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Abstract

abstract

The spring 1979 breeding population of southern elephant seals Mirounga leonina at Kerguelen (49�'S, 69�' E) numbered 2 993 bulls and 38 181 cows, with 36291 live pups present during the census period. Mean harem structure (beachmasters : assistant beachmasters : challengers : bachelors : cows) was I: 0,43:2,98: 1,54:76, 13�1,55; harem sex ratio and overall sex ratio being 1:53,1 and 1:12,8 respectively. Significant differences in the overall sex ratio, harem structure and frequency distribution exist in comparison with previous surveys, particularly that of 1977. These differences may be related to the difference in the mean census dates of the different years, but were also accompanied by a continued decrease in both the cow and bull components, corroborating the slump noted since 1970. An analysis based on subdivisions of the study area into distinct regions, showed area specific increases and decreases in cow numbers, in spite of the net decrease in cow numbers. The linear regression of area specific percentage pup mortality on the density of cows was highly significant, which could explain the area specific fluctuation in cow numbers, elephant seal cows reputedly showing fidelity to their birthsites. This density dependent mechanism acts primarily through pup mortality as a function of the density of bulls active in and around the harems, bull numbers being positively correlated with cow numbers.