Regulation of Numbers of an Apparently Isolated Population of Weddell Seals (Leptonychotes weddelli)

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Abstract
A population of Weddell seals (Leptonychotes weddelli) at White Island on the McMurdo Ice Shelf, Antarctica, appears to be geographically isolated. Its origin is still uncertain but three explanatory theories are mentioned. The breeding and nonbreeding fractions of the population separate during the pupping season (October to mid-December) and mix later as do their counterparts in eastern McMurdo Sound. Despite a greater reproductive potential, only two or three pups have been born annually for the past five summers. Natality clearly exceeds mortality of adults and subadults, but known survival of pups is of the same order as known mortality of adults and subadults. Censuses of seals visible on the sea ice in 1961 and 1968 were identical (11 seals), which also indicate stability in numbers of the population. The extreme fatness of all seals at White Island suggests that availability of food does not regulate numbers. Probably the availability of breathing holes in winter does. Intrasexual social strife between females also appears to lower the reproductive rate below its theoretical potential.