the proportion of all females in an age-class that give 

 birth each year), FEC IX „ were calculated as the prod- 

 uct of the age-specific pregnancy rates for mature 

 females (Bigg, 1969; Olesiuk 1 ) and the estimated 

 proportion of females in each age-class that were 

 mature. 



The stable sex- and age-structure of the study popu- 

 lation was estimated from the sex- and age-composi- 

 tion of a subsample of 245 animals aged >6.0 months 

 that were considered to have been collected in a ran- 

 dom fashion (Bigg, 1969). Following Bigg ( 1969), speci- 

 mens aged 0-5.9 months were excluded from the analy- 

 sis because of potential sampling biases, specimens 

 aged 6.0-17.9 months were tallied as 1-year-olds, speci- 

 mens aged 18.0-29.9 months as 2-year-olds, etc. The 

 exponential rate of decline, r, in the relative number of 

 animals of each sex, S (/^female and w=male) in age- 

 classes, iV sflJ , with age, X, was smoothed by regressing 

 the logarithm of the estimated number of survivors in 

 each age-class on age, such that 



Ws,x + u=-/V s *e-". 



[6] 



The residuals from the log-linear regressions indicated 

 that the rates of decline differed between sexes and 



