Comparative life histories of East Atlantic and other harbour seal populations

Abstract

During the seal epizootic in 1988 tissue samples were collected from harbour seals, Phoca vitulina vitulina, that died in the Kattegat-Skagerrak area. The material is used for describing life history and estimating vital parameters of East Atlantic harbour seals. From a similar size at birth of 81 em and 8.7 kg, females and males become sexually mature at 127 and 130 ern, which is close to 85% of the asymptotic lengths of 146 and 156 ern. Asymptotic weight is 67 and 75 kg for females and males, respectively, but shows considerable fluctuation during seasons of parturition, mating and moulting. During lactation females lose 26 kg (35%), and males decrease from 72 to 62 kg during theJuly mating period. Females have a mean age at sexual maturity of 3.72 years and a mean age at first parturition of 4.64. Males mature one year later. The overall pregnancy rate of females was 92% from age 3 to age 36, with lowered reproductive success after the age of 25 years. Ovulation occurs mainly during July, with a distinct peak during the last two weeks of the month. Maximum age for females was 36 and for males 31 years, and the sex ratio is skewed in favour of females in older segments. Based on an exponential rate of population increase (r=0.11) survival rates were calculated at 0.91 for adult males, whereas age-specific pregnancy rates were used to demonstrate possible combinations of juvenile and adult survival in females. The East Atlantic harbour seal is identical in the schedule of its reproductive cycle to the subspecies Phoca vitulina concolor and P. v. richardsi, but differs in seasonal timing of reproduction. Harbour seals in Alaska exhibit greater asymptotic length and weight, but no statistically significant differences in growth or reproduction could be detected between subspecies. The harbour seals in the Kattegat-Skagerrak are slightly smaller than the other two subspecies and have a relatively high rate of reproduction.