Population Recovery of Northern Elephant Seals on the Southern California Channel Islands

Abstract

Northern elephant seals (Mirounga angustirostris) were exterminated from the southern California Channel Islands by commercial and scientific hunters by the mid-18508 and the species was believed extinct by 1892. But a small number of seals survived and were the founders of a remarkable population recovery and range expansion that continues today. Colonies on the southern California Channel Islands, which now account for over 65% of the species’ reproduction, have increased about 14% per year since breeding was re-established there in the 1950s. Pup mortality has remained low and independent of density of breeding seals in all habitats. Fit of a generalized form of the logistic equation to seal births in some breeding habitats indicates that the growth fits an asymmetric logistic growth curve. The inflection point of the logistic curve occurs at about 0.75 of carrying capacity (K). Thus, annual per capita rates of increase of births are not influenced by density of breeding females until K is closely approached. These results support theoretical predictions about density-dependent population growth of marine mammals that they should show greatest rates of increase at population sizes near K. But there is no evidence to indicate whether or not nonlinearity in density dependence will apply to resources other than breeding space as elephant seals approach K of their widespread foraging habitat.