INTERANNUAL VARIATION IN BIRTH MASS AND POSTNATAL-GROWTH RATE OF FERNANDEZ,JUAN FUR SEALS

The objectives of this study were to establish body mass at birth, pos tnatal growth rate, and the factors that influence these parameters fo r the Juan Fernandez fur seal, Arctocephalus philippii. Females of thi s species have an unusual attendance pattern in which foraging trips a nd shore visits last, on average, 12.3 and 5.3 days, respectively. Pup mass was obtained from cohorts born during the reproductive seasons i n 1988 through 1992. Birth masses of male and female pups were signifi cantly different, averaging 6.1 and 5.5 kg, respectively (F = 13.2, P < 0.0003, n = 238). Birth masses also differed among cohorts, being lo west in 1992 and highest in 1990. During the first 2 months of life, m ale and female pups grew at the same rate (79 +/- 61.5 g.day(-1) (mean +/- SD); F-[1] = 0.03, P = 0.8562). Interannual differences in growth rate during the first month were significant (F-[4] = 8.14, P < 0.000 1), as was the interaction between month and year effects (F-[2] = 6.8 1, P = 0.0012). Growth rates for the 1990 cohort were lower than those in all other years except 1992. Birth masses and postnatal growth rat es of Juan Fernandez fur seal pups are comparable to those of other ot ariid species.