Estimates of births and standing age-distributions were combined to estimate the size of the southern elephant seal population at Marion Island at various times during its decline. To estimate births each year from 1986 through 1997 we used the number of adult females hauled out on 15 October, which is the peak haulout date for breeding elephant seal females at all breeding sites in the Indian Ocean. A conversion factor (3.15) was derived from the standing age-distributions, and applied to estimates of annual births to yield total population size. The population at Marion Island declined 37.5% overall from 1986–97 at an annual rate of 4.3% from 1986–91 and c. 2.5% yr−1 afterwards.