Temporal variation in reproduction and body mass of polar bears in western Hudson Bay

Abstract

We studied annual and long-term variation in rates of reproduction in female polar bears (Ursus maritimus) in western Hudson Bay from 1966 to 1992. In addition, long-term trends in body mass were examined from 3146 captured polar bears. Reproduction rates declined in the late 1980s from higher levels in 1966–1984. Litter production decreased from 0.48 to 0.34 litters per female each year over the study. Recruitment to autumn declined from 0.75 to 0.52 cubs per female each year, in part because of cub mortality between spring and autumn, which increased from 25.0% in 1980–1984 to 50.9% in 1987–1992. Spring and autumn litter sizes of females with cubs did not vary annually, but litter sizes of females with yearlings varied between years. The proportion of yearlings independent of their mother in autumn dropped from 81% prior to 1980 to 34% in 1980–1992. After 1986, offspring remained with their mothers longer, resulting in the birth interval increasing from 2.1 to 2.9 years. Body mass of most age-classes of females and males declined in the 1980s. Unlike earlier studies that found reproduction rates to be higher in western Hudson Bay than in higher latitude populations, reproduction rates in 1986–1992 in western Hudson Bay were similar to those of other populations. Insufficient information was available to determine the cause of declines in reproduction and body mass.